[PATCH v27 12/31] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

Yu-cheng Yu
2021-05-21T22:14:02Z

When Shadow Stack is introduced, [R/O + _PAGE_DIRTY] PTE is reserved for
shadow stack. Copy-on-write PTEs have [R/O + _PAGE_COW].

When a PTE goes from [R/W + _PAGE_DIRTY] to [R/O + _PAGE_COW], it could
become a transient shadow stack PTE in two cases:

The first case is that some processors can start a write but end up seeing
a read-only 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.

The second case is that 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.

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

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/include/asm/pgtable.h | 36 ++++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index f7bc20780fee..e61ad0946212 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -1306,6 +1306,24 @@ 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)
{
+ /*
+ * If Shadow Stack is enabled, pte_wrprotect() moves _PAGE_DIRTY
+ * to _PAGE_COW (see comments at pte_wrprotect()).
+ * When a thread reads a RW=1, Dirty=0 PTE and before changing it
+ * to RW=0, Dirty=0, another thread could have written to the page
+ * and the PTE is RW=1, Dirty=1 now. Use try_cmpxchg() to detect
+ * PTE changes and update old_pte, then try again.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_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;
+ }
clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
}

@@ -1350,6 +1368,24 @@ 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)
{
+ /*
+ * 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. Use try_cmpxchg() to detect
+ * PMD changes and update old_pmd, then try again.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ pmd_t old_pmd, new_pmd;
+
+ old_pmd = READ_ONCE(*pmdp);
+ do {
+ new_pmd = pmd_wrprotect(old_pmd);
+ } while (!try_cmpxchg((pmdval_t *)pmdp, (pmdval_t *)&old_pmd, pmd_val(new_pmd)));
+
+ return;
+ }
clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
}

--
2.21.0