From c7af5300d78460fc5037ddc77113ba3dbfe77dc0 Mon Sep 17 00:00:00 2001 From: Hao Ge Date: Tue, 21 Oct 2025 09:03:53 +0800 Subject: [PATCH] slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts 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: Suggested-by: Suren Baghdasaryan Signed-off-by: Hao Ge Reviewed-by: Harry Yoo Reviewed-by: Suren Baghdasaryan Link: https://patch.msgid.link/20251021010353.1187193-1-hao.ge@linux.dev Signed-off-by: Vlastimil Babka Signed-off-by: Greg Kroah-Hartman --- mm/slub.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 24e65d7048ba2..c4e05a6c7c861 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -1943,7 +1943,7 @@ 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, @@ -2008,6 +2008,7 @@ 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) { @@ -2017,8 +2018,7 @@ 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 @@ -2027,6 +2027,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, 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); -- 2.47.3