On 24.08.22 05:03, Alistair Popple wrote:
> 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@xxxxxxxxxx>
> Reported-by: Nadav Amit <nadav.amit@xxxxxxxxx>
> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> Cc: stable@xxxxxxxxxxxxxxx
>
> ---
>
> 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
I'm not a TLB-flushing expert, but this matches my understanding (and a
TLB flushing Linux documentation I stumbled over some while ago but
cannot quickly find).
In the ordinary try_to_migrate_one() path, flushing would happen via
ptep_clear_flush() (just like we do for the anon_exclusive case here as
well), correct?
--
Thanks,
David / dhildenb
