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

Commit Message

Edgecombe, Rick P Dec. 3, 2022, 12:35 a.m. UTC

From: Yu-cheng Yu <[e-mail]>

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 <[e-mail]>
Tested-by: John Allen <[e-mail]>
Signed-off-by: Yu-cheng Yu <[e-mail]>
Reviewed-by: Kirill A. Shutemov <[e-mail]>
Signed-off-by: Rick Edgecombe <[e-mail]>
---

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(+)

Comments

Kees Cook Dec. 3, 2022, 2:38 a.m. UTC | #1

On Fri, Dec 02, 2022 at 04:35:48PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <[e-mail]>
> 
> 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 <[e-mail]>
> Tested-by: John Allen <[e-mail]>
> Signed-off-by: Yu-cheng Yu <[e-mail]>

Reviewed-by: Kees Cook <[e-mail]>

Patch

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.