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

Rick Edgecombe
2022-12-03T00:40:53Z

From: Yu-cheng Yu <yu-cheng.yu@intel.com>

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@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---

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