[PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

[Alistair Popple]

When clearing a PTE the TLB should be flushed whilst still holding the
PTL to avoid a potential race with madvise/munmap/etc. For example
consider the following sequence:

  CPU0                          CPU1
  ----                          ----

  migrate_vma_collect_pmd()
  pte_unmap_unlock()
                                madvise(MADV_DONTNEED)
                                -> zap_pte_range()
                                pte_offset_map_lock()
                                [ PTE not present, TLB not flushed ]
                                pte_unmap_unlock()
                                [ page is still accessible via stale TLB ]
  flush_tlb_range()

In this case the page may still be accessed via the stale TLB entry
after madvise returns. Fix this by flushing the TLB while holding the
PTL.

Signed-off-by: Alistair Popple <apopple at nvidia.com>
Reported-by: Nadav Amit <nadav.amit at gmail.com>
Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
Cc: stable at vger.kernel.org

---

Changes for v3:

 - New for v3
---
 mm/migrate_device.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 27fb37d..6a5ef9f 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -254,13 +254,14 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		migrate->dst[migrate->npages] = 0;
 		migrate->src[migrate->npages++] = mpfn;
 	}
-	arch_leave_lazy_mmu_mode();
-	pte_unmap_unlock(ptep - 1, ptl);
 
 	/* Only flush the TLB if we actually modified any entries */
 	if (unmapped)
 		flush_tlb_range(walk->vma, start, end);
 
+	arch_leave_lazy_mmu_mode();
+	pte_unmap_unlock(ptep - 1, ptl);
+
 	return 0;
 }
 

base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136
-- 
git-series 0.9.1