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

[Huang, Ying]

Alistair Popple <apopple@xxxxxxxxxx> writes:

> 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);

It appears that we can increase "unmapped" only if ptep_get_and_clear()
is used?

Best Regards,
Huang, Ying

> +	arch_leave_lazy_mmu_mode();
> +	pte_unmap_unlock(ptep - 1, ptl);
> +
>  	return 0;
>  }
>  
>
> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136