Re: [PATCH v3 2/6] mm/migrate_device: Do not write past the end of the src_pfns array

[Balbir Singh]

On 8/6/26 9:10 AM, Matthew Brost wrote:
> migrate_device_range() and migrate_device_pfns() zero the tail entries
> of a large folio without checking them against @npages:
> 
> 	for (j = 1; j < nr; j++)
> 		src_pfns[i+j] = 0;
> 
> @nr comes from the folio, not from the array, so a folio that extends
> past the end of the range being migrated writes beyond src_pfns[].
> Callers size that array for @npages entries, so this corrupts whatever
> follows it.
> 
> Bound the loop by @npages. The subsequent "i += j - 1" still terminates
> the outer loop correctly: on a bounded exit j is @npages - i, leaving i
> at @npages after the increment.
> 
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
> Cc: Nico Pache <nico.pache@xxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Dev Jain <dev.jain@xxxxxxx>
> Cc: Barry Song <baohua@xxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Usama Arif <usama.arif@xxxxxxxxx>
> Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> Cc: Rakie Kim <rakie.kim@xxxxxx>
> Cc: Byungchul Park <byungchul@xxxxxx>
> Cc: Gregory Price <gourry@xxxxxxxxxx>
> Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Balbir Singh <balbirs@xxxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Simona Vetter <simona@xxxxxxxx>
> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: GitHub_Copilot:claude-opus-5
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> ---
>  mm/migrate_device.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 162d29b2807a..ae9027421b80 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -1415,7 +1415,7 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
>  		nr = folio_nr_pages(folio);
>  		if (nr > 1) {
>  			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
> -			for (j = 1; j < nr; j++)
> +			for (j = 1; j < nr && (i + j) < npages; j++)
>  				src_pfns[i+j] = 0;

I have a similar patch lined up in my clean ups (that I am yet to send out), but
the patch was more along the lines of

  		nr = folio_nr_pages(folio);
+		if (nr > npages - i) {
+			migrate_device_folio_unlock(folio);
+			src_pfns[i] = 0;
+			continue;
+		}
+

This prevents partial selection, migrate_device_unmap() does take npages as an
argument. We could change the increment of i here to skip past the entire folio.

I am OK with this change as well

>  			i += j - 1;
>  			pfn += j - 1;
> @@ -1449,7 +1449,7 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages)
>  		nr = folio_nr_pages(folio);
>  		if (nr > 1) {
>  			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
> -			for (j = 1; j < nr; j++)
> +			for (j = 1; j < nr && (i + j) < npages; j++)
>  				src_pfns[i+j] = 0;
>  			i += j - 1;
>  		}

Reviewed-by: Balbir Singh <balbirs@xxxxxxxxxx>