Re: [PATCH v3] slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts

[Vlastimil Babka <vbabka@suse.cz>]

On 10/21/25 03:03, Hao Ge wrote:
> From: Hao Ge <gehao@kylinos.cn>
> 
> 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>

Added to slab/for-next-fixes, thanks!

> ---
> v3: According to Suren's suggestion, simplify the commit message and the code comments.
>     Thanks for Suren.
> 
> v2: Incorporate handling for the scenario where, if mark_failed_objexts_alloc wins the race,
>     the other process (that previously succeeded in allocation) will lose the race, based on Suren's suggestion.
>     Add Suggested-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  mm/slub.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2e4340c75be2..d4403341c9df 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)