[PATCH v25 19/30] mm: Update can_follow_write_pte() for shadow stack

[Yu-cheng Yu]

Can_follow_write_pte() ensures a read-only page is COWed by checking the
FOLL_COW flag, and uses pte_dirty() to validate the flag is still valid.

Like a writable data page, a shadow stack page is writable, and becomes
read-only during copy-on-write, but it is always dirty.  Thus, in the
can_follow_write_pte() check, it belongs to the writable page case and
should be excluded from the read-only page pte_dirty() check.  Apply
the same changes to can_follow_write_pmd().

While at it, also split the long line into smaller ones.

Signed-off-by: Yu-cheng Yu <yu-cheng...@intel.com>
Cc: Kees Cook <keesc...@chromium.org>
Cc: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
---
v25:
- Split long line into smaller ones.
v24:
- Change arch_shadow_stack_mapping() to is_shadow_stack_mapping().

 mm/gup.c         | 16 ++++++++++++----
 mm/huge_memory.c | 16 ++++++++++++----
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ef7d2da9f03f..f2813cf4d07b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -356,10 +356,18 @@ 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(pte_t pte, unsigned int flags,
+                                       vm_flags_t vm_flags)
 {
-       return pte_write(pte) ||
-               ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+       if (pte_write(pte))
+               return true;
+       if ((flags & (FOLL_FORCE | FOLL_COW)) != (FOLL_FORCE | FOLL_COW))
+               return false;
+       if (!pte_dirty(pte))
+               return false;
+       if (is_shadow_stack_mapping(vm_flags))
+               return false;
+       return true;
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -402,7 +410,7 @@ 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(pte, flags, 
vma->vm_flags)) {
                pte_unmap_unlock(ptep, ptl);
                return NULL;
        }
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 044029ef45cd..a0858eac0320 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1338,10 +1338,18 @@ 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(pmd_t pmd, unsigned int flags,
+                                       vm_flags_t vm_flags)
 {
-       return pmd_write(pmd) ||
-              ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+       if (pmd_write(pmd))
+               return true;
+       if ((flags & (FOLL_FORCE | FOLL_COW)) != (FOLL_FORCE | FOLL_COW))
+               return false;
+       if (!pmd_dirty(pmd))
+               return false;
+       if (is_shadow_stack_mapping(vm_flags))
+               return false;
+       return true;
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
@@ -1354,7 +1362,7 @@ 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(*pmd, flags, 
vma->vm_flags))
                goto out;
 
        /* Avoid dumping huge zero page */
-- 
2.21.0