linux-kernel - [tip: x86/shstk] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED

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Message-ID: <168980685940.28540.1641055647652693070.tip-bot2@tip-bot2>
Date:   Wed, 19 Jul 2023 22:47:39 -0000
From:   "tip-bot2 for Rick Edgecombe" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Yu-cheng Yu" <yu-cheng.yu@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Pengfei Xu <pengfei.xu@...el.com>,
        John Allen <john.allen@....com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/shstk] x86/mm: Update ptep/pmdp_set_wrprotect() for
 _PAGE_SAVED_DIRTY

The following commit has been merged into the x86/shstk branch of tip:

Commit-ID:     1f6f66f62e8cba909abc4fb59de3b57d8c5a9783
Gitweb:        https://git.kernel.org/tip/1f6f66f62e8cba909abc4fb59de3b57d8c5a9783
Author:        Rick Edgecombe <rick.p.edgecombe@...el.com>
AuthorDate:    Mon, 12 Jun 2023 17:10:37 -07:00
Committer:     Rick Edgecombe <rick.p.edgecombe@...el.com>
CommitterDate: Tue, 11 Jul 2023 14:12:19 -07:00

x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED_DIRTY

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 <yu-cheng.yu@...el.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Acked-by: Mike Rapoport (IBM) <rppt@...nel.org>
Tested-by: Pengfei Xu <pengfei.xu@...el.com>
Tested-by: John Allen <john.allen@....com>
Tested-by: Kees Cook <keescook@...omium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-12-rick.p.edgecombe%40intel.com
---
 arch/x86/include/asm/pgtable.h | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 0c0747c..b48d07a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1190,7 +1190,17 @@ 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)
@@ -1242,7 +1252,17 @@ 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