[RFC PATCH v4 16/27] mm: Update can_follow_write_pte/pmd for shadow stack

Yu-cheng Yu
2018-09-21T15:10:34Z

can_follow_write_pte/pmd look for the (RO & DIRTY) PTE/PMD to
verify an exclusive RO page still exists after a broken COW.

A shadow stack PTE is RO & PAGE_DIRTY_SW when it is shared,
otherwise RO & PAGE_DIRTY_HW.

Introduce pte_exclusive() and pmd_exclusive() to also verify a
shadow stack PTE is exclusive.

Also rename can_follow_write_pte/pmd() to can_follow_write() to
make their meaning clear; i.e. "Can we write to the page?", not
"Is the PTE writable?"

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/mm/pgtable.c | 19 +++++++++++++++++++
include/asm-generic/pgtable.h | 4 ++++
mm/gup.c | 8 +++++---
mm/huge_memory.c | 8 +++++---
4 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index ccdfd3dd7163..e13a020e37db 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -894,4 +894,23 @@ inline bool arch_copy_pte_mapping(vm_flags_t vm_flags)
{
return (vm_flags & VM_SHSTK);
}
+
+inline bool pte_exclusive(pte_t pte, struct vm_area_struct *vma)
+{
+ if (vma->vm_flags & VM_SHSTK)
+ return pte_dirty_hw(pte);
+ else
+ return pte_dirty(pte);
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+inline bool pmd_exclusive(pmd_t pmd, struct vm_area_struct *vma)
+{
+ if (vma->vm_flags & VM_SHSTK)
+ return pmd_dirty_hw(pmd);
+ else
+ return pmd_dirty(pmd);
+}
+#endif
+
#endif /* CONFIG_X86_INTEL_SHADOW_STACK_USER */
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index a91f07454ced..6223017929be 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -1131,10 +1131,14 @@ static inline bool arch_has_pfn_modify_check(void)
#define pte_set_vma_features(pte, vma) pte
#define pmd_set_vma_features(pmd, vma) pmd
#define arch_copy_pte_mapping(vma_flags) false
+#define pte_exclusive(pte, vma) pte_dirty(pte)
+#define pmd_exclusive(pmd, vma) pmd_dirty(pmd)
#else
inline pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma);
inline pmd_t pmd_set_vma_features(pmd_t pmd, struct vm_area_struct *vma);
bool arch_copy_pte_mapping(vm_flags_t vm_flags);
+bool pte_exclusive(pte_t pte, struct vm_area_struct *vma);
+bool pmd_exclusive(pmd_t pmd, struct vm_area_struct *vma);
#endif

#endif /* _ASM_GENERIC_PGTABLE_H */
diff --git a/mm/gup.c b/mm/gup.c
index 1abc8b4afff6..03cb2e331f80 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -64,10 +64,12 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
* FOLL_FORCE can write to even unwritable pte's, but only
* after we've gone through a COW cycle and they are dirty.
*/
-static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+static inline bool can_follow_write(pte_t pte, unsigned int flags,
+ struct vm_area_struct *vma)
{
return pte_write(pte) ||
- ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+ ((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
+ pte_exclusive(pte, vma));
}

static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -105,7 +107,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
}
if ((flags & FOLL_NUMA) && pte_protnone(pte))
goto no_page;
- if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
+ if ((flags & FOLL_WRITE) && !can_follow_write(pte, flags, vma)) {
pte_unmap_unlock(ptep, ptl);
return NULL;
}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index df39ae20fe40..c70aa8fa4cb2 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1387,10 +1387,12 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
* FOLL_FORCE can write to even unwritable pmd's, but only
* after we've gone through a COW cycle and they are dirty.
*/
-static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+static inline bool can_follow_write(pmd_t pmd, unsigned int flags,
+ struct vm_area_struct *vma)
{
return pmd_write(pmd) ||
- ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+ ((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
+ pmd_exclusive(pmd, vma));
}

struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
@@ -1403,7 +1405,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,

assert_spin_locked(pmd_lockptr(mm, pmd));

- if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
+ if (flags & FOLL_WRITE && !can_follow_write(*pmd, flags, vma))
goto out;

/* Avoid dumping huge zero page */
--
2.17.1