VM_HIGH_ARCH_5 is used for riscv

Reviewed-by: Zong Li zong.li@sifive.com Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com Acked-by: David Hildenbrand david@redhat.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- include/linux/mm.h | 7 +++++++ 1 file changed, 7 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h index d16b33bacc32..2032d3f195f1 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -380,6 +380,13 @@ extern unsigned int kobjsize(const void *objp); # define VM_SHADOW_STACK VM_HIGH_ARCH_6 #endif

+#if defined(CONFIG_RISCV_USER_CFI) +/* + * Following x86 and picking up the same bitpos. + */ +# define VM_SHADOW_STACK VM_HIGH_ARCH_5 +#endif + #ifndef VM_SHADOW_STACK # define VM_SHADOW_STACK VM_NONE #endif

This patch adds support for detecting zicfiss and zicfilp. zicfiss and zicfilp stands for unprivleged integer spec extension for shadow stack and branch tracking on indirect branches, respectively.

This patch looks for zicfiss and zicfilp in device tree and accordinlgy lights up bit in cpu feature bitmap. Furthermore this patch adds detection utility functions to return whether shadow stack or landing pads are supported by cpu.

Reviewed-by: Zong Li zong.li@sifive.com Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/cpufeature.h | 12 ++++++++++++ arch/riscv/include/asm/hwcap.h | 2 ++ arch/riscv/kernel/cpufeature.c | 22 ++++++++++++++++++++++ 3 files changed, 36 insertions(+)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h index fbd0e4306c93..481f483ebf15 100644 --- a/arch/riscv/include/asm/cpufeature.h +++ b/arch/riscv/include/asm/cpufeature.h @@ -150,4 +150,16 @@ static __always_inline bool riscv_cpu_has_extension_unlikely(int cpu, const unsi return __riscv_isa_extension_available(hart_isa[cpu].isa, ext); }

+static inline bool cpu_supports_shadow_stack(void) +{ + return (IS_ENABLED(CONFIG_RISCV_USER_CFI) && + riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICFISS)); +} + +static inline bool cpu_supports_indirect_br_lp_instr(void) +{ + return (IS_ENABLED(CONFIG_RISCV_USER_CFI) && + riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICFILP)); +} + #endif diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h index affd63e11b0a..7c4619a6d70d 100644 --- a/arch/riscv/include/asm/hwcap.h +++ b/arch/riscv/include/asm/hwcap.h @@ -106,6 +106,8 @@ #define RISCV_ISA_EXT_ZAAMO 97 #define RISCV_ISA_EXT_ZALRSC 98 #define RISCV_ISA_EXT_ZICBOP 99 +#define RISCV_ISA_EXT_ZICFILP 100 +#define RISCV_ISA_EXT_ZICFISS 101

#define RISCV_ISA_EXT_XLINUXENVCFG 127

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 67b59699357d..17b9e77bafc3 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -274,6 +274,24 @@ static int riscv_ext_svadu_validate(const struct riscv_isa_ext_data *data, return 0; }

+static int riscv_cfilp_validate(const struct riscv_isa_ext_data *data, + const unsigned long *isa_bitmap) +{ + if (!IS_ENABLED(CONFIG_RISCV_USER_CFI)) + return -EINVAL; + + return 0; +} + +static int riscv_cfiss_validate(const struct riscv_isa_ext_data *data, + const unsigned long *isa_bitmap) +{ + if (!IS_ENABLED(CONFIG_RISCV_USER_CFI)) + return -EINVAL; + + return 0; +} + static const unsigned int riscv_a_exts[] = { RISCV_ISA_EXT_ZAAMO, RISCV_ISA_EXT_ZALRSC, @@ -461,6 +479,10 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = { __RISCV_ISA_EXT_DATA_VALIDATE(zicbop, RISCV_ISA_EXT_ZICBOP, riscv_ext_zicbop_validate), __RISCV_ISA_EXT_SUPERSET_VALIDATE(zicboz, RISCV_ISA_EXT_ZICBOZ, riscv_xlinuxenvcfg_exts, riscv_ext_zicboz_validate), __RISCV_ISA_EXT_DATA(ziccrse, RISCV_ISA_EXT_ZICCRSE), + __RISCV_ISA_EXT_SUPERSET_VALIDATE(zicfilp, RISCV_ISA_EXT_ZICFILP, riscv_xlinuxenvcfg_exts, + riscv_cfilp_validate), + __RISCV_ISA_EXT_SUPERSET_VALIDATE(zicfiss, RISCV_ISA_EXT_ZICFISS, riscv_xlinuxenvcfg_exts, + riscv_cfiss_validate), __RISCV_ISA_EXT_DATA(zicntr, RISCV_ISA_EXT_ZICNTR), __RISCV_ISA_EXT_DATA(zicond, RISCV_ISA_EXT_ZICOND), __RISCV_ISA_EXT_DATA(zicsr, RISCV_ISA_EXT_ZICSR),

Carves out space in arch specific thread struct for cfi status and shadow stack in usermode on riscv.

This patch does following - defines a new structure cfi_status with status bit for cfi feature - defines shadow stack pointer, base and size in cfi_status structure - defines offsets to new member fields in thread in asm-offsets.c - Saves and restore shadow stack pointer on trap entry (U --> S) and exit (S --> U)

Shadow stack save/restore is gated on feature availiblity and implemented using alternative. CSR can be context switched in `switch_to` as well but soon as kernel shadow stack support gets rolled in, shadow stack pointer will need to be switched at trap entry/exit point (much like `sp`). It can be argued that kernel using shadow stack deployment scenario may not be as prevalant as user mode using this feature. But even if there is some minimal deployment of kernel shadow stack, that means that it needs to be supported. And thus save/restore of shadow stack pointer in entry.S instead of in `switch_to.h`.

Reviewed-by: Charlie Jenkins charlie@rivosinc.com Reviewed-by: Zong Li zong.li@sifive.com Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/processor.h | 1 + arch/riscv/include/asm/thread_info.h | 3 +++ arch/riscv/include/asm/usercfi.h | 23 +++++++++++++++++++++++ arch/riscv/kernel/asm-offsets.c | 4 ++++ arch/riscv/kernel/entry.S | 31 +++++++++++++++++++++++++++++++ 5 files changed, 62 insertions(+)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h index da5426122d28..4c3dd94d0f63 100644 --- a/arch/riscv/include/asm/processor.h +++ b/arch/riscv/include/asm/processor.h @@ -16,6 +16,7 @@ #include <asm/insn-def.h> #include <asm/alternative-macros.h> #include <asm/hwcap.h> +#include <asm/usercfi.h>

#define arch_get_mmap_end(addr, len, flags) \ ({ \ diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h index 836d80dd2921..36918c9200c9 100644 --- a/arch/riscv/include/asm/thread_info.h +++ b/arch/riscv/include/asm/thread_info.h @@ -73,6 +73,9 @@ struct thread_info { */ unsigned long a0, a1, a2; #endif +#ifdef CONFIG_RISCV_USER_CFI + struct cfi_state user_cfi_state; +#endif };

#ifdef CONFIG_SHADOW_CALL_STACK diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h new file mode 100644 index 000000000000..4c5233e8f3f9 --- /dev/null +++ b/arch/riscv/include/asm/usercfi.h @@ -0,0 +1,23 @@ +/* SPDX-License-Identifier: GPL-2.0 + * Copyright (C) 2024 Rivos, Inc. + * Deepak Gupta debug@rivosinc.com + */ +#ifndef _ASM_RISCV_USERCFI_H +#define _ASM_RISCV_USERCFI_H + +#ifndef __ASSEMBLER__ +#include <linux/types.h> + +#ifdef CONFIG_RISCV_USER_CFI +struct cfi_state { + unsigned long ubcfi_en : 1; /* Enable for backward cfi. */ + unsigned long user_shdw_stk; /* Current user shadow stack pointer */ + unsigned long shdw_stk_base; /* Base address of shadow stack */ + unsigned long shdw_stk_size; /* size of shadow stack */ +}; + +#endif /* CONFIG_RISCV_USER_CFI */ + +#endif /* __ASSEMBLER__ */ + +#endif /* _ASM_RISCV_USERCFI_H */ diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c index 7d42d3b8a32a..8a2b2656cb2f 100644 --- a/arch/riscv/kernel/asm-offsets.c +++ b/arch/riscv/kernel/asm-offsets.c @@ -51,6 +51,10 @@ void asm_offsets(void) #endif

OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu); +#ifdef CONFIG_RISCV_USER_CFI + OFFSET(TASK_TI_CFI_STATE, task_struct, thread_info.user_cfi_state); + OFFSET(TASK_TI_USER_SSP, task_struct, thread_info.user_cfi_state.user_shdw_stk); +#endif OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]); OFFSET(TASK_THREAD_F1, task_struct, thread.fstate.f[1]); OFFSET(TASK_THREAD_F2, task_struct, thread.fstate.f[2]); diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S index d3d92a4becc7..8410850953d6 100644 --- a/arch/riscv/kernel/entry.S +++ b/arch/riscv/kernel/entry.S @@ -92,6 +92,35 @@ REG_L a0, TASK_TI_A0(tp) .endm

+/* + * If previous mode was U, capture shadow stack pointer and save it away + * Zero CSR_SSP at the same time for sanitization. + */ +.macro save_userssp tmp, status + ALTERNATIVE("nops(4)", + __stringify( \ + andi \tmp, \status, SR_SPP; \ + bnez \tmp, skip_ssp_save; \ + csrrw \tmp, CSR_SSP, x0; \ + REG_S \tmp, TASK_TI_USER_SSP(tp); \ + skip_ssp_save:), + 0, + RISCV_ISA_EXT_ZICFISS, + CONFIG_RISCV_USER_CFI) +.endm + +.macro restore_userssp tmp, status + ALTERNATIVE("nops(4)", + __stringify( \ + andi \tmp, \status, SR_SPP; \ + bnez \tmp, skip_ssp_restore; \ + REG_L \tmp, TASK_TI_USER_SSP(tp); \ + csrw CSR_SSP, \tmp; \ + skip_ssp_restore:), + 0, + RISCV_ISA_EXT_ZICFISS, + CONFIG_RISCV_USER_CFI) +.endm

SYM_CODE_START(handle_exception) /* @@ -148,6 +177,7 @@ SYM_CODE_START(handle_exception)

REG_L s0, TASK_TI_USER_SP(tp) csrrc s1, CSR_STATUS, t0 + save_userssp s2, s1 csrr s2, CSR_EPC csrr s3, CSR_TVAL csrr s4, CSR_CAUSE @@ -243,6 +273,7 @@ SYM_CODE_START_NOALIGN(ret_from_exception) call riscv_v_context_nesting_end #endif REG_L a0, PT_STATUS(sp) + restore_userssp s3, a0 /* * The current load reservation is effectively part of the processor's * state, in the sense that load reservations cannot be shared between

This patch implements creating shadow stack pte (on riscv). Creating shadow stack PTE on riscv means that clearing RWX and then setting W=1.

Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/pgtable.h | 10 ++++++++++ 1 file changed, 10 insertions(+)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h index 4c4057a2550e..e4eb4657e1b6 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -425,6 +425,11 @@ static inline pte_t pte_mkwrite_novma(pte_t pte) return __pte(pte_val(pte) | _PAGE_WRITE); }

+static inline pte_t pte_mkwrite_shstk(pte_t pte) +{ + return __pte((pte_val(pte) & ~(_PAGE_LEAF)) | _PAGE_WRITE); +} + /* static inline pte_t pte_mkexec(pte_t pte) */

static inline pte_t pte_mkdirty(pte_t pte) @@ -765,6 +770,11 @@ static inline pmd_t pmd_mkwrite_novma(pmd_t pmd) return pte_pmd(pte_mkwrite_novma(pmd_pte(pmd))); }

+static inline pmd_t pmd_mkwrite_shstk(pmd_t pte) +{ + return __pmd((pmd_val(pte) & ~(_PAGE_LEAF)) | _PAGE_WRITE); +} + static inline pmd_t pmd_wrprotect(pmd_t pmd) { return pte_pmd(pte_wrprotect(pmd_pte(pmd)));

`fork` implements copy on write (COW) by making pages readonly in child and parent both.

ptep_set_wrprotect and pte_wrprotect clears _PAGE_WRITE in PTE. Assumption is that page is readable and on fault copy on write happens.

To implement COW on shadow stack pages, clearing up W bit makes them XWR = 000. This will result in wrong PTE setting which says no perms but V=1 and PFN field pointing to final page. Instead desired behavior is to turn it into a readable page, take an access (load/store) fault on sspush/sspop (shadow stack) and then perform COW on such pages. This way regular reads would still be allowed and not lead to COW maintaining current behavior of COW on non-shadow stack but writeable memory.

On the other hand it doesn't interfere with existing COW for read-write memory. Assumption is always that _PAGE_READ must have been set and thus setting _PAGE_READ is harmless.

Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/pgtable.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h index b03e8f85221f..df4a04b64944 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -415,7 +415,7 @@ static inline int pte_special(pte_t pte)

static inline pte_t pte_wrprotect(pte_t pte) { - return __pte(pte_val(pte) & ~(_PAGE_WRITE)); + return __pte((pte_val(pte) & ~(_PAGE_WRITE)) | (_PAGE_READ)); }

/* static inline pte_t pte_mkread(pte_t pte) */ @@ -611,7 +611,15 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm, static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long address, pte_t *ptep) { - atomic_long_and(~(unsigned long)_PAGE_WRITE, (atomic_long_t *)ptep); + pte_t read_pte = READ_ONCE(*ptep); + /* + * ptep_set_wrprotect can be called for shadow stack ranges too. + * shadow stack memory is XWR = 010 and thus clearing _PAGE_WRITE will lead to + * encoding 000b which is wrong encoding with V = 1. This should lead to page fault + * but we dont want this wrong configuration to be set in page tables. + */ + atomic_long_set((atomic_long_t *)ptep, + ((pte_val(read_pte) & ~(unsigned long)_PAGE_WRITE) | _PAGE_READ)); }

#define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH

Userspace specifies CLONE_VM to share address space and spawn new thread. `clone` allow userspace to specify a new stack for new thread. However there is no way to specify new shadow stack base address without changing API. This patch allocates a new shadow stack whenever CLONE_VM is given.

In case of CLONE_VFORK, parent is suspended until child finishes and thus can child use parent shadow stack. In case of !CLONE_VM, COW kicks in because entire address space is copied from parent to child.

`clone3` is extensible and can provide mechanisms using which shadow stack as an input parameter can be provided. This is not settled yet and being extensively discussed on mailing list. Once that's settled, this commit will adapt to that.

Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/mmu_context.h | 7 ++ arch/riscv/include/asm/usercfi.h | 25 ++++++++ arch/riscv/kernel/process.c | 10 +++ arch/riscv/kernel/usercfi.c | 120 +++++++++++++++++++++++++++++++++++ 4 files changed, 162 insertions(+)

diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h index 8c4bc49a3a0f..dbf27a78df6c 100644 --- a/arch/riscv/include/asm/mmu_context.h +++ b/arch/riscv/include/asm/mmu_context.h @@ -48,6 +48,13 @@ static inline unsigned long mm_untag_mask(struct mm_struct *mm) } #endif

+#define deactivate_mm deactivate_mm +static inline void deactivate_mm(struct task_struct *tsk, + struct mm_struct *mm) +{ + shstk_release(tsk); +} + #include <asm-generic/mmu_context.h>

#endif /* _ASM_RISCV_MMU_CONTEXT_H */ diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h index 4c5233e8f3f9..a16a5dff8b0e 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -8,6 +8,9 @@ #ifndef __ASSEMBLER__ #include <linux/types.h>

+struct task_struct; +struct kernel_clone_args; + #ifdef CONFIG_RISCV_USER_CFI struct cfi_state { unsigned long ubcfi_en : 1; /* Enable for backward cfi. */ @@ -16,6 +19,28 @@ struct cfi_state { unsigned long shdw_stk_size; /* size of shadow stack */ };

+unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, + const struct kernel_clone_args *args); +void shstk_release(struct task_struct *tsk); +void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size); +unsigned long get_shstk_base(struct task_struct *task, unsigned long *size); +void set_active_shstk(struct task_struct *task, unsigned long shstk_addr); +bool is_shstk_enabled(struct task_struct *task); + +#else + +#define shstk_alloc_thread_stack(tsk, args) 0 + +#define shstk_release(tsk) + +#define get_shstk_base(task, size) 0UL + +#define set_shstk_base(task, shstk_addr, size) do {} while (0) + +#define set_active_shstk(task, shstk_addr) do {} while (0) + +#define is_shstk_enabled(task) false + #endif /* CONFIG_RISCV_USER_CFI */

#endif /* __ASSEMBLER__ */ diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index 31a392993cb4..72d35adc6e0e 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -31,6 +31,7 @@ #include <asm/vector.h> #include <asm/cpufeature.h> #include <asm/exec.h> +#include <asm/usercfi.h>

#if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK) #include <linux/stackprotector.h> @@ -226,6 +227,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) u64 clone_flags = args->flags; unsigned long usp = args->stack; unsigned long tls = args->tls; + unsigned long ssp = 0; struct pt_regs *childregs = task_pt_regs(p);

/* Ensure all threads in this mm have the same pointer masking mode. */ @@ -245,11 +247,19 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) p->thread.s[1] = (unsigned long)args->fn_arg; p->thread.ra = (unsigned long)ret_from_fork_kernel_asm; } else { + /* allocate new shadow stack if needed. In case of CLONE_VM we have to */ + ssp = shstk_alloc_thread_stack(p, args); + if (IS_ERR_VALUE(ssp)) + return PTR_ERR((void *)ssp); + *childregs = *(current_pt_regs()); /* Turn off status.VS */ riscv_v_vstate_off(childregs); if (usp) /* User fork */ childregs->sp = usp; + /* if needed, set new ssp */ + if (ssp) + set_active_shstk(p, ssp); if (clone_flags & CLONE_SETTLS) childregs->tp = tls; childregs->a0 = 0; /* Return value of fork() */ diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index 251c3faccbf8..872e0c52e034 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -19,6 +19,41 @@

#define SHSTK_ENTRY_SIZE sizeof(void *)

+bool is_shstk_enabled(struct task_struct *task) +{ + return task->thread_info.user_cfi_state.ubcfi_en; +} + +void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size) +{ + task->thread_info.user_cfi_state.shdw_stk_base = shstk_addr; + task->thread_info.user_cfi_state.shdw_stk_size = size; +} + +unsigned long get_shstk_base(struct task_struct *task, unsigned long *size) +{ + if (size) + *size = task->thread_info.user_cfi_state.shdw_stk_size; + return task->thread_info.user_cfi_state.shdw_stk_base; +} + +void set_active_shstk(struct task_struct *task, unsigned long shstk_addr) +{ + task->thread_info.user_cfi_state.user_shdw_stk = shstk_addr; +} + +/* + * If size is 0, then to be compatible with regular stack we want it to be as big as + * regular stack. Else PAGE_ALIGN it and return back + */ +static unsigned long calc_shstk_size(unsigned long size) +{ + if (size) + return PAGE_ALIGN(size); + + return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G)); +} + /* * Writes on shadow stack can either be `sspush` or `ssamoswap`. `sspush` can happen * implicitly on current shadow stack pointed to by CSR_SSP. `ssamoswap` takes pointer to @@ -140,3 +175,88 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi

return allocate_shadow_stack(addr, aligned_size, size, set_tok); } + +/* + * This gets called during clone/clone3/fork. And is needed to allocate a shadow stack for + * cases where CLONE_VM is specified and thus a different stack is specified by user. We + * thus need a separate shadow stack too. How does separate shadow stack is specified by + * user is still being debated. Once that's settled, remove this part of the comment. + * This function simply returns 0 if shadow stack are not supported or if separate shadow + * stack allocation is not needed (like in case of !CLONE_VM) + */ +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, + const struct kernel_clone_args *args) +{ + unsigned long addr, size; + + /* If shadow stack is not supported, return 0 */ + if (!cpu_supports_shadow_stack()) + return 0; + + /* + * If shadow stack is not enabled on the new thread, skip any + * switch to a new shadow stack. + */ + if (!is_shstk_enabled(tsk)) + return 0; + + /* + * For CLONE_VFORK the child will share the parents shadow stack. + * Set base = 0 and size = 0, this is special means to track this state + * so the freeing logic run for child knows to leave it alone. + */ + if (args->flags & CLONE_VFORK) { + set_shstk_base(tsk, 0, 0); + return 0; + } + + /* + * For !CLONE_VM the child will use a copy of the parents shadow + * stack. + */ + if (!(args->flags & CLONE_VM)) + return 0; + + /* + * reaching here means, CLONE_VM was specified and thus a separate shadow + * stack is needed for new cloned thread. Note: below allocation is happening + * using current mm. + */ + size = calc_shstk_size(args->stack_size); + addr = allocate_shadow_stack(0, size, 0, false); + if (IS_ERR_VALUE(addr)) + return addr; + + set_shstk_base(tsk, addr, size); + + return addr + size; +} + +void shstk_release(struct task_struct *tsk) +{ + unsigned long base = 0, size = 0; + /* If shadow stack is not supported or not enabled, nothing to release */ + if (!cpu_supports_shadow_stack() || !is_shstk_enabled(tsk)) + return; + + /* + * When fork() with CLONE_VM fails, the child (tsk) already has a + * shadow stack allocated, and exit_thread() calls this function to + * free it. In this case the parent (current) and the child share + * the same mm struct. Move forward only when they're same. + */ + if (!tsk->mm || tsk->mm != current->mm) + return; + + /* + * We know shadow stack is enabled but if base is NULL, then + * this task is not managing its own shadow stack (CLONE_VFORK). So + * skip freeing it. + */ + base = get_shstk_base(tsk, &size); + if (!base) + return; + + vm_munmap(base, size); + set_shstk_base(tsk, 0, 0); +}

Three architectures (x86, aarch64, riscv) have support for indirect branch tracking feature in a very similar fashion. On a very high level, indirect branch tracking is a CPU feature where CPU tracks branches which uses memory operand to perform control transfer in program. As part of this tracking on indirect branches, CPU goes in a state where it expects a landing pad instr on target and if not found then CPU raises some fault (architecture dependent)

x86 landing pad instr - `ENDBRANCH` arch64 landing pad instr - `BTI` riscv landing instr - `lpad`

Given that three major arches have support for indirect branch tracking, This patch makes `prctl` for indirect branch tracking arch agnostic.

