diff options
| author | Deepak Gupta <debug@rivosinc.com> | 2026-01-25 21:09:54 -0700 |
|---|---|---|
| committer | Paul Walmsley <pjw@kernel.org> | 2026-01-25 21:09:54 -0700 |
| commit | 540de7ade1e1327119e367ec5f662a627b3fe9f9 (patch) | |
| tree | 2cb0e7b26c1cae5481bbd600f5172663a4ff48db | |
| parent | c68c2ef9d64169317a6e0e6f0506953637760409 (diff) | |
riscv/mm: update write protect to work on shadow stacks
'fork' implements copy-on-write (COW) by making pages readonly in both
child and parent.
ptep_set_wrprotect() and pte_wrprotect() clear _PAGE_WRITE in PTE.
The assumption is that the page is readable and, on a fault,
copy-on-write happens.
To implement COW on shadow stack pages, clearing the W bit makes them
XWR = 000. This will result in the wrong PTE setting, which allows no
permissions, but with V=1 and the PFN field pointing to the final
page. Instead, the 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, this doesn't interfere with existing COW for
read-write memory. The 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>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-9-b55691eacf4f@rivosinc.com
[pjw@kernel.org: clarify patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>
| -rw-r--r-- | arch/riscv/include/asm/pgtable.h | 12 |
1 files changed, 10 insertions, 2 deletions
diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h index 7f81eecac243..1340aa398a74 100644 --- a/arch/riscv/include/asm/pgtable.h +++ b/arch/riscv/include/asm/pgtable.h @@ -411,7 +411,7 @@ 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)); } #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP @@ -683,7 +683,15 @@ 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 |