[PATCH v4 1/4] mm/migrate_device.c: Flush TLB while holding PTL

Alistair Popple
2022-09-02T00:36:34Z

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@nvidia.com>
Reported-by: Nadav Amit <nadav.amit@gmail.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
Cc: stable@vger.kernel.org

---

Changes for v4:

- Added Review-by

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