To allow userspace to enable this feature for itself, following prtcls are defined: - PR_GET_INDIR_BR_LP_STATUS: Gets current configured status for indirect branch tracking. - PR_SET_INDIR_BR_LP_STATUS: Sets a configuration for indirect branch tracking. Following status options are allowed - PR_INDIR_BR_LP_ENABLE: Enables indirect branch tracking on user thread. - PR_INDIR_BR_LP_DISABLE; Disables indirect branch tracking on user thread. - PR_LOCK_INDIR_BR_LP_STATUS: Locks configured status for indirect branch tracking for user thread.

Reviewed-by: Mark Brown broonie@kernel.org Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- include/linux/cpu.h | 4 ++++ include/uapi/linux/prctl.h | 27 +++++++++++++++++++++++++++ kernel/sys.c | 30 ++++++++++++++++++++++++++++++ 3 files changed, 61 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h index 487b3bf2e1ea..8239cd95a005 100644 --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -229,4 +229,8 @@ static inline bool cpu_attack_vector_mitigated(enum cpu_attack_vectors v) #define smt_mitigations SMT_MITIGATIONS_OFF #endif

+int arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status); +int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status); +int arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long status); + #endif /* _LINUX_CPU_H_ */ diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 51c4e8c82b1e..9b4afdc85099 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -386,4 +386,31 @@ struct prctl_mm_map { # define PR_FUTEX_HASH_SET_SLOTS 1 # define PR_FUTEX_HASH_GET_SLOTS 2

+/* + * Get the current indirect branch tracking configuration for the current + * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS. + */ +#define PR_GET_INDIR_BR_LP_STATUS 79 + +/* + * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will + * enable cpu feature for user thread, to track all indirect branches and ensure + * they land on arch defined landing pad instruction. + * x86 - If enabled, an indirect branch must land on `ENDBRANCH` instruction. + * arch64 - If enabled, an indirect branch must land on `BTI` instruction. + * riscv - If enabled, an indirect branch must land on `lpad` instruction. + * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect + * branches will no more be tracked by cpu to land on arch defined landing pad + * instruction. + */ +#define PR_SET_INDIR_BR_LP_STATUS 80 +# define PR_INDIR_BR_LP_ENABLE (1UL << 0) + +/* + * Prevent further changes to the specified indirect branch tracking + * configuration. All bits may be locked via this call, including + * undefined bits. + */ +#define PR_LOCK_INDIR_BR_LP_STATUS 81 + #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/sys.c b/kernel/sys.c index 8b58eece4e58..9071422c1609 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2388,6 +2388,21 @@ int __weak arch_lock_shadow_stack_status(struct task_struct *t, unsigned long st return -EINVAL; }

+int __weak arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status) +{ + return -EINVAL; +} + +int __weak arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status) +{ + return -EINVAL; +} + +int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long status) +{ + return -EINVAL; +} + #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)

static int prctl_set_vma(unsigned long opt, unsigned long addr, @@ -2868,6 +2883,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, case PR_FUTEX_HASH: error = futex_hash_prctl(arg2, arg3, arg4); break; + case PR_GET_INDIR_BR_LP_STATUS: + if (arg3 || arg4 || arg5) + return -EINVAL; + error = arch_get_indir_br_lp_status(me, (unsigned long __user *)arg2); + break; + case PR_SET_INDIR_BR_LP_STATUS: + if (arg3 || arg4 || arg5) + return -EINVAL; + error = arch_set_indir_br_lp_status(me, arg2); + break; + case PR_LOCK_INDIR_BR_LP_STATUS: + if (arg3 || arg4 || arg5) + return -EINVAL; + error = arch_lock_indir_br_lp_status(me, arg2); + break; default: trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5); error = -EINVAL;

zicfiss / zicfilp introduces a new exception to priv isa `software check exception` with cause code = 18. This patch implements software check exception.

Additionally it implements a cfi violation handler which checks for code in xtval. If xtval=2, it means that sw check exception happened because of an indirect branch not landing on 4 byte aligned PC or not landing on `lpad` instruction or label value embedded in `lpad` not matching label value setup in `x7`. If xtval=3, it means that sw check exception happened because of mismatch between link register (x1 or x5) and top of shadow stack (on execution of `sspopchk`).

In case of cfi violation, SIGSEGV is raised with code=SEGV_CPERR. SEGV_CPERR was introduced by x86 shadow stack patches.

To keep uprobes working, handle the uprobe event first before reporting the CFI violation in software-check exception handler. Because when the landing pad is activated, if the uprobe point is set at the lpad instruction at the beginning of a function, the system triggers a software -check exception instead of an ebreak exception due to the exception priority, then uprobe can't work successfully.

Co-developed-by: Zong Li zong.li@sifive.com Reviewed-by: Zong Li zong.li@sifive.com Signed-off-by: Zong Li zong.li@sifive.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/asm-prototypes.h | 1 + arch/riscv/include/asm/entry-common.h | 2 ++ arch/riscv/kernel/entry.S | 3 ++ arch/riscv/kernel/traps.c | 54 +++++++++++++++++++++++++++++++++ 4 files changed, 60 insertions(+)

diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h index a9988bf21ec8..41ec5cdec367 100644 --- a/arch/riscv/include/asm/asm-prototypes.h +++ b/arch/riscv/include/asm/asm-prototypes.h @@ -51,6 +51,7 @@ DECLARE_DO_ERROR_INFO(do_trap_ecall_u); DECLARE_DO_ERROR_INFO(do_trap_ecall_s); DECLARE_DO_ERROR_INFO(do_trap_ecall_m); DECLARE_DO_ERROR_INFO(do_trap_break); +DECLARE_DO_ERROR_INFO(do_trap_software_check);

asmlinkage void ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), struct pt_regs *regs); asmlinkage void ret_from_fork_user(struct pt_regs *regs); diff --git a/arch/riscv/include/asm/entry-common.h b/arch/riscv/include/asm/entry-common.h index b28ccc6cdeea..34ed149af5d1 100644 --- a/arch/riscv/include/asm/entry-common.h +++ b/arch/riscv/include/asm/entry-common.h @@ -40,4 +40,6 @@ static inline int handle_misaligned_store(struct pt_regs *regs) } #endif

+bool handle_user_cfi_violation(struct pt_regs *regs); + #endif /* _ASM_RISCV_ENTRY_COMMON_H */ diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S index 036a6ca7641f..53c5aa0b6a16 100644 --- a/arch/riscv/kernel/entry.S +++ b/arch/riscv/kernel/entry.S @@ -495,6 +495,9 @@ SYM_DATA_START_LOCAL(excp_vect_table) RISCV_PTR do_page_fault /* load page fault */ RISCV_PTR do_trap_unknown RISCV_PTR do_page_fault /* store page fault */ + RISCV_PTR do_trap_unknown /* cause=16 */ + RISCV_PTR do_trap_unknown /* cause=17 */ + RISCV_PTR do_trap_software_check /* cause=18 is sw check exception */ SYM_DATA_END_LABEL(excp_vect_table, SYM_L_LOCAL, excp_vect_table_end)

#ifndef CONFIG_MMU diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c index 80230de167de..d939a8dbdb15 100644 --- a/arch/riscv/kernel/traps.c +++ b/arch/riscv/kernel/traps.c @@ -366,6 +366,60 @@ void do_trap_ecall_u(struct pt_regs *regs)

}

+#define CFI_TVAL_FCFI_CODE 2 +#define CFI_TVAL_BCFI_CODE 3 +/* handle cfi violations */ +bool handle_user_cfi_violation(struct pt_regs *regs) +{ + unsigned long tval = csr_read(CSR_TVAL); + bool is_fcfi = (tval == CFI_TVAL_FCFI_CODE && cpu_supports_indirect_br_lp_instr()); + bool is_bcfi = (tval == CFI_TVAL_BCFI_CODE && cpu_supports_shadow_stack()); + + /* + * Handle uprobe event first. The probe point can be a valid target + * of indirect jumps or calls, in this case, forward cfi violation + * will be triggered instead of breakpoint exception. Clear ELP flag + * on sstatus image as well to avoid recurring fault. + */ + if (is_fcfi && probe_breakpoint_handler(regs)) { + regs->status &= ~SR_ELP; + return true; + } + + if (is_fcfi || is_bcfi) { + do_trap_error(regs, SIGSEGV, SEGV_CPERR, regs->epc, + "Oops - control flow violation"); + return true; + } + + return false; +} + +/* + * software check exception is defined with risc-v cfi spec. Software check + * exception is raised when:- + * a) An indirect branch doesn't land on 4 byte aligned PC or `lpad` + * instruction or `label` value programmed in `lpad` instr doesn't + * match with value setup in `x7`. reported code in `xtval` is 2. + * b) `sspopchk` instruction finds a mismatch between top of shadow stack (ssp) + * and x1/x5. reported code in `xtval` is 3. + */ +asmlinkage __visible __trap_section void do_trap_software_check(struct pt_regs *regs) +{ + if (user_mode(regs)) { + irqentry_enter_from_user_mode(regs); + + /* not a cfi violation, then merge into flow of unknown trap handler */ + if (!handle_user_cfi_violation(regs)) + do_trap_unknown(regs); + + irqentry_exit_to_user_mode(regs); + } else { + /* sw check exception coming from kernel is a bug in kernel */ + die(regs, "Kernel BUG"); + } +} + #ifdef CONFIG_MMU asmlinkage __visible noinstr void do_page_fault(struct pt_regs *regs) {

Save shadow stack pointer in sigcontext structure while delivering signal. Restore shadow stack pointer from sigcontext on sigreturn.

As part of save operation, kernel uses `ssamoswap` to save snapshot of current shadow stack on shadow stack itself (can be called as a save token). During restore on sigreturn, kernel retrieves token from top of shadow stack and validates it. This allows that user mode can't arbitrary pivot to any shadow stack address without having a token and thus provide strong security assurance between signaly delivery and sigreturn window.

