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>

1 files changed, 8 insertions, 4 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index 6ccecc0..30d7d24 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);