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 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 b7f13f087954..3487f28fa0bf 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -352,6 +352,13 @@ 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 Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/cpufeature.h | 12 ++++++++++++ arch/riscv/include/asm/hwcap.h | 2 ++ arch/riscv/include/asm/processor.h | 1 + arch/riscv/kernel/cpufeature.c | 22 ++++++++++++++++++++++ 4 files changed, 37 insertions(+)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h index 3a87f612035c..100f4b53ba5d 100644 --- a/arch/riscv/include/asm/cpufeature.h +++ b/arch/riscv/include/asm/cpufeature.h @@ -146,4 +146,16 @@ 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 e3cbf203cdde..abc33ca1302e 100644 --- a/arch/riscv/include/asm/hwcap.h +++ b/arch/riscv/include/asm/hwcap.h @@ -105,6 +105,8 @@ #define RISCV_ISA_EXT_ZVFBFWMA 96 #define RISCV_ISA_EXT_ZAAMO 97 #define RISCV_ISA_EXT_ZALRSC 98 +#define RISCV_ISA_EXT_ZICFILP 99 +#define RISCV_ISA_EXT_ZICFISS 100

#define RISCV_ISA_EXT_XLINUXENVCFG 127

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h index 5f56eb9d114a..e3aba3336e63 100644 --- a/arch/riscv/include/asm/processor.h +++ b/arch/riscv/include/asm/processor.h @@ -13,6 +13,7 @@ #include <vdso/processor.h>

#include <asm/ptrace.h> +#include <asm/hwcap.h>

#define arch_get_mmap_end(addr, len, flags) \ ({ \ diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 2054f6c4b0ae..c54de1bbe206 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -258,6 +258,24 @@ 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, @@ -444,6 +462,10 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = { __RISCV_ISA_EXT_SUPERSET_VALIDATE(zicbom, RISCV_ISA_EXT_ZICBOM, riscv_xlinuxenvcfg_exts, riscv_ext_zicbom_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 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 | 28 ++++++++++++++++++++++++++++ 5 files changed, 59 insertions(+)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h index e3aba3336e63..d851bb5c6da0 100644 --- a/arch/riscv/include/asm/processor.h +++ b/arch/riscv/include/asm/processor.h @@ -14,6 +14,7 @@

#include <asm/ptrace.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 f5916a70879a..e066f41176ca 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..94b214c295c0 --- /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 __ASSEMBLY__ +#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 /* __ASSEMBLY__ */ + +#endif /* _ASM_RISCV_USERCFI_H */ diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c index 16490755304e..f33945432f8f 100644 --- a/arch/riscv/kernel/asm-offsets.c +++ b/arch/riscv/kernel/asm-offsets.c @@ -49,6 +49,10 @@ 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 33a5a9f2a0d4..c4bfe2085c41 100644 --- a/arch/riscv/kernel/entry.S +++ b/arch/riscv/kernel/entry.S @@ -91,6 +91,32 @@ 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 + ALTERNATIVE("nops(2)", + __stringify( \ + REG_L \tmp, TASK_TI_USER_SSP(tp); \ + csrw CSR_SSP, \tmp), + 0, + RISCV_ISA_EXT_ZICFISS, + CONFIG_RISCV_USER_CFI) +.endm

SYM_CODE_START(handle_exception) /* @@ -147,6 +173,7 @@ 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 @@ -236,6 +263,7 @@ SYM_CODE_START_NOALIGN(ret_from_exception) * structures again. */ csrw CSR_SCRATCH, tp + restore_userssp s3 1: #ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE move a0, sp

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 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 dba257cc4e2d..f21c888f59eb 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -433,6 +433,11 @@ 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) @@ -778,6 +783,11 @@ 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 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 60d4821627d2..4e3431ccf634 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -423,7 +423,7 @@ static inline int pte_devmap(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) */ @@ -624,7 +624,15 @@ 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 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 | 9 +++ arch/riscv/kernel/usercfi.c | 120 +++++++++++++++++++++++++++++++++++ 4 files changed, 161 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 94b214c295c0..0581587490c2 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -8,6 +8,9 @@ #ifndef __ASSEMBLY__ #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) + +#define set_active_shstk(task, shstk_addr) + +#define is_shstk_enabled(task) false + #endif /* CONFIG_RISCV_USER_CFI */

