[PATCH v4 21/39] mm/mprotect: Exclude shadow stack from preserve_write

[Rick Edgecombe]

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

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.

In change_pte_range(), when a PTE is changed for prot_numa, _PAGE_RW is
preserved to avoid the additional write fault after the NUMA hinting fault.
However, pte_write() now includes both normal writable and shadow stack
(Write=0, Dirty=1) PTEs, but the latter does not have _PAGE_RW and has no
need to preserve it.

Exclude shadow stack from preserve_write test, and apply the same change to
change_huge_pmd().

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---

v4:
 - Add "why" to comments in code (Peterz)

Yu-cheng v25:
 - Move is_shadow_stack_mapping() to a separate line.

Yu-cheng v24:
 - Change arch_shadow_stack_mapping() to is_shadow_stack_mapping().

 mm/huge_memory.c | 8 ++++++++
 mm/mprotect.c    | 8 ++++++++
 2 files changed, 16 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 60451e588955..b6294c4ad471 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1803,6 +1803,14 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		return 0;
 
 	preserve_write = prot_numa && pmd_write(*pmd);
+
+	/*
+	 * Preserve only normal writable huge PMD, but not shadow
+	 * stack (RW=0, Dirty=1), so the restoring code doesn't
+	 * make the shadow stack memory conventionally writable.
+	 */
+	if (vma->vm_flags & VM_SHADOW_STACK)
+		preserve_write = false;
 	ret = 1;
 
 #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
diff --git a/mm/mprotect.c b/mm/mprotect.c
index de7351631e21..026347f1f1ee 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -115,6 +115,14 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
 			pte_t ptent;
 			bool preserve_write = prot_numa && pte_write(oldpte);
 
+			/*
+			 * Preserve only normal writable PTE, but not shadow
+			 * stack (RW=0, Dirty=1), so the restoring code doesn't
+			 * make the shadow stack memory conventionally writable.
+			 */
+			if (vma->vm_flags & VM_SHADOW_STACK)
+				preserve_write = false;
+
 			/*
 			 * Avoid trapping faults against the zero or KSM
 			 * pages. See similar comment in change_huge_pmd.
-- 
2.17.1