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

Commit Message

Alistair Popple Sept. 2, 2022, 12:35 a.m. UTC

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 <[e-mail]>
Reported-by: Nadav Amit <[e-mail]>
Reviewed-by: "Huang, Ying" <[e-mail]>
Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
Cc: [e-mail]

---

Changes for v4:

 - Added Review-by

Changes for v3:

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


base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136

Comments

David Hildenbrand Sept. 2, 2022, 6:51 a.m. UTC | #1

On 02.09.22 02:35, 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 <[e-mail]>
> Reported-by: Nadav Amit <[e-mail]>
> Reviewed-by: "Huang, Ying" <[e-mail]>
> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> Cc: [e-mail]
> 
> ---
> 
> 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


Acked-by: David Hildenbrand <[e-mail]>

Peter Xu Sept. 2, 2022, 8:35 p.m. UTC | #2

On Fri, Sep 02, 2022 at 10:35:51AM +1000, 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 <[e-mail]>
> Reported-by: Nadav Amit <[e-mail]>
> Reviewed-by: "Huang, Ying" <[e-mail]>
> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> Cc: [e-mail]

Acked-by: Peter Xu <[e-mail]>

Patch

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;
 }