Use ABI compatible way of saving/restoring shadow stack pointer into signal stack. This follows what Vector extension, where extra registers are placed in a form of extension header + extension body in the stack. The extension header indicates the size of the extra architectural states plus the size of header itself, and a magic identifier of the extension. Then, the extensions body contains the new architectural states in the form defined by uapi.

Signed-off-by: Andy Chiu andy.chiu@sifive.com Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/usercfi.h | 10 ++++ arch/riscv/include/uapi/asm/ptrace.h | 4 ++ arch/riscv/include/uapi/asm/sigcontext.h | 1 + arch/riscv/kernel/signal.c | 86 ++++++++++++++++++++++++++++++++ arch/riscv/kernel/usercfi.c | 57 +++++++++++++++++++++ 5 files changed, 158 insertions(+)

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h index 4501d741a609..ec4b8a53eb74 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -8,6 +8,7 @@ #ifndef __ASSEMBLER__ #include <linux/types.h> #include <linux/prctl.h> +#include <linux/errno.h>

struct task_struct; struct kernel_clone_args; @@ -34,6 +35,9 @@ bool is_shstk_locked(struct task_struct *task); bool is_shstk_allocated(struct task_struct *task); void set_shstk_lock(struct task_struct *task); void set_shstk_status(struct task_struct *task, bool enable); +unsigned long get_active_shstk(struct task_struct *task); +int restore_user_shstk(struct task_struct *tsk, unsigned long shstk_ptr); +int save_user_shstk(struct task_struct *tsk, unsigned long *saved_shstk_ptr); bool is_indir_lp_enabled(struct task_struct *task); bool is_indir_lp_locked(struct task_struct *task); void set_indir_lp_status(struct task_struct *task, bool enable); @@ -71,6 +75,12 @@ void set_indir_lp_lock(struct task_struct *task);

#define set_indir_lp_lock(task) do {} while (0)

+#define restore_user_shstk(tsk, shstk_ptr) -EINVAL + +#define save_user_shstk(tsk, saved_shstk_ptr) -EINVAL + +#define get_active_shstk(task) 0UL + #endif /* CONFIG_RISCV_USER_CFI */

#endif /* __ASSEMBLER__ */ diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h index beff8df80ac9..261bfe70f60a 100644 --- a/arch/riscv/include/uapi/asm/ptrace.h +++ b/arch/riscv/include/uapi/asm/ptrace.h @@ -127,6 +127,10 @@ struct __riscv_v_regset_state { */ #define RISCV_MAX_VLENB (8192)

+struct __sc_riscv_cfi_state { + unsigned long ss_ptr; /* shadow stack pointer */ +}; + #endif /* __ASSEMBLER__ */

#endif /* _UAPI_ASM_RISCV_PTRACE_H */ diff --git a/arch/riscv/include/uapi/asm/sigcontext.h b/arch/riscv/include/uapi/asm/sigcontext.h index 748dffc9ae19..d22d0815d605 100644 --- a/arch/riscv/include/uapi/asm/sigcontext.h +++ b/arch/riscv/include/uapi/asm/sigcontext.h @@ -10,6 +10,7 @@

/* The Magic number for signal context frame header. */ #define RISCV_V_MAGIC 0x53465457 +#define RISCV_ZICFISS_MAGIC 0x9487 #define END_MAGIC 0x0

/* The size of END signal context header. */ diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c index a5e3d54fe54b..1bcda11e0680 100644 --- a/arch/riscv/kernel/signal.c +++ b/arch/riscv/kernel/signal.c @@ -22,11 +22,13 @@ #include <asm/vector.h> #include <asm/csr.h> #include <asm/cacheflush.h> +#include <asm/usercfi.h>

unsigned long signal_minsigstksz __ro_after_init;

extern u32 __user_rt_sigreturn[2]; static size_t riscv_v_sc_size __ro_after_init; +static size_t riscv_zicfiss_sc_size __ro_after_init;

#define DEBUG_SIG 0

@@ -140,6 +142,62 @@ static long __restore_v_state(struct pt_regs *regs, void __user *sc_vec) return copy_from_user(current->thread.vstate.datap, datap, riscv_v_vsize); }

+static long save_cfiss_state(struct pt_regs *regs, void __user *sc_cfi) +{ + struct __sc_riscv_cfi_state __user *state = sc_cfi; + unsigned long ss_ptr = 0; + long err = 0; + + if (!is_shstk_enabled(current)) + return 0; + + /* + * Save a pointer to shadow stack itself on shadow stack as a form of token. + * A token on shadow gives following properties + * - Safe save and restore for shadow stack switching. Any save of shadow stack + * must have had saved a token on shadow stack. Similarly any restore of shadow + * stack must check the token before restore. Since writing to shadow stack with + * address of shadow stack itself is not easily allowed. A restore without a save + * is quite difficult for an attacker to perform. + * - A natural break. A token in shadow stack provides a natural break in shadow stack + * So a single linear range can be bucketed into different shadow stack segments. Any + * sspopchk will detect the condition and fault to kernel as sw check exception. + */ + err |= save_user_shstk(current, &ss_ptr); + err |= __put_user(ss_ptr, &state->ss_ptr); + if (unlikely(err)) + return -EFAULT; + + return riscv_zicfiss_sc_size; +} + +static long __restore_cfiss_state(struct pt_regs *regs, void __user *sc_cfi) +{ + struct __sc_riscv_cfi_state __user *state = sc_cfi; + unsigned long ss_ptr = 0; + long err; + + /* + * Restore shadow stack as a form of token stored on shadow stack itself as a safe + * way to restore. + * A token on shadow gives following properties + * - Safe save and restore for shadow stack switching. Any save of shadow stack + * must have had saved a token on shadow stack. Similarly any restore of shadow + * stack must check the token before restore. Since writing to shadow stack with + * address of shadow stack itself is not easily allowed. A restore without a save + * is quite difficult for an attacker to perform. + * - A natural break. A token in shadow stack provides a natural break in shadow stack + * So a single linear range can be bucketed into different shadow stack segments. + * sspopchk will detect the condition and fault to kernel as sw check exception. + */ + err = __copy_from_user(&ss_ptr, &state->ss_ptr, sizeof(unsigned long)); + + if (unlikely(err)) + return err; + + return restore_user_shstk(current, ss_ptr); +} + struct arch_ext_priv { __u32 magic; long (*save)(struct pt_regs *regs, void __user *sc_vec); @@ -150,6 +208,10 @@ struct arch_ext_priv arch_ext_list[] = { .magic = RISCV_V_MAGIC, .save = &save_v_state, }, + { + .magic = RISCV_ZICFISS_MAGIC, + .save = &save_cfiss_state, + }, };

const size_t nr_arch_exts = ARRAY_SIZE(arch_ext_list); @@ -202,6 +264,12 @@ static long restore_sigcontext(struct pt_regs *regs,

err = __restore_v_state(regs, sc_ext_ptr); break; + case RISCV_ZICFISS_MAGIC: + if (!is_shstk_enabled(current) || size != riscv_zicfiss_sc_size) + return -EINVAL; + + err = __restore_cfiss_state(regs, sc_ext_ptr); + break; default: return -EINVAL; } @@ -223,6 +291,16 @@ static size_t get_rt_frame_size(bool cal_all) total_context_size += riscv_v_sc_size; }

+ if (is_shstk_enabled(current)) + total_context_size += riscv_zicfiss_sc_size; + + /* + * Preserved a __riscv_ctx_hdr for END signal context header if an + * extension uses __riscv_extra_ext_header + */ + if (total_context_size) + total_context_size += sizeof(struct __riscv_ctx_hdr); + frame_size += total_context_size;

frame_size = round_up(frame_size, 16); @@ -359,6 +437,11 @@ static int setup_rt_frame(struct ksignal *ksig, sigset_t *set, #ifdef CONFIG_MMU regs->ra = (unsigned long)VDSO_SYMBOL( current->mm->context.vdso, rt_sigreturn); + + /* if bcfi is enabled x1 (ra) and x5 (t0) must match. not sure if we need this? */ + if (is_shstk_enabled(current)) + regs->t0 = regs->ra; + #else /* * For the nommu case we don't have a VDSO. Instead we push two @@ -487,6 +570,9 @@ void __init init_rt_signal_env(void) { riscv_v_sc_size = sizeof(struct __riscv_ctx_hdr) + sizeof(struct __sc_riscv_v_state) + riscv_v_vsize; + + riscv_zicfiss_sc_size = sizeof(struct __riscv_ctx_hdr) + + sizeof(struct __sc_riscv_cfi_state); /* * Determine the stack space required for guaranteed signal delivery. * The signal_minsigstksz will be populated into the AT_MINSIGSTKSZ entry diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index a384ce9ed25c..7f01befabd11 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -52,6 +52,11 @@ void set_active_shstk(struct task_struct *task, unsigned long shstk_addr) task->thread_info.user_cfi_state.user_shdw_stk = shstk_addr; }

+unsigned long get_active_shstk(struct task_struct *task) +{ + return task->thread_info.user_cfi_state.user_shdw_stk; +} + void set_shstk_status(struct task_struct *task, bool enable) { if (!cpu_supports_shadow_stack()) @@ -168,6 +173,58 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) return 0; }

+/* + * Save user shadow stack pointer on shadow stack itself and return pointer to saved location + * returns -EFAULT if operation was unsuccessful + */ +int save_user_shstk(struct task_struct *tsk, unsigned long *saved_shstk_ptr) +{ + unsigned long ss_ptr = 0; + unsigned long token_loc = 0; + int ret = 0; + + if (!saved_shstk_ptr) + return -EINVAL; + + ss_ptr = get_active_shstk(tsk); + ret = create_rstor_token(ss_ptr, &token_loc); + + if (!ret) { + *saved_shstk_ptr = token_loc; + set_active_shstk(tsk, token_loc); + } + + return ret; +} + +/* + * Restores user shadow stack pointer from token on shadow stack for task `tsk` + * returns -EFAULT if operation was unsuccessful + */ +int restore_user_shstk(struct task_struct *tsk, unsigned long shstk_ptr) +{ + unsigned long token = 0; + + token = amo_user_shstk((unsigned long __user *)shstk_ptr, 0); + + if (token == -1) + return -EFAULT; + + /* invalid token, return EINVAL */ + if ((token - shstk_ptr) != SHSTK_ENTRY_SIZE) { + pr_info_ratelimited("%s[%d]: bad restore token in %s: pc=%p sp=%p, token=%p, " + "shstk_ptr=%p\n", tsk->comm, task_pid_nr(tsk), __func__, + (void *)(task_pt_regs(tsk)->epc), + (void *)(task_pt_regs(tsk)->sp), + (void *)token, (void *)shstk_ptr); + return -EINVAL; + } + + /* all checks passed, set active shstk and return success */ + set_active_shstk(tsk, token); + return 0; +} + static unsigned long allocate_shadow_stack(unsigned long addr, unsigned long size, unsigned long token_offset, bool set_tok) {

Expose a new register type NT_RISCV_USER_CFI for risc-v cfi status and state. Intentionally both landing pad and shadow stack status and state are rolled into cfi state. Creating two different NT_RISCV_USER_XXX would not be useful and wastage of a note type. Enabling, disabling and locking of feature is not allowed via ptrace set interface. However setting `elp` state or setting shadow stack pointer are allowed via ptrace set interface . It is expected `gdb` might have use to fixup `elp` state or `shadow stack` pointer.

Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/uapi/asm/ptrace.h | 30 ++++++++++++ arch/riscv/kernel/ptrace.c | 95 ++++++++++++++++++++++++++++++++++++ include/uapi/linux/elf.h | 2 + 3 files changed, 127 insertions(+)

diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h index 261bfe70f60a..b2a18dfeb2fb 100644 --- a/arch/riscv/include/uapi/asm/ptrace.h +++ b/arch/riscv/include/uapi/asm/ptrace.h @@ -131,6 +131,36 @@ struct __sc_riscv_cfi_state { unsigned long ss_ptr; /* shadow stack pointer */ };

+#define PTRACE_CFI_LP_EN_BIT 0 +#define PTRACE_CFI_LP_LOCK_BIT 1 +#define PTRACE_CFI_ELP_BIT 2 +#define PTRACE_CFI_SS_EN_BIT 3 +#define PTRACE_CFI_SS_LOCK_BIT 4 +#define PTRACE_CFI_SS_PTR_BIT 5 + +#define PTRACE_CFI_LP_EN_STATE (1 << PTRACE_CFI_LP_EN_BIT) +#define PTRACE_CFI_LP_LOCK_STATE (1 << PTRACE_CFI_LP_LOCK_BIT) +#define PTRACE_CFI_ELP_STATE (1 << PTRACE_CFI_ELP_BIT) +#define PTRACE_CFI_SS_EN_STATE (1 << PTRACE_CFI_SS_EN_BIT) +#define PTRACE_CFI_SS_LOCK_STATE (1 << PTRACE_CFI_SS_LOCK_BIT) +#define PTRACE_CFI_SS_PTR_STATE (1 << PTRACE_CFI_SS_PTR_BIT) + +#define PRACE_CFI_STATE_INVALID_MASK ~(PTRACE_CFI_LP_EN_STATE | \ + PTRACE_CFI_LP_LOCK_STATE | \ + PTRACE_CFI_ELP_STATE | \ + PTRACE_CFI_SS_EN_STATE | \ + PTRACE_CFI_SS_LOCK_STATE | \ + PTRACE_CFI_SS_PTR_STATE) + +struct __cfi_status { + __u64 cfi_state; +}; + +struct user_cfi_state { + struct __cfi_status cfi_status; + __u64 shstk_ptr; +}; + #endif /* __ASSEMBLER__ */

#endif /* _UAPI_ASM_RISCV_PTRACE_H */ diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c index 8e86305831ea..56b9e3871862 100644 --- a/arch/riscv/kernel/ptrace.c +++ b/arch/riscv/kernel/ptrace.c @@ -19,6 +19,7 @@ #include <linux/regset.h> #include <linux/sched.h> #include <linux/sched/task_stack.h> +#include <asm/usercfi.h>

enum riscv_regset { REGSET_X, @@ -31,6 +32,9 @@ enum riscv_regset { #ifdef CONFIG_RISCV_ISA_SUPM REGSET_TAGGED_ADDR_CTRL, #endif +#ifdef CONFIG_RISCV_USER_CFI + REGSET_CFI, +#endif };

static int riscv_gpr_get(struct task_struct *target, @@ -184,6 +188,87 @@ static int tagged_addr_ctrl_set(struct task_struct *target, } #endif

+#ifdef CONFIG_RISCV_USER_CFI +static int riscv_cfi_get(struct task_struct *target, + const struct user_regset *regset, + struct membuf to) +{ + struct user_cfi_state user_cfi; + struct pt_regs *regs; + + memset(&user_cfi, 0, sizeof(user_cfi)); + regs = task_pt_regs(target); + + if (is_indir_lp_enabled(target)) { + user_cfi.cfi_status.cfi_state |= PTRACE_CFI_LP_EN_STATE; + user_cfi.cfi_status.cfi_state |= is_indir_lp_locked(target) ? + PTRACE_CFI_LP_LOCK_STATE : 0; + user_cfi.cfi_status.cfi_state |= (regs->status & SR_ELP) ? + PTRACE_CFI_ELP_STATE : 0; + } + + if (is_shstk_enabled(target)) { + user_cfi.cfi_status.cfi_state |= (PTRACE_CFI_SS_EN_STATE | + PTRACE_CFI_SS_PTR_STATE); + user_cfi.cfi_status.cfi_state |= is_shstk_locked(target) ? + PTRACE_CFI_SS_LOCK_STATE : 0; + user_cfi.shstk_ptr = get_active_shstk(target); + } + + return membuf_write(&to, &user_cfi, sizeof(user_cfi)); +} + +/* + * Does it make sense to allowing enable / disable of cfi via ptrace? + * Not allowing enable / disable / locking control via ptrace for now. + * Setting shadow stack pointer is allowed. GDB might use it to unwind or + * some other fixup. Similarly gdb might want to suppress elp and may want + * to reset elp state. + */ +static int riscv_cfi_set(struct task_struct *target, + const struct user_regset *regset, + unsigned int pos, unsigned int count, + const void *kbuf, const void __user *ubuf) +{ + int ret; + struct user_cfi_state user_cfi; + struct pt_regs *regs; + + regs = task_pt_regs(target); + + ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &user_cfi, 0, -1); + if (ret) + return ret; + + /* + * Not allowing enabling or locking shadow stack or landing pad + * There is no disabling of shadow stack or landing pad via ptrace + * rsvd field should be set to zero so that if those fields are needed in future + */ + if ((user_cfi.cfi_status.cfi_state & + (PTRACE_CFI_LP_EN_STATE | PTRACE_CFI_LP_LOCK_STATE | + PTRACE_CFI_SS_EN_STATE | PTRACE_CFI_SS_LOCK_STATE)) || + (user_cfi.cfi_status.cfi_state & PRACE_CFI_STATE_INVALID_MASK)) + return -EINVAL; + + /* If lpad is enabled on target and ptrace requests to set / clear elp, do that */ + if (is_indir_lp_enabled(target)) { + if (user_cfi.cfi_status.cfi_state & + PTRACE_CFI_ELP_STATE) /* set elp state */ + regs->status |= SR_ELP; + else + regs->status &= ~SR_ELP; /* clear elp state */ + } + + /* If shadow stack enabled on target, set new shadow stack pointer */ + if (is_shstk_enabled(target) && + (user_cfi.cfi_status.cfi_state & PTRACE_CFI_SS_PTR_STATE)) + set_active_shstk(target, user_cfi.shstk_ptr); + + return 0; +} +#endif + static const struct user_regset riscv_user_regset[] = { [REGSET_X] = { USER_REGSET_NOTE_TYPE(PRSTATUS), @@ -224,6 +309,16 @@ static const struct user_regset riscv_user_regset[] = { .set = tagged_addr_ctrl_set, }, #endif +#ifdef CONFIG_RISCV_USER_CFI + [REGSET_CFI] = { + .core_note_type = NT_RISCV_USER_CFI, + .align = sizeof(__u64), + .n = sizeof(struct user_cfi_state) / sizeof(__u64), + .size = sizeof(__u64), + .regset_get = riscv_cfi_get, + .set = riscv_cfi_set, + }, +#endif };

static const struct user_regset_view riscv_user_native_view = { diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h index 819ded2d39de..ee30dcd80901 100644 --- a/include/uapi/linux/elf.h +++ b/include/uapi/linux/elf.h @@ -545,6 +545,8 @@ typedef struct elf64_shdr { #define NT_RISCV_VECTOR 0x901 /* RISC-V vector registers */ #define NN_RISCV_TAGGED_ADDR_CTRL "LINUX" #define NT_RISCV_TAGGED_ADDR_CTRL 0x902 /* RISC-V tagged address control (prctl()) */ +#define NN_RISCV_USER_CFI "LINUX" +#define NT_RISCV_USER_CFI 0x903 /* RISC-V shadow stack state */ #define NN_LOONGARCH_CPUCFG "LINUX" #define NT_LOONGARCH_CPUCFG 0xa00 /* LoongArch CPU config registers */ #define NN_LOONGARCH_CSR "LINUX"

This commit adds a kernel command line option to disable part or all of user cfi. User backward cfi and forward cfi can be controlled independently. Kernel command line parameter "riscv_nousercfi" can take the following values: - "all" : Disable forward and backward cfi both. - "bcfi" : Disable backward cfi. - "fcfi" : Disable forward cfi

Signed-off-by: Paul Walmsley pjw@kernel.org Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- Documentation/admin-guide/kernel-parameters.txt | 8 ++++ arch/riscv/include/asm/usercfi.h | 10 +++++ arch/riscv/kernel/cpufeature.c | 7 ++- arch/riscv/kernel/usercfi.c | 59 ++++++++++++++++++++----- 4 files changed, 71 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 6c42061ca20e..453127ef8746 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -6453,6 +6453,14 @@ replacement properties are not found. See the Kconfig entry for RISCV_ISA_FALLBACK.

