[v9,11/42] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED_DIRTY

Commit Message

Edgecombe, Rick P June 13, 2023, 12:10 a.m. UTC

When shadow stack is in use, Write=0,Dirty=1 PTE are preserved for
shadow stack. Copy-on-write PTEs then have Write=0,SavedDirty=1.

When a PTE goes from Write=1,Dirty=1 to Write=0,SavedDirty=1, it could
become a transient shadow stack PTE in two cases:

1. Some processors can start a write but end up seeing a Write=0 PTE by
   the time they get to the Dirty bit, creating a transient shadow stack
   PTE. However, this will not occur on processors supporting shadow
   stack, and a TLB flush is not necessary.

2. When _PAGE_DIRTY is replaced with _PAGE_SAVED_DIRTY non-atomically, a
   transient shadow stack PTE can be created as a result.

Prevent the second case when doing a write protection and Dirty->SavedDirty
shift at the same time with a CMPXCHG loop. The first case

Note, in the PAE case CMPXCHG will need to operate on 8 byte, but
try_cmpxchg() will not use CMPXCHG8B, so it cannot operate on a full PAE
PTE. However the exiting logic is not operating on a full 8 byte region
either, and relies on the fact that the Write bit is in the first 4
bytes when doing the clear_bit(). Since both the Dirty, SavedDirty and
Write bits are in the first 4 bytes, casting to a long will be similar to
the existing behavior which also casts to a long.

Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many
insights to the issue. Jann Horn provided the CMPXCHG solution.

Co-developed-by: Yu-cheng Yu <[e-mail]>
Signed-off-by: Yu-cheng Yu <[e-mail]>
Signed-off-by: Rick Edgecombe <[e-mail]>
Acked-by: Mike Rapoport (IBM) <[e-mail]>
Tested-by: Pengfei Xu <[e-mail]>
Tested-by: John Allen <[e-mail]>
Tested-by: Kees Cook <[e-mail]>
---
v9:
 - Use bit shifting helpers that don't need any extra conditional
   logic. (Linus)
 - Always do the SavedDirty shifting (Linus)
---
 arch/x86/include/asm/pgtable.h | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

Comments

Linus Torvalds June 13, 2023, 6:01 p.m. UTC | #1

On Mon, Jun 12, 2023 at 5:14 PM Rick Edgecombe
<[e-mail]> wrote:
>
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1189,7 +1189,17 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
>  static inline void ptep_set_wrprotect(struct mm_struct *mm,
>                                       unsigned long addr, pte_t *ptep)
>  {
> -       clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> +       /*
> +        * Avoid accidentally creating shadow stack PTEs
> +        * (Write=0,Dirty=1).  Use cmpxchg() to prevent races with
> +        * the hardware setting Dirty=1.
> +        */
> +       pte_t old_pte, new_pte;
> +
> +       old_pte = READ_ONCE(*ptep);
> +       do {
> +               new_pte = pte_wrprotect(old_pte);
> +       } while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
>  }

Thanks. Much nicer with this all being done just one way and no need
for ifdeffery on config options and runtime static branches.

                  Linus

Patch

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index a95f872c7429..99b54ab0a919 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1189,7 +1189,17 @@  static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
 static inline void ptep_set_wrprotect(struct mm_struct *mm,
 				      unsigned long addr, pte_t *ptep)
 {
-	clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
+	/*
+	 * Avoid accidentally creating shadow stack PTEs
+	 * (Write=0,Dirty=1).  Use cmpxchg() to prevent races with
+	 * the hardware setting Dirty=1.
+	 */
+	pte_t old_pte, new_pte;
+
+	old_pte = READ_ONCE(*ptep);
+	do {
+		new_pte = pte_wrprotect(old_pte);
+	} while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
 }
 
 #define flush_tlb_fix_spurious_fault(vma, address, ptep) do { } while (0)
@@ -1241,7 +1251,17 @@  static inline pud_t pudp_huge_get_and_clear(struct mm_struct *mm,
 static inline void pmdp_set_wrprotect(struct mm_struct *mm,
 				      unsigned long addr, pmd_t *pmdp)
 {
-	clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
+	/*
+	 * Avoid accidentally creating shadow stack PTEs
+	 * (Write=0,Dirty=1).  Use cmpxchg() to prevent races with
+	 * the hardware setting Dirty=1.
+	 */
+	pmd_t old_pmd, new_pmd;
+
+	old_pmd = READ_ONCE(*pmdp);
+	do {
+		new_pmd = pmd_wrprotect(old_pmd);
+	} while (!try_cmpxchg((long *)pmdp, (long *)&old_pmd, *(long *)&new_pmd));
 }
 
 #ifndef pmdp_establish