#endif /* __ASSEMBLY__ */ diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index 7c244de77180..99acb6342a37 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -29,6 +29,7 @@ #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> @@ -211,6 +212,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) unsigned long 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. */ @@ -229,11 +231,18 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) p->thread.s[0] = (unsigned long)args->fn; p->thread.s[1] = (unsigned long)args->fn_arg; } 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 */ + ssp ? set_active_shstk(p, ssp) : 0; 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 0b3bbb41490a..ec3d78efd6f3 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 @@ -141,3 +176,88 @@ 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 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 e3049543008b..ea8b351b5bc5 100644 --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -204,4 +204,8 @@ static inline bool cpu_mitigations_auto_nosmt(void) } #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 15c18ef4eb11..2e09b19317a3 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -364,4 +364,31 @@ struct prctl_mm_map { # define PR_TIMER_CREATE_RESTORE_IDS_ON 1 # define PR_TIMER_CREATE_RESTORE_IDS_GET 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 78 + +/* + * 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 79 +# 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 80 + #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/sys.c b/kernel/sys.c index c434968e9f5d..91a1dc093c2a 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2340,6 +2340,21 @@ 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)

#ifdef CONFIG_ANON_VMA_NAME @@ -2820,6 +2835,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, return -EINVAL; error = posixtimer_create_prctl(arg2); 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.

Reviewed-by: Zong Li zong.li@sifive.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 | 43 +++++++++++++++++++++++++++++++++ 4 files changed, 49 insertions(+)

diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h index cd627ec289f1..5a27cefd7805 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 handle_bad_stack(struct pt_regs *regs); asmlinkage void do_page_fault(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 978115567bca..8d25837a9384 100644 --- a/arch/riscv/kernel/entry.S +++ b/arch/riscv/kernel/entry.S @@ -474,6 +474,9 @@ 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 8ff8e8b36524..3f7709f4595a 100644 --- a/arch/riscv/kernel/traps.c +++ b/arch/riscv/kernel/traps.c @@ -354,6 +354,49 @@ 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) +{ + bool ret = false; + unsigned long tval = csr_read(CSR_TVAL); + + if ((tval == CFI_TVAL_FCFI_CODE && cpu_supports_indirect_br_lp_instr()) || + (tval == CFI_TVAL_BCFI_CODE && cpu_supports_shadow_stack())) { + do_trap_error(regs, SIGSEGV, SEGV_CPERR, regs->epc, + "Oops - control flow violation"); + ret = true; + } + + return ret; +} + +/* + * 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 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 | 56 +++++++++++++++++++++ 5 files changed, 157 insertions(+)

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h index cea7908cdb3a..68da5b7b79fd 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -8,6 +8,7 @@ #ifndef __ASSEMBLY__ #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)

+#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 /* __ASSEMBLY__ */ diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h index a38268b19c3d..659ea3af5680 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 /* __ASSEMBLY__ */

#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 cd4f175dc837..f37e4beffe03 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 2ebe789caa6b..8bc3e1e3f712 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()) @@ -169,6 +174,57 @@ 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 == NULL) + 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.

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 659ea3af5680..42c3fc8bd513 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 /* __ASSEMBLY__ */

#endif /* _UAPI_ASM_RISCV_PTRACE_H */ diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c index ea67e9fb7a58..933a3d26d33c 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] = { .core_note_type = NT_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 using which user cfi can be disabled. User backward cfi and forward cfi can be enabled independently. Kernel command line parameter "riscv_nousercfi" can take below values: - "all" : Disable forward and backward cfi both. - "bcfi" : Disable backward cfi. - "fcfi" : Disable forward cfi

Signed-off-by: Deepak Gupta debug@rivosinc.com --- Documentation/admin-guide/kernel-parameters.txt | 8 ++++ arch/riscv/include/asm/usercfi.h | 7 +++ arch/riscv/kernel/cpufeature.c | 9 +++- arch/riscv/kernel/usercfi.c | 59 ++++++++++++++++++++----- 4 files changed, 70 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 76e538c77e31..f75d50420a56 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -6237,6 +6237,14 @@ 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 68da5b7b79fd..6867ba6bd5a5 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 __ASSEMBLY__ #include <linux/types.h> #include <linux/prctl.h> @@ -83,6 +87,9 @@ 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 /* __ASSEMBLY__ */

#endif /* _ASM_RISCV_USERCFI_H */ diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index c54de1bbe206..bf2035c0c136 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)

@@ -44,6 +45,8 @@ struct riscv_isainfo hart_isa[NR_CPUS];

u32 thead_vlenb_of;

+extern unsigned int riscv_nousercfi; + /** * riscv_isa_extension_base() - Get base extension word * @@ -261,7 +264,8 @@ 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; @@ -270,7 +274,8 @@ 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 8bc3e1e3f712..5ef357f43ad7 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 int 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);

From: Jim Shu jim.shu@sifive.com

user mode tasks compiled with zicfilp may call indirectly into vdso (like hwprobe indirect calls). Add landing pad compile support in vdso. vdso with landing pad in it will be nop for tasks which have not enabled landing pad. This patch allows to run user mode tasks with cfi eanbled and do no harm.

