mremap: move_ptes: check pte dirty after its removal

Linus found there still is a race in mremap after commit 5d1904204c99
("mremap: fix race between mremap() and page cleanning").

As described by Linus:
 "the issue is that another thread might make the pte be dirty (in the
  hardware walker, so no locking of ours will make any difference)
  *after* we checked whether it was dirty, but *before* we removed it
  from the page tables"

Fix it by moving the check after we removed it from the page table.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 10 insertions, 6 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index eff3de359d50..d4a6e4001512 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1456,9 +1456,9 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
                 new_ptl = pmd_lockptr(mm, new_pmd);
                 if (new_ptl != old_ptl)
                         spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
-                if (pmd_present(*old_pmd) && pmd_dirty(*old_pmd))
-                        force_flush = true;
                 pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
+                if (pmd_present(pmd) && pmd_dirty(pmd))
+                        force_flush = true;
                 VM_BUG_ON(!pmd_none(*new_pmd));
 
                 if (pmd_move_must_withdraw(new_ptl, old_ptl) &&
diff --git a/mm/mremap.c b/mm/mremap.c
index 6ccecc03f56a..30d7d2482eea 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -149,14 +149,18 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
                 if (pte_none(*old_pte))
                         continue;
 
+                pte = ptep_get_and_clear(mm, old_addr, old_pte);
                 /*
-                 * We are remapping a dirty PTE, make sure to
-                 * flush TLB before we drop the PTL for the
+                 * If we are remapping a dirty PTE, make sure
+                 * to flush TLB before we drop the PTL for the
                  * old PTE or we may race with page_mkclean().
+                 *
+                 * This check has to be done after we removed the
+                 * old PTE from page tables or another thread may
+                 * dirty it after the check and before the removal.
                  */
-                if (pte_present(*old_pte) && pte_dirty(*old_pte))
+                if (pte_present(pte) && pte_dirty(pte))
                         force_flush = true;
-                pte = ptep_get_and_clear(mm, old_addr, old_pte);
                 pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
                 pte = move_soft_dirty_pte(pte);
                 set_pte_at(mm, new_addr, new_pte, pte);