6.17-stable review patch. If anyone has any objections, please let me know.
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From: Hao Ge gehao@kylinos.cn
commit 6ed8bfd24ce1cb31742b09a3eb557cd008533eec upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/slub.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
--- a/mm/slub.c +++ b/mm/slub.c @@ -1978,7 +1978,7 @@ static inline void mark_objexts_empty(st
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, @@ -2043,6 +2043,7 @@ int alloc_slab_obj_exts(struct slab *sla #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) { @@ -2052,8 +2053,7 @@ int alloc_slab_obj_exts(struct slab *sla * 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 @@ -2062,6 +2062,9 @@ int alloc_slab_obj_exts(struct slab *sla mark_objexts_empty(vec); 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; }
kmemleak_not_leak(vec);