Even though the KMSAN warnings generated by memchr_inv() are suppressed
by metadata_access_enable(), its return value may still be poisoned.

The reason is that the last iteration of memchr_inv() returns
`*start != value ? start : NULL`, where *start is poisoned. Because of
this, somewhat counterintuitively, the shadow value computed by
visitSelectInst() is equal to `(uintptr_t)start`.

The intention behind guarding memchr_inv() behind
metadata_access_enable() is to touch poisoned metadata without
triggering KMSAN, so unpoison its return value.

Acked-by: Vlastimil Babka <redacted>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 mm/slub.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index a290f6c63e7b..b9101b2dc9aa 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -1185,6 +1185,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
 	metadata_access_enable();
 	fault = memchr_inv(kasan_reset_tag(start), value, bytes);
 	metadata_access_disable();
+	kmsan_unpoison_memory(&fault, sizeof(fault));
 	if (!fault)
 		return 1;
 

@@ -1291,6 +1292,7 @@ static void slab_pad_check(struct kmem_cache *s, struct slab *slab)
 	metadata_access_enable();
 	fault = memchr_inv(kasan_reset_tag(pad), POISON_INUSE, remainder);
 	metadata_access_disable();
+	kmsan_unpoison_memory(&fault, sizeof(fault));
 	if (!fault)
 		return;
 	while (end > fault && end[-1] == POISON_INUSE)

-- 
2.45.1