[PATCH v7 18/38] mm: slub: Disable KMSAN when checking the padding bytes

[Ilya Leoshkevich]

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`.

One possibility to fix this, since the intention behind guarding
memchr_inv() behind metadata_access_enable() is to touch poisoned
metadata without triggering KMSAN, is to unpoison its return value.
However, this approach is too fragile. So simply disable the KMSAN
checks in the respective functions.

Reviewed-by: Alexander Potapenko <glider@xxxxxxxxxx>
Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
---
 mm/slub.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index b050e528112c..fcd68fcea4ab 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1176,9 +1176,16 @@ static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
 	memset(from, data, to - from);
 }
 
-static int check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
-			u8 *object, char *what,
-			u8 *start, unsigned int value, unsigned int bytes)
+#ifdef CONFIG_KMSAN
+#define pad_check_attributes noinline __no_kmsan_checks
+#else
+#define pad_check_attributes
+#endif
+
+static pad_check_attributes int
+check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
+		       u8 *object, char *what,
+		       u8 *start, unsigned int value, unsigned int bytes)
 {
 	u8 *fault;
 	u8 *end;
@@ -1270,7 +1277,8 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
 }
 
 /* Check the pad bytes at the end of a slab page */
-static void slab_pad_check(struct kmem_cache *s, struct slab *slab)
+static pad_check_attributes void
+slab_pad_check(struct kmem_cache *s, struct slab *slab)
 {
 	u8 *start;
 	u8 *fault;
-- 
2.45.1