Future work can be done on this to do below - labeled landing pad on vdso functions (whenever labeling support shows up in gnu-toolchain) - emit shadow stack instructions only in vdso compiled objects as part of kernel compile.

Signed-off-by: Jim Shu jim.shu@sifive.com Reviewed-by: Zong Li zong.li@sifive.com Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/Makefile | 5 +++- arch/riscv/include/asm/assembler.h | 44 +++++++++++++++++++++++++++++++++++ arch/riscv/kernel/vdso/Makefile | 6 +++++ arch/riscv/kernel/vdso/flush_icache.S | 4 ++++ arch/riscv/kernel/vdso/getcpu.S | 4 ++++ arch/riscv/kernel/vdso/rt_sigreturn.S | 4 ++++ arch/riscv/kernel/vdso/sys_hwprobe.S | 4 ++++ 7 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile index 539d2aef5cab..c2dd09bb9db3 100644 --- a/arch/riscv/Makefile +++ b/arch/riscv/Makefile @@ -88,9 +88,12 @@ riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZACAS) := $(riscv-march-y)_zacas # Check if the toolchain supports Zabha riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZABHA) := $(riscv-march-y)_zabha

+KBUILD_BASE_ISA = -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/') +export KBUILD_BASE_ISA + # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by # matching non-v and non-multi-letter extensions out with the filter ([^v_]*) -KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/') +KBUILD_CFLAGS += $(KBUILD_BASE_ISA)

KBUILD_AFLAGS += -march=$(riscv-march-y)

diff --git a/arch/riscv/include/asm/assembler.h b/arch/riscv/include/asm/assembler.h index 44b1457d3e95..a058ea5e9c58 100644 --- a/arch/riscv/include/asm/assembler.h +++ b/arch/riscv/include/asm/assembler.h @@ -80,3 +80,47 @@ .endm

#endif /* __ASM_ASSEMBLER_H */ + +#if defined(CONFIG_RISCV_USER_CFI) && (__riscv_xlen == 64) +.macro vdso_lpad +lpad 0 +.endm +#else +.macro vdso_lpad +.endm +#endif + +/* + * This macro emits a program property note section identifying + * architecture features which require special handling, mainly for + * use in assembly files included in the VDSO. + */ +#define NT_GNU_PROPERTY_TYPE_0 5 +#define GNU_PROPERTY_RISCV_FEATURE_1_AND 0xc0000000 + +#define GNU_PROPERTY_RISCV_FEATURE_1_ZICFILP (1U << 0) +#define GNU_PROPERTY_RISCV_FEATURE_1_ZICFISS (1U << 1) + +#if defined(CONFIG_RISCV_USER_CFI) && (__riscv_xlen == 64) +#define GNU_PROPERTY_RISCV_FEATURE_1_DEFAULT \ + (GNU_PROPERTY_RISCV_FEATURE_1_ZICFILP) +#endif + +#ifdef GNU_PROPERTY_RISCV_FEATURE_1_DEFAULT +.macro emit_riscv_feature_1_and, feat = GNU_PROPERTY_RISCV_FEATURE_1_DEFAULT + .pushsection .note.gnu.property, "a" + .p2align 3 + .word 4 + .word 16 + .word NT_GNU_PROPERTY_TYPE_0 + .asciz "GNU" + .word GNU_PROPERTY_RISCV_FEATURE_1_AND + .word 4 + .word \feat + .word 0 + .popsection +.endm +#else +.macro emit_riscv_feature_1_and, feat = 0 +.endm +#endif diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile index ad73607abc28..441c5431d27e 100644 --- a/arch/riscv/kernel/vdso/Makefile +++ b/arch/riscv/kernel/vdso/Makefile @@ -13,12 +13,18 @@ vdso-syms += flush_icache vdso-syms += hwprobe vdso-syms += sys_hwprobe