+ riscv_nousercfi= + all Disable user cfi ABI to userspace even if cpu extension + are available. + bcfi Disable user backward cfi ABI to userspace even if + shadow stack extension is available. + fcfi Disable user forward cfi ABI to userspace even if landing + pad extension is available. + ro [KNL] Mount root device read-only on boot

rodata= [KNL,EARLY] diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h index ec4b8a53eb74..356f2c4d087d 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -5,6 +5,10 @@ #ifndef _ASM_RISCV_USERCFI_H #define _ASM_RISCV_USERCFI_H

+#define CMDLINE_DISABLE_RISCV_USERCFI_FCFI 1 +#define CMDLINE_DISABLE_RISCV_USERCFI_BCFI 2 +#define CMDLINE_DISABLE_RISCV_USERCFI 3 + #ifndef __ASSEMBLER__ #include <linux/types.h> #include <linux/prctl.h> @@ -14,6 +18,9 @@ struct task_struct; struct kernel_clone_args;

#ifdef CONFIG_RISCV_USER_CFI + +extern unsigned long riscv_nousercfi; + struct cfi_state { unsigned long ubcfi_en : 1; /* Enable for backward cfi. */ unsigned long ubcfi_locked : 1; @@ -83,6 +90,9 @@ void set_indir_lp_lock(struct task_struct *task);

#endif /* CONFIG_RISCV_USER_CFI */

+bool is_user_shstk_enabled(void); +bool is_user_lpad_enabled(void); + #endif /* __ASSEMBLER__ */

#endif /* _ASM_RISCV_USERCFI_H */ diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 17b9e77bafc3..32ad463545fa 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -28,6 +28,7 @@ #include <asm/vector.h> #include <asm/vendor_extensions.h> #include <asm/vendor_extensions/thead.h> +#include <asm/usercfi.h>

#define NUM_ALPHA_EXTS ('z' - 'a' + 1)

@@ -277,7 +278,8 @@ static int riscv_ext_svadu_validate(const struct riscv_isa_ext_data *data, static int riscv_cfilp_validate(const struct riscv_isa_ext_data *data, const unsigned long *isa_bitmap) { - if (!IS_ENABLED(CONFIG_RISCV_USER_CFI)) + if (!IS_ENABLED(CONFIG_RISCV_USER_CFI) || + (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_FCFI)) return -EINVAL;

return 0; @@ -286,7 +288,8 @@ static int riscv_cfilp_validate(const struct riscv_isa_ext_data *data, static int riscv_cfiss_validate(const struct riscv_isa_ext_data *data, const unsigned long *isa_bitmap) { - if (!IS_ENABLED(CONFIG_RISCV_USER_CFI)) + if (!IS_ENABLED(CONFIG_RISCV_USER_CFI) || + (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_BCFI)) return -EINVAL;

return 0; diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index 7f01befabd11..27b36034ea85 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -17,6 +17,8 @@ #include <asm/csr.h> #include <asm/usercfi.h>

+unsigned long riscv_nousercfi; + #define SHSTK_ENTRY_SIZE sizeof(void *)

bool is_shstk_enabled(struct task_struct *task) @@ -59,7 +61,7 @@ unsigned long get_active_shstk(struct task_struct *task)

void set_shstk_status(struct task_struct *task, bool enable) { - if (!cpu_supports_shadow_stack()) + if (!is_user_shstk_enabled()) return;

task->thread_info.user_cfi_state.ubcfi_en = enable ? 1 : 0; @@ -89,7 +91,7 @@ bool is_indir_lp_locked(struct task_struct *task)

void set_indir_lp_status(struct task_struct *task, bool enable) { - if (!cpu_supports_indirect_br_lp_instr()) + if (!is_user_lpad_enabled()) return;

task->thread_info.user_cfi_state.ufcfi_en = enable ? 1 : 0; @@ -259,7 +261,7 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi bool set_tok = flags & SHADOW_STACK_SET_TOKEN; unsigned long aligned_size = 0;

- if (!cpu_supports_shadow_stack()) + if (!is_user_shstk_enabled()) return -EOPNOTSUPP;

/* Anything other than set token should result in invalid param */ @@ -306,7 +308,7 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long addr, size;

/* If shadow stack is not supported, return 0 */ - if (!cpu_supports_shadow_stack()) + if (!is_user_shstk_enabled()) return 0;

/* @@ -352,7 +354,7 @@ void shstk_release(struct task_struct *tsk) { unsigned long base = 0, size = 0; /* If shadow stack is not supported or not enabled, nothing to release */ - if (!cpu_supports_shadow_stack() || !is_shstk_enabled(tsk)) + if (!is_user_shstk_enabled() || !is_shstk_enabled(tsk)) return;

/* @@ -381,7 +383,7 @@ int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user *st { unsigned long bcfi_status = 0;

- if (!cpu_supports_shadow_stack()) + if (!is_user_shstk_enabled()) return -EINVAL;

/* this means shadow stack is enabled on the task */ @@ -395,7 +397,7 @@ int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status) unsigned long size = 0, addr = 0; bool enable_shstk = false;

- if (!cpu_supports_shadow_stack()) + if (!is_user_shstk_enabled()) return -EINVAL;

/* Reject unknown flags */ @@ -448,7 +450,7 @@ int arch_lock_shadow_stack_status(struct task_struct *task, unsigned long arg) { /* If shtstk not supported or not enabled on task, nothing to lock here */ - if (!cpu_supports_shadow_stack() || + if (!is_user_shstk_enabled() || !is_shstk_enabled(task) || arg != 0) return -EINVAL;

@@ -461,7 +463,7 @@ int arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *sta { unsigned long fcfi_status = 0;

- if (!cpu_supports_indirect_br_lp_instr()) + if (!is_user_lpad_enabled()) return -EINVAL;

/* indirect branch tracking is enabled on the task or not */ @@ -474,7 +476,7 @@ int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status) { bool enable_indir_lp = false;

- if (!cpu_supports_indirect_br_lp_instr()) + if (!is_user_lpad_enabled()) return -EINVAL;

/* indirect branch tracking is locked and further can't be modified by user */ @@ -498,7 +500,7 @@ int arch_lock_indir_br_lp_status(struct task_struct *task, * If indirect branch tracking is not supported or not enabled on task, * nothing to lock here */ - if (!cpu_supports_indirect_br_lp_instr() || + if (!is_user_lpad_enabled() || !is_indir_lp_enabled(task) || arg != 0) return -EINVAL;

@@ -506,3 +508,38 @@ int arch_lock_indir_br_lp_status(struct task_struct *task,

return 0; } + +bool is_user_shstk_enabled(void) +{ + return (cpu_supports_shadow_stack() && + !(riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_BCFI)); +} + +bool is_user_lpad_enabled(void) +{ + return (cpu_supports_indirect_br_lp_instr() && + !(riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_FCFI)); +} + +static int __init setup_global_riscv_enable(char *str) +{ + if (strcmp(str, "all") == 0) + riscv_nousercfi = CMDLINE_DISABLE_RISCV_USERCFI; + + if (strcmp(str, "fcfi") == 0) + riscv_nousercfi |= CMDLINE_DISABLE_RISCV_USERCFI_FCFI; + + if (strcmp(str, "bcfi") == 0) + riscv_nousercfi |= CMDLINE_DISABLE_RISCV_USERCFI_BCFI; + + if (riscv_nousercfi) + pr_info("riscv user cfi disabled via cmdline " + "shadow stack status : %s, landing pad status : %s\n", + (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_BCFI) ? "disabled" : + "enabled", (riscv_nousercfi & CMDLINE_DISABLE_RISCV_USERCFI_FCFI) ? + "disabled" : "enabled"); + + return 1; +} + +__setup("riscv_nousercfi=", setup_global_riscv_enable);

Kernel will have to perform shadow stack operations on user shadow stack. Like during signal delivery and sigreturn, shadow stack token must be created and validated respectively. Thus shadow stack access for kernel must be enabled.

In future when kernel shadow stacks are enabled for linux kernel, it must be enabled as early as possible for better coverage and prevent imbalance between regular stack and shadow stack. After `relocate_enable_mmu` has been done, this is as early as possible it can enabled.

Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/kernel/asm-offsets.c | 6 ++++++ arch/riscv/kernel/head.S | 27 +++++++++++++++++++++++++++ 2 files changed, 33 insertions(+)

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c index 8a2b2656cb2f..af827448a609 100644 --- a/arch/riscv/kernel/asm-offsets.c +++ b/arch/riscv/kernel/asm-offsets.c @@ -533,4 +533,10 @@ void asm_offsets(void) DEFINE(FREGS_A6, offsetof(struct __arch_ftrace_regs, a6)); DEFINE(FREGS_A7, offsetof(struct __arch_ftrace_regs, a7)); #endif +#ifdef CONFIG_RISCV_SBI + DEFINE(SBI_EXT_FWFT, SBI_EXT_FWFT); + DEFINE(SBI_EXT_FWFT_SET, SBI_EXT_FWFT_SET); + DEFINE(SBI_FWFT_SHADOW_STACK, SBI_FWFT_SHADOW_STACK); + DEFINE(SBI_FWFT_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK); +#endif } diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S index bdf3352acf4c..9c99c5ad6fe8 100644 --- a/arch/riscv/kernel/head.S +++ b/arch/riscv/kernel/head.S @@ -15,6 +15,7 @@ #include <asm/image.h> #include <asm/scs.h> #include <asm/xip_fixup.h> +#include <asm/usercfi.h> #include "efi-header.S"

__HEAD @@ -170,6 +171,19 @@ secondary_start_sbi: call relocate_enable_mmu #endif call .Lsetup_trap_vector +#if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_USER_CFI) + li a7, SBI_EXT_FWFT + li a6, SBI_EXT_FWFT_SET + li a0, SBI_FWFT_SHADOW_STACK + li a1, 1 /* enable supervisor to access shadow stack access */ + li a2, SBI_FWFT_SET_FLAG_LOCK + ecall + beqz a0, 1f + la a1, riscv_nousercfi + li a0, CMDLINE_DISABLE_RISCV_USERCFI_BCFI + REG_S a0, (a1) +1: +#endif scs_load_current call smp_callin #endif /* CONFIG_SMP */ @@ -330,6 +344,19 @@ SYM_CODE_START(_start_kernel) la tp, init_task la sp, init_thread_union + THREAD_SIZE addi sp, sp, -PT_SIZE_ON_STACK +#if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_USER_CFI) + li a7, SBI_EXT_FWFT + li a6, SBI_EXT_FWFT_SET + li a0, SBI_FWFT_SHADOW_STACK + li a1, 1 /* enable supervisor to access shadow stack access */ + li a2, SBI_FWFT_SET_FLAG_LOCK + ecall + beqz a0, 1f + la a1, riscv_nousercfi + li a0, CMDLINE_DISABLE_RISCV_USERCFI_BCFI + REG_S a0, (a1) +1: +#endif scs_load_current

#ifdef CONFIG_KASAN

Shadow stack instructions are taken from zimop (mandated on RVA23). Any hardware prior to RVA23 profile will fault on shadow stack instruction. Any userspace with shadow stack instruction in it will fault on such hardware. Thus such userspace can't be brought onto such a hardware.

It's not known how userspace will respond to such binary fragmentation. However in order to keep kernel portable across such different hardware, `arch/riscv/kernel/vdso_cfi` is created which has logic (Makefile) to compile `arch/riscv/kernel/vdso` sources with cfi flags and then changes in `arch/riscv/kernel/vdso.c` for selecting appropriate vdso depending on whether underlying hardware(cpu) implements zimop extension. Offset of vdso symbols will change due to having two different vdso binaries, there is added logic to include new generated vdso offset header and dynamically select offset (like for rt_sigreturn).

