[PATCH v5 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

[Rick Edgecombe]

From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

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

When a PTE goes from Write=1,Dirty=1 to Write=0,Cow=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_COW non-atomically, a transient
   shadow stack PTE can be created as a result. Thus, prevent that with
   cmpxchg.

In the case of pmdp_set_wrprotect(), for nopmd configs the ->pmd operated
on does not exist and the logic would need to be different. Although the
extra functionality will normally be optimized out when user shadow
stacks are not configured, also exclude it in the preprocessor stage so
that it will still compile. User shadow stack is not supported there by
Linux anyway. Leave the cpu_feature_enabled() check so that the
functionality also gets disabled based on runtime detection of the
feature.

Similarly, compile it out in ptep_set_wrprotect() due to a clang warning
on i386. Like above, the code path should get optimized out on i386
since shadow stack is not supported on 32 bit kernels, but this makes
the compiler happy.

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

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---

v5:
 - Commit log verbiage and formatting (Boris)
 - Remove capitalization on shadow stack (Boris)
 - Fix i386 warning on recent clang

v3:
 - Remove unnecessary #ifdef (Dave Hansen)

v2:
 - Compile out some code due to clang build error
 - Clarify commit log (dhansen)
 - Normalize PTE bit descriptions between patches (dhansen)
 - Update comment with text from (dhansen)

Yu-cheng v30:
 - Replace (pmdval_t) cast with CONFIG_PGTABLE_LEVELES > 2 (Borislav Petkov).

 arch/x86/include/asm/pgtable.h | 37 ++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 7942eff2af50..c5047eb5f406 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1232,6 +1232,23 @@ 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)
 {
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+	/*
+	 * Avoid accidentally creating shadow stack PTEs
+	 * (Write=0,Dirty=1).  Use cmpxchg() to prevent races with
+	 * the hardware setting Dirty=1.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) {
+		pte_t old_pte, new_pte;
+
+		old_pte = READ_ONCE(*ptep);
+		do {
+			new_pte = pte_wrprotect(old_pte);
+		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
+
+		return;
+	}
+#endif
 	clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
 }
 
@@ -1284,6 +1301,26 @@ 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)
 {
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+	/*
+	 * If shadow stack is enabled, pmd_wrprotect() moves _PAGE_DIRTY
+	 * to _PAGE_COW (see comments at pmd_wrprotect()).
+	 * When a thread reads a RW=1, Dirty=0 PMD and before changing it
+	 * to RW=0, Dirty=0, another thread could have written to the page
+	 * and the PMD is RW=1, Dirty=1 now.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) {
+		pmd_t old_pmd, new_pmd;
+
+		old_pmd = READ_ONCE(*pmdp);
+		do {
+			new_pmd = pmd_wrprotect(old_pmd);
+		} while (!try_cmpxchg(&pmdp->pmd, &old_pmd.pmd, new_pmd.pmd));
+
+		return;
+	}
+#endif
+
 	clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
 }
 
-- 
2.17.1