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slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts

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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: f7381b9116 ("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>
This commit is contained in:
Hao Ge 2025-10-21 09:03:53 +08:00 committed by Vlastimil Babka
parent 86f54f9b6c
commit 6ed8bfd24c
1 changed files with 6 additions and 3 deletions
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9
mm/slub.c
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@ -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)