+ifdef CONFIG_RISCV_USER_CFI +LPAD_MARCH = _zicfilp_zicfiss -fcf-protection=full +endif + # Files to link into the vdso obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o

ccflags-y := -fno-stack-protector ccflags-y += -DDISABLE_BRANCH_PROFILING ccflags-y += -fno-builtin +ccflags-y += $(KBUILD_BASE_ISA)$(LPAD_MARCH) +asflags-y += $(KBUILD_BASE_ISA)$(LPAD_MARCH)

ifneq ($(c-gettimeofday-y),) CFLAGS_vgettimeofday.o += -fPIC -include $(c-gettimeofday-y) diff --git a/arch/riscv/kernel/vdso/flush_icache.S b/arch/riscv/kernel/vdso/flush_icache.S index 8f884227e8bc..e4c56970905e 100644 --- a/arch/riscv/kernel/vdso/flush_icache.S +++ b/arch/riscv/kernel/vdso/flush_icache.S @@ -5,11 +5,13 @@

#include <linux/linkage.h> #include <asm/unistd.h> +#include <asm/assembler.h>

.text /* int __vdso_flush_icache(void *start, void *end, unsigned long flags); */ SYM_FUNC_START(__vdso_flush_icache) .cfi_startproc + vdso_lpad #ifdef CONFIG_SMP li a7, __NR_riscv_flush_icache ecall @@ -20,3 +22,5 @@ SYM_FUNC_START(__vdso_flush_icache) ret .cfi_endproc SYM_FUNC_END(__vdso_flush_icache) + +emit_riscv_feature_1_and diff --git a/arch/riscv/kernel/vdso/getcpu.S b/arch/riscv/kernel/vdso/getcpu.S index 9c1bd531907f..5c1ecc4e1465 100644 --- a/arch/riscv/kernel/vdso/getcpu.S +++ b/arch/riscv/kernel/vdso/getcpu.S @@ -5,14 +5,18 @@

#include <linux/linkage.h> #include <asm/unistd.h> +#include <asm/assembler.h>

.text /* int __vdso_getcpu(unsigned *cpu, unsigned *node, void *unused); */ SYM_FUNC_START(__vdso_getcpu) .cfi_startproc + vdso_lpad /* For now, just do the syscall. */ li a7, __NR_getcpu ecall ret .cfi_endproc SYM_FUNC_END(__vdso_getcpu) + +emit_riscv_feature_1_and diff --git a/arch/riscv/kernel/vdso/rt_sigreturn.S b/arch/riscv/kernel/vdso/rt_sigreturn.S index 3dc022aa8931..e82987dc3739 100644 --- a/arch/riscv/kernel/vdso/rt_sigreturn.S +++ b/arch/riscv/kernel/vdso/rt_sigreturn.S @@ -5,12 +5,16 @@

#include <linux/linkage.h> #include <asm/unistd.h> +#include <asm/assembler.h>

.text SYM_FUNC_START(__vdso_rt_sigreturn) .cfi_startproc .cfi_signal_frame + vdso_lpad li a7, __NR_rt_sigreturn ecall .cfi_endproc SYM_FUNC_END(__vdso_rt_sigreturn) + +emit_riscv_feature_1_and diff --git a/arch/riscv/kernel/vdso/sys_hwprobe.S b/arch/riscv/kernel/vdso/sys_hwprobe.S index 77e57f830521..f1694451a60c 100644 --- a/arch/riscv/kernel/vdso/sys_hwprobe.S +++ b/arch/riscv/kernel/vdso/sys_hwprobe.S @@ -3,13 +3,17 @@

#include <linux/linkage.h> #include <asm/unistd.h> +#include <asm/assembler.h>

.text SYM_FUNC_START(riscv_hwprobe) .cfi_startproc + vdso_lpad li a7, __NR_riscv_hwprobe ecall ret

.cfi_endproc SYM_FUNC_END(riscv_hwprobe) + +emit_riscv_feature_1_and

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 Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/Kconfig | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index bbec87b79309..147ae201823e 100644 --- a/arch/riscv/Kconfig +++ b/arch/riscv/Kconfig @@ -256,6 +256,27 @@ config ARCH_HAS_BROKEN_DWARF5 # https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471... depends on LD_IS_LLD && LLD_VERSION < 180000

+config RISCV_USER_CFI + def_bool n + bool "riscv userspace control flow integrity" + depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss) + 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. + 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 + config ARCH_MMAP_RND_BITS_MIN default 18 if 64BIT default 8

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 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..b50089e6a52b --- /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:`SIGSEV`` 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.