slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts
If two competing threads enter alloc_slab_obj_exts() and one of them
fails to allocate the object extension vector, it might override the
valid slab->obj_exts allocated by the other thread with
OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and
expects a valid pointer to dereference a NULL pointer later on.
Update slab->obj_exts atomically using cmpxchg() to avoid
slab->obj_exts overrides by racing threads.
Thanks for Vlastimil and Suren's help with debugging.
Fixes: f7381b911640 ("slab: mark slab->obj_exts allocation failures unconditionally")
Cc: <stable@vger.kernel.org>
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Link: https://patch.msgid.link/20251021010353.1187193-1-hao.ge@linux.dev
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
diff --git a/mm/slub.c b/mm/slub.c
index a8fcc7e..23d8f54 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2054,7 +2054,7 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
static inline void mark_failed_objexts_alloc(struct slab *slab)
{
- slab->obj_exts = OBJEXTS_ALLOC_FAIL;
+ cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
}
static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
@@ -2136,6 +2136,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
#ifdef CONFIG_MEMCG
new_exts |= MEMCG_DATA_OBJEXTS;
#endif
+retry:
old_exts = READ_ONCE(slab->obj_exts);
handle_failed_objexts_alloc(old_exts, vec, objects);
if (new_slab) {
@@ -2145,8 +2146,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
* be simply assigned.
*/
slab->obj_exts = new_exts;
- } else if ((old_exts & ~OBJEXTS_FLAGS_MASK) ||
- cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
+ } else if (old_exts & ~OBJEXTS_FLAGS_MASK) {
/*
* If the slab is already in use, somebody can allocate and
* assign slabobj_exts in parallel. In this case the existing
@@ -2158,6 +2158,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
else
kfree(vec);
return 0;
+ } else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
+ /* Retry if a racing thread changed slab->obj_exts from under us. */
+ goto retry;
}
if (allow_spin)
