From: Deepak Gupta debug@rivosinc.com
This patch implements new risc-v specific protection flag `PROT_SHADOWSTACK` (only for kernel) on riscv.
`PROT_SHADOWSTACK` protection flag is only limited to kernel and not exposed to userspace. Shadow stack is a security construct to prevent against ROP attacks. `map_shadow_stack` is a new syscall to manufacture shadow stack. In order to avoid multiple methods to create shadow stack, `PROT_SHADOWSTACK` is not allowed for user space `mmap` call. `mprotect` wouldn't allow because `arch_validate_prot` already takes care of this for risc-v.
`arch_calc_vm_prot_bits` is implemented on risc-v to return VM_SHADOW_STACK (alias for VM_WRITE) if PROT_SHADOWSTACK is supplied (such as call to `do_mmap` will) and underlying CPU supports shadow stack. `PROT_WRITE` will be converted to `VM_READ | `VM_WRITE` so that existing case where `PROT_WRITE` is specified keep working but don't collide with `VM_WRITE` only encoding which now denotes a shadow stack.
risc-v `mmap` wrapper enforces if PROT_WRITE is specified and PROT_READ is left out then PROT_READ is enforced.
Earlier `protection_map[VM_WRITE]` used to pick read-write (and copy on write) PTE encodings. Now all non-shadow stack writeable mappings will pick `protection_map[VM_WRITE | VM_READ] PTE encodings. `protection[VM_WRITE]` are programmed to pick PAGE_SHADOWSTACK PTE encordings.
Signed-off-by: Deepak Gupta debug@rivosinc.com --- arch/riscv/include/asm/mman.h | 17 +++++++++++++++++ arch/riscv/include/asm/pgtable.h | 1 + arch/riscv/kernel/sys_riscv.c | 19 +++++++++++++++++++ arch/riscv/mm/init.c | 2 +- 4 files changed, 38 insertions(+), 1 deletion(-)
diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/asm/mman.h index 4902d837e93c..bc09a9c0e81f 100644 --- a/arch/riscv/include/asm/mman.h +++ b/arch/riscv/include/asm/mman.h @@ -22,4 +22,21 @@ */ #define PROT_SHADOWSTACK 0x40
+static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot, + unsigned long pkey __always_unused) +{ + unsigned long ret = 0; + + if (cpu_supports_shadow_stack()) + ret = (prot & PROT_SHADOWSTACK) ? VM_SHADOW_STACK : 0; + /* + * If PROT_WRITE was specified, force it to VM_READ | VM_WRITE. + * Only VM_WRITE means shadow stack. + */ + if (prot & PROT_WRITE) + ret = (VM_READ | VM_WRITE); + return ret; +} +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey) + #endif /* ! __ASM_MMAN_H__ */ diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h index 294044429e8e..54a8dde29504 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -184,6 +184,7 @@ extern struct pt_alloc_ops pt_ops __initdata; #define PAGE_READ_EXEC __pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC) #define PAGE_WRITE_EXEC __pgprot(_PAGE_BASE | _PAGE_READ | \ _PAGE_EXEC | _PAGE_WRITE) +#define PAGE_SHADOWSTACK __pgprot(_PAGE_BASE | _PAGE_WRITE)
#define PAGE_COPY PAGE_READ #define PAGE_COPY_EXEC PAGE_READ_EXEC diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c index a2ca5b7756a5..2a7cf28a6fe0 100644 --- a/arch/riscv/kernel/sys_riscv.c +++ b/arch/riscv/kernel/sys_riscv.c @@ -16,6 +16,7 @@ #include <asm/unistd.h> #include <asm-generic/mman-common.h> #include <vdso/vsyscall.h> +#include <asm/mman.h>
static long riscv_sys_mmap(unsigned long addr, unsigned long len, unsigned long prot, unsigned long flags, @@ -25,6 +26,24 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len, if (unlikely(offset & (~PAGE_MASK >> page_shift_offset))) return -EINVAL;
+ /* + * If only PROT_WRITE is specified then extend that to PROT_READ + * protection_map[VM_WRITE] is now going to select shadow stack encodings. + * So specifying PROT_WRITE actually should select protection_map [VM_WRITE | VM_READ] + * If user wants to create shadow stack then they should use `map_shadow_stack` syscall. + */ + if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ))) + prot |= PROT_READ; + + /* + * PROT_SHADOWSTACK is a kernel only protection flag on risc-v. + * mmap doesn't expect PROT_SHADOWSTACK to be set by user space. + * User space can rely on `map_shadow_stack` syscall to create + * shadow stack pages. + */ + if (unlikely(prot & PROT_SHADOWSTACK)) + return -EINVAL; + return ksys_mmap_pgoff(addr, len, prot, flags, fd, offset >> (PAGE_SHIFT - page_shift_offset)); } diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c index 2e011cbddf3a..f71c2d2c6cbf 100644 --- a/arch/riscv/mm/init.c +++ b/arch/riscv/mm/init.c @@ -296,7 +296,7 @@ pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE); static const pgprot_t protection_map[16] = { [VM_NONE] = PAGE_NONE, [VM_READ] = PAGE_READ, - [VM_WRITE] = PAGE_COPY, + [VM_WRITE] = PAGE_SHADOWSTACK, [VM_WRITE | VM_READ] = PAGE_COPY, [VM_EXEC] = PAGE_EXEC, [VM_EXEC | VM_READ] = PAGE_READ_EXEC,