Acked-by: Charles Mirabile cmirabil@redhat.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/Makefile | 3 +++ arch/riscv/include/asm/vdso.h | 13 ++++++++++++- arch/riscv/kernel/Makefile | 1 + arch/riscv/kernel/vdso.c | 7 +++++++ arch/riscv/kernel/vdso/Makefile | 29 ++++++++++++++++++++--------- arch/riscv/kernel/vdso/gen_vdso_offsets.sh | 4 +++- arch/riscv/kernel/vdso_cfi/Makefile | 25 +++++++++++++++++++++++++ arch/riscv/kernel/vdso_cfi/vdso-cfi.S | 11 +++++++++++ 8 files changed, 82 insertions(+), 11 deletions(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile index f60c2de0ca08..b74b63da16a7 100644 --- a/arch/riscv/Makefile +++ b/arch/riscv/Makefile @@ -176,6 +176,8 @@ ifeq ($(CONFIG_MMU),y) prepare: vdso_prepare vdso_prepare: prepare0 $(Q)$(MAKE) $(build)=arch/riscv/kernel/vdso include/generated/vdso-offsets.h + $(if $(CONFIG_RISCV_USER_CFI),$(Q)$(MAKE) \ + $(build)=arch/riscv/kernel/vdso_cfi include/generated/vdso-cfi-offsets.h) $(if $(CONFIG_COMPAT),$(Q)$(MAKE) \ $(build)=arch/riscv/kernel/compat_vdso include/generated/compat_vdso-offsets.h)

@@ -183,6 +185,7 @@ endif endif

vdso-install-y += arch/riscv/kernel/vdso/vdso.so.dbg +vdso-install-$(CONFIG_RISCV_USER_CFI) += arch/riscv/kernel/vdso_cfi/vdso-cfi.so.dbg vdso-install-$(CONFIG_COMPAT) += arch/riscv/kernel/compat_vdso/compat_vdso.so.dbg

BOOT_TARGETS := Image Image.gz Image.bz2 Image.lz4 Image.lzma Image.lzo Image.zst Image.xz loader loader.bin xipImage vmlinuz.efi diff --git a/arch/riscv/include/asm/vdso.h b/arch/riscv/include/asm/vdso.h index f80357fe24d1..35bf830a5576 100644 --- a/arch/riscv/include/asm/vdso.h +++ b/arch/riscv/include/asm/vdso.h @@ -18,9 +18,19 @@

#ifndef __ASSEMBLER__ #include <generated/vdso-offsets.h> +#ifdef CONFIG_RISCV_USER_CFI +#include <generated/vdso-cfi-offsets.h> +#endif

+#ifdef CONFIG_RISCV_USER_CFI #define VDSO_SYMBOL(base, name) \ - (void __user *)((unsigned long)(base) + __vdso_##name##_offset) + (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZIMOP) ? \ + (void __user *)((unsigned long)(base) + __vdso_##name##_cfi_offset) : \ + (void __user *)((unsigned long)(base) + __vdso_##name##_offset)) +#else +#define VDSO_SYMBOL(base, name) \ + ((void __user *)((unsigned long)(base) + __vdso_##name##_offset)) +#endif

#ifdef CONFIG_COMPAT #include <generated/compat_vdso-offsets.h> @@ -33,6 +43,7 @@ extern char compat_vdso_start[], compat_vdso_end[]; #endif /* CONFIG_COMPAT */

extern char vdso_start[], vdso_end[]; +extern char vdso_cfi_start[], vdso_cfi_end[];

#endif /* !__ASSEMBLER__ */

diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile index 2d0e0dcedbd3..9026400cba10 100644 --- a/arch/riscv/kernel/Makefile +++ b/arch/riscv/kernel/Makefile @@ -72,6 +72,7 @@ obj-y += vendor_extensions/ obj-y += probes/ obj-y += tests/ obj-$(CONFIG_MMU) += vdso.o vdso/ +obj-$(CONFIG_RISCV_USER_CFI) += vdso_cfi/

obj-$(CONFIG_RISCV_MISALIGNED) += traps_misaligned.o obj-$(CONFIG_RISCV_MISALIGNED) += unaligned_access_speed.o diff --git a/arch/riscv/kernel/vdso.c b/arch/riscv/kernel/vdso.c index 3a8e038b10a2..43f70198ac3c 100644 --- a/arch/riscv/kernel/vdso.c +++ b/arch/riscv/kernel/vdso.c @@ -98,6 +98,13 @@ static struct __vdso_info compat_vdso_info __ro_after_init = {

static int __init vdso_init(void) { + /* Hart implements zimop, expose cfi compiled vdso */ + if (IS_ENABLED(CONFIG_RISCV_USER_CFI) && + riscv_has_extension_unlikely(RISCV_ISA_EXT_ZIMOP)) { + vdso_info.vdso_code_start = vdso_cfi_start; + vdso_info.vdso_code_end = vdso_cfi_end; + } + __vdso_init(&vdso_info); #ifdef CONFIG_COMPAT __vdso_init(&compat_vdso_info); diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile index 272f1d837a80..a842dc034571 100644 --- a/arch/riscv/kernel/vdso/Makefile +++ b/arch/riscv/kernel/vdso/Makefile @@ -20,6 +20,10 @@ endif ifdef VDSO_CFI_BUILD CFI_MARCH = _zicfilp_zicfiss CFI_FULL = -fcf-protection=full +CFI_SUFFIX = -cfi +OFFSET_SUFFIX = _cfi +ccflags-y += -DVDSO_CFI=1 +asflags-y += -DVDSO_CFI=1 endif

# Files to link into the vdso @@ -48,13 +52,20 @@ endif CFLAGS_hwprobe.o += -fPIC

# Build rules -targets := $(obj-vdso) vdso.so vdso.so.dbg vdso.lds +vdso_offsets := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),)-offsets.h +vdso_o := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).o +vdso_so := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).so +vdso_so_dbg := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).so.dbg +vdso_lds := vdso.lds + +targets := $(obj-vdso) $(vdso_so) $(vdso_so_dbg) $(vdso_lds) + obj-vdso := $(addprefix $(obj)/, $(obj-vdso))

-obj-y += vdso.o -CPPFLAGS_vdso.lds += -P -C -U$(ARCH) +obj-y += vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).o +CPPFLAGS_$(vdso_lds) += -P -C -U$(ARCH) ifneq ($(filter vgettimeofday, $(vdso-syms)),) -CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY +CPPFLAGS_$(vdso_lds) += -DHAS_VGETTIMEOFDAY endif

# Disable -pg to prevent insert call site @@ -63,12 +74,12 @@ CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)

# Force dependency -$(obj)/vdso.o: $(obj)/vdso.so +$(obj)/$(vdso_o): $(obj)/$(vdso_so)

# link rule for the .so file, .lds has to be first -$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE +$(obj)/$(vdso_so_dbg): $(obj)/$(vdso_lds) $(obj-vdso) FORCE $(call if_changed,vdsold_and_check) -LDFLAGS_vdso.so.dbg = -shared -soname=linux-vdso.so.1 \ +LDFLAGS_$(vdso_so_dbg) = -shared -soname=linux-vdso.so.1 \ --build-id=sha1 --eh-frame-hdr

# strip rule for the .so file @@ -79,9 +90,9 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE # Generate VDSO offsets using helper script gen-vdsosym := $(src)/gen_vdso_offsets.sh quiet_cmd_vdsosym = VDSOSYM $@ - cmd_vdsosym = $(NM) $< | $(gen-vdsosym) | LC_ALL=C sort > $@ + cmd_vdsosym = $(NM) $< | $(gen-vdsosym) $(OFFSET_SUFFIX) | LC_ALL=C sort > $@

-include/generated/vdso-offsets.h: $(obj)/vdso.so.dbg FORCE +include/generated/$(vdso_offsets): $(obj)/$(vdso_so_dbg) FORCE $(call if_changed,vdsosym)

# actual build commands diff --git a/arch/riscv/kernel/vdso/gen_vdso_offsets.sh b/arch/riscv/kernel/vdso/gen_vdso_offsets.sh index c2e5613f3495..bd5d5afaaa14 100755 --- a/arch/riscv/kernel/vdso/gen_vdso_offsets.sh +++ b/arch/riscv/kernel/vdso/gen_vdso_offsets.sh @@ -2,4 +2,6 @@ # SPDX-License-Identifier: GPL-2.0

LC_ALL=C -sed -n -e 's/^[0]+(0[0-9a-fA-F]*) . (__vdso_[a-zA-Z0-9_]*)$/#define \2_offset\t0x\1/p' +SUFFIX=${1:-""} +sed -n -e \ +'s/^[0]+(0[0-9a-fA-F]*) . (__vdso_[a-zA-Z0-9_]*)$/#define \2'$SUFFIX'_offset\t0x\1/p' diff --git a/arch/riscv/kernel/vdso_cfi/Makefile b/arch/riscv/kernel/vdso_cfi/Makefile new file mode 100644 index 000000000000..8ebd190782b0 --- /dev/null +++ b/arch/riscv/kernel/vdso_cfi/Makefile @@ -0,0 +1,25 @@ +# SPDX-License-Identifier: GPL-2.0-only +# RISC-V VDSO CFI Makefile +# This Makefile builds the VDSO with CFI support when CONFIG_RISCV_USER_CFI is enabled + +# setting VDSO_CFI_BUILD triggers build for vdso differently +VDSO_CFI_BUILD := 1 + +# Set the source directory to the main vdso directory +src := $(srctree)/arch/riscv/kernel/vdso + +# Copy all .S and .c files from vdso directory to vdso_cfi object build directory +vdso_c_sources := $(wildcard $(src)/*.c) +vdso_S_sources := $(wildcard $(src)/*.S) +vdso_c_objects := $(addprefix $(obj)/, $(notdir $(vdso_c_sources))) +vdso_S_objects := $(addprefix $(obj)/, $(notdir $(vdso_S_sources))) + +$(vdso_S_objects): $(obj)/%.S: $(src)/%.S + $(Q)cp $< $@ + +$(vdso_c_objects): $(obj)/%.c: $(src)/%.c + $(Q)cp $< $@ + +# Include the main VDSO Makefile which contains all the build rules and sources +# The VDSO_CFI_BUILD variable will be passed to it to enable CFI compilation +include $(src)/Makefile diff --git a/arch/riscv/kernel/vdso_cfi/vdso-cfi.S b/arch/riscv/kernel/vdso_cfi/vdso-cfi.S new file mode 100644 index 000000000000..d426f6accb35 --- /dev/null +++ b/arch/riscv/kernel/vdso_cfi/vdso-cfi.S @@ -0,0 +1,11 @@ +/* SPDX-License-Identifier: GPL-2.0-only */ +/* + * Copyright 2025 Rivos, Inc + */ + +#define vdso_start vdso_cfi_start +#define vdso_end vdso_cfi_end + +#define __VDSO_PATH "arch/riscv/kernel/vdso_cfi/vdso-cfi.so" + +#include "../vdso/vdso.S"

Adding documentation on shadow stack for user mode on riscv and kernel interfaces exposed so that user tasks can enable it.

Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- Documentation/arch/riscv/index.rst | 1 + Documentation/arch/riscv/zicfiss.rst | 179 +++++++++++++++++++++++++++++++++++ 2 files changed, 180 insertions(+)

diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst index be7237b69682..e240eb0ceb70 100644 --- a/Documentation/arch/riscv/index.rst +++ b/Documentation/arch/riscv/index.rst @@ -15,6 +15,7 @@ RISC-V architecture vector cmodx zicfilp + zicfiss

features

diff --git a/Documentation/arch/riscv/zicfiss.rst b/Documentation/arch/riscv/zicfiss.rst new file mode 100644 index 000000000000..7fb86d5ba120 --- /dev/null +++ b/Documentation/arch/riscv/zicfiss.rst @@ -0,0 +1,179 @@ +.. SPDX-License-Identifier: GPL-2.0 + +:Author: Deepak Gupta debug@rivosinc.com +:Date: 12 January 2024 + +========================================================= +Shadow stack to protect function returns on RISC-V Linux +========================================================= + +This document briefly describes the interface provided to userspace by Linux +to enable shadow stack for user mode applications on RISC-V + +1. Feature Overview +-------------------- + +Memory corruption issues usually result into crashes, however when in hands of +an adversary and if used creatively can result into a variety security issues. + +One of those security issues can be code re-use attacks on program where +adversary can use corrupt return addresses present on stack and chain them +together to perform return oriented programming (ROP) and thus compromising +control flow integrity (CFI) of the program. + +Return addresses live on stack and thus in read-write memory and thus are +susceptible to corruption and which allows an adversary to reach any program +counter (PC) in address space. On RISC-V ``zicfiss`` extension provides an +alternate stack termed as shadow stack on which return addresses can be safely +placed in prolog of the function and retrieved in epilog. ``zicfiss`` extension +makes following changes: + +- PTE encodings for shadow stack virtual memory + An earlier reserved encoding in first stage translation i.e. + PTE.R=0, PTE.W=1, PTE.X=0 becomes PTE encoding for shadow stack pages. + +- ``sspush x1/x5`` instruction pushes (stores) ``x1/x5`` to shadow stack. + +- ``sspopchk x1/x5`` instruction pops (loads) from shadow stack and compares + with ``x1/x5`` and if un-equal, CPU raises ``software check exception`` with + ``*tval = 3`` + +Compiler toolchain makes sure that function prologue have ``sspush x1/x5`` to +save return address on shadow stack in addition to regular stack. Similarly +function epilogs have ``ld x5, offset(x2)`` followed by ``sspopchk x5`` to +ensure that popped value from regular stack matches with popped value from +shadow stack. + +2. Shadow stack protections and linux memory manager +----------------------------------------------------- + +As mentioned earlier, shadow stacks get new page table encodings and thus have +some special properties assigned to them and instructions that operate on them +as below: + +- Regular stores to shadow stack memory raises access store faults. This way + shadow stack memory is protected from stray inadvertent writes. + +- Regular loads to shadow stack memory are allowed. This allows stack trace + utilities or backtrace functions to read true callstack (not tampered). + +- Only shadow stack instructions can generate shadow stack load or shadow stack + store. + +- Shadow stack load / shadow stack store on read-only memory raises AMO/store + page fault. Thus both ``sspush x1/x5`` and ``sspopchk x1/x5`` will raise AMO/ + store page fault. This simplies COW handling in kernel during fork, kernel + can convert shadow stack pages into read-only memory (as it does for regular + read-write memory) and as soon as subsequent ``sspush`` or ``sspopchk`` in + userspace is encountered, then kernel can perform COW. + +- Shadow stack load / shadow stack store on read-write, read-write-execute + memory raises an access fault. This is a fatal condition because shadow stack + should never be operating on read-write, read-write-execute memory. + +3. ELF and psABI +----------------- + +Toolchain sets up :c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_BCFI` for property +:c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file. + +4. Linux enabling +------------------ + +User space programs can have multiple shared objects loaded in its address space +and it's a difficult task to make sure all the dependencies have been compiled +with support of shadow stack. Thus it's left to dynamic loader to enable +shadow stack for the program. + +5. prctl() enabling +-------------------- + +:c:macro:`PR_SET_SHADOW_STACK_STATUS` / :c:macro:`PR_GET_SHADOW_STACK_STATUS` / +:c:macro:`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage shadow +stack enabling for tasks. prctls are arch agnostic and returns -EINVAL on other +arches. + +* prctl(PR_SET_SHADOW_STACK_STATUS, unsigned long arg) + +If arg1 :c:macro:`PR_SHADOW_STACK_ENABLE` and if CPU supports ``zicfiss`` then +kernel will enable shadow stack for the task. Dynamic loader can issue this +:c:macro:`prctl` once it has determined that all the objects loaded in address +space have support for shadow stack. Additionally if there is a +:c:macro:`dlopen` to an object which wasn't compiled with ``zicfiss``, dynamic +loader can issue this prctl with arg1 set to 0 (i.e. +:c:macro:`PR_SHADOW_STACK_ENABLE` being clear) + +* prctl(PR_GET_SHADOW_STACK_STATUS, unsigned long * arg) + +Returns current status of indirect branch tracking. If enabled it'll return +:c:macro:`PR_SHADOW_STACK_ENABLE`. + +* prctl(PR_LOCK_SHADOW_STACK_STATUS, unsigned long arg) + +Locks current status of shadow stack enabling on the task. User space may want +to run with strict security posture and wouldn't want loading of objects +without ``zicfiss`` support in it and thus would want to disallow disabling of +shadow stack on current task. In that case user space can use this prctl to +lock current settings. + +5. violations related to returns with shadow stack enabled +----------------------------------------------------------- + +Pertaining to shadow stack, CPU raises software check exception in following +condition: + +- On execution of ``sspopchk x1/x5``, ``x1/x5`` didn't match top of shadow + stack. If mismatch happens then cpu does ``*tval = 3`` and raise software + check exception. + +Linux kernel will treat this as :c:macro:`SIGSEGV` with code = +:c:macro:`SEGV_CPERR` and follow normal course of signal delivery. + +6. Shadow stack tokens +----------------------- +Regular stores on shadow stacks are not allowed and thus can't be tampered +with via arbitrary stray writes due to bugs. However method of pivoting / +switching to shadow stack is simply writing to csr ``CSR_SSP`` and that will +change active shadow stack for the program. Instances of writes to ``CSR_SSP`` +in the address space of the program should be mostly limited to context +switching, stack unwind, longjmp or similar mechanisms (like context switching +of green threads) in languages like go, rust. This can be problematic because +an attacker can use memory corruption bugs and eventually use such context +switching routines to pivot to any shadow stack. Shadow stack tokens can help +mitigate this problem by making sure that: + +- When software is switching away from a shadow stack, shadow stack pointer + should be saved on shadow stack itself and call it ``shadow stack token`` + +- When software is switching to a shadow stack, it should read the + ``shadow stack token`` from shadow stack pointer and verify that + ``shadow stack token`` itself is pointer to shadow stack itself. + +- Once the token verification is done, software can perform the write to + ``CSR_SSP`` to switch shadow stack. + +Here software can be user mode task runtime itself which is managing various +contexts as part of single thread. Software can be kernel as well when kernel +has to deliver a signal to user task and must save shadow stack pointer. Kernel +can perform similar procedure by saving a token on user shadow stack itself. +This way whenever :c:macro:`sigreturn` happens, kernel can read the token and +verify the token and then switch to shadow stack. Using this mechanism, kernel +helps user task so that any corruption issue in user task is not exploited by +adversary by arbitrarily using :c:macro:`sigreturn`. Adversary will have to +make sure that there is a ``shadow stack token`` in addition to invoking +:c:macro:`sigreturn` + +7. Signal shadow stack +----------------------- +Following structure has been added to sigcontext for RISC-V:: + + struct __sc_riscv_cfi_state { + unsigned long ss_ptr; + }; + +As part of signal delivery, shadow stack token is saved on current shadow stack +itself and updated pointer is saved away in :c:macro:`ss_ptr` field in +:c:macro:`__sc_riscv_cfi_state` under :c:macro:`sigcontext`. Existing shadow +stack allocation is used for signal delivery. During :c:macro:`sigreturn`, +kernel will obtain :c:macro:`ss_ptr` from :c:macro:`sigcontext` and verify the +saved token on shadow stack itself and switch shadow stack.

On Thu, Dec 04, 2025 at 02:17:27PM -0800, Randy Dunlap wrote:

On 12/4/25 12:04 PM, Deepak Gupta wrote:

...

This patch creates a config for shadow stack support and landing pad instr support. Shadow stack support and landing instr support can be enabled by selecting `CONFIG_RISCV_USER_CFI`. Selecting `CONFIG_RISCV_USER_CFI` wires up path to enumerate CPU support and if cpu support exists, kernel will support cpu assisted user mode cfi.

If CONFIG_RISCV_USER_CFI is selected, select `ARCH_USES_HIGH_VMA_FLAGS`, `ARCH_HAS_USER_SHADOW_STACK` and DYNAMIC_SIGFRAME for riscv.

Reviewed-by: Zong Li zong.li@sifive.com Tested-by: Andreas Korb andreas.korb@aisec.fraunhofer.de Tested-by: Valentin Haudiquet valentin.haudiquet@canonical.com Signed-off-by: Deepak Gupta debug@rivosinc.com

---

arch/riscv/Kconfig | 22 ++++++++++++++++++++++ arch/riscv/configs/hardening.config | 4 ++++ 2 files changed, 26 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index 0c6038dc5dfd..f5574c6f66d8 100644 --- a/arch/riscv/Kconfig +++ b/arch/riscv/Kconfig @@ -1146,6 +1146,28 @@ config RANDOMIZE_BASE

       If unsure, say N.

+config RISCV_USER_CFI

• def_bool y
• bool "riscv userspace control flow integrity"
• depends on 64BIT && \

• $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss_zicfilp -fcf-protection=full)
  

• depends on RISCV_ALTERNATIVE
• select RISCV_SBI
• select ARCH_HAS_USER_SHADOW_STACK
• select ARCH_USES_HIGH_VMA_FLAGS
• select DYNAMIC_SIGFRAME
• help

•  Provides CPU assisted control flow integrity to userspace tasks.
  

          CPU-assisted

...

•  Control flow integrity is provided by implementing shadow stack for
  

•  backward edge and indirect branch tracking for forward edge in program.
  

•  Shadow stack protection is a hardware feature that detects function
  

•  return address corruption. This helps mitigate ROP attacks.
  

•  Indirect branch tracking enforces that all indirect branches must land
  

•  on a landing pad instruction else CPU will fault. This mitigates against
  

•  JOP / COP attacks. Applications must be enabled to use it, and old user-
  

•  space does not get protection "for free".
  

•  default y.
  

 Default is y if hardware supports it.

?

No default Y means support is built in the kernel for cfi. If hardware doesn't support CFI instructions, then kernel will do following

- prctls to manage shadow stack/landing pad enable/disable will fail. - vDSO will not have shadow stack instructions in it.

...

endmenu # "Kernel features"

-- ~Randy