[PATCH v4 24/39] mm: Warn on shadow stack memory in wrong vma

[Rick Edgecombe]

The x86 Control-flow Enforcement Technology (CET) feature includes a new
type of memory called shadow stack. This shadow stack memory has some
unusual properties, which requires some core mm changes to function
properly.

One sharp edge is that PTEs that are both Write=0 and Dirty=1 are
treated as shadow by the CPU, but this combination used to be created by
the kernel on x86. Previous patches have changed the kernel to now avoid
creating these PTEs unless they are for shadow stack memory. In case any
missed corners of the kernel are still creating PTEs like this for
non-shadow stack memory, and to catch any re-introductions of the logic,
warn if any shadow stack PTEs (Write=0, Dirty=1) are found in non-shadow
stack VMAs when they are being zapped. This won't catch transient cases
but should have decent coverage. It will be compiled out when shadow
stack is not configured.

In order to check if a pte is shadow stack in core mm code, add default
implmentations for pte_shstk() and pmd_shstk().

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

v3:
 - New patch

 arch/x86/include/asm/pgtable.h |  2 ++
 include/linux/pgtable.h        | 14 ++++++++++++++
 mm/huge_memory.c               |  2 ++
 mm/memory.c                    |  2 ++
 4 files changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 945d58681a87..519a266ace10 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -129,6 +129,7 @@ static inline bool pte_dirty(pte_t pte)
 	return pte_flags(pte) & _PAGE_DIRTY_BITS;
 }
 
+#define pte_shstk pte_shstk
 static inline bool pte_shstk(pte_t pte)
 {
 	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
@@ -147,6 +148,7 @@ static inline bool pmd_dirty(pmd_t pmd)
 	return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
 }
 
+#define pmd_shstk pmd_shstk
 static inline bool pmd_shstk(pmd_t pmd)
 {
 	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index b4a9d9936463..34aa57941d57 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -531,6 +531,20 @@ static inline pte_t pte_mkwrite_shstk(pte_t pte)
 }
 #endif
 
+#ifndef pte_shstk
+static inline bool pte_shstk(pte_t pte)
+{
+	return false;
+}
+#endif
+
+#ifndef pmd_shstk
+static inline bool pmd_shstk(pmd_t pte)
+{
+	return false;
+}
+#endif
+
 #ifndef pte_clear_savedwrite
 #define pte_clear_savedwrite pte_wrprotect
 #endif
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b6294c4ad471..64d81aa97a61 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1656,6 +1656,8 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	 */
 	orig_pmd = pmdp_huge_get_and_clear_full(vma, addr, pmd,
 						tlb->fullmm);
+	VM_WARN_ON_ONCE(!(vma->vm_flags & VM_SHADOW_STACK) &&
+			pmd_shstk(orig_pmd));
 	tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
 	if (vma_is_special_huge(vma)) {
 		if (arch_needs_pgtable_deposit())
diff --git a/mm/memory.c b/mm/memory.c
index c02b6421241d..bae62b2d6696 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1437,6 +1437,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 				continue;
 			ptent = ptep_get_and_clear_full(mm, addr, pte,
 							tlb->fullmm);
+			VM_WARN_ON_ONCE(!(vma->vm_flags & VM_SHADOW_STACK) &&
+					pte_shstk(ptent));
 			tlb_remove_tlb_entry(tlb, pte, addr);
 			zap_install_uffd_wp_if_needed(vma, addr, pte, details,
 						      ptent);
-- 
2.17.1