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

[Matthew Brost]

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 += 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;
 		}
-- 
2.34.1