Re: [PATCH v2] mm/slab: sample slab_state once in kmem_cache_destroy()

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 10/14/26 17:08, Imre Kaloz wrote:
> kmem_cache_release() tests slab_state >= FULL after kmem_cache_destroy()
> has dropped slab_mutex and unlinked the cache, and for a
> SLAB_TYPESAFE_BY_RCU cache after rcu_barrier(). If slab_late_init() runs
> before that test it sets slab_state to FULL but skips the unlinked cache,
> which never got a sysfs entry, so sysfs_slab_release() calls kobject_put()
> on a kobject that was never initialized:
> 
>   WARNING: lib/kobject.c:734 at kobject_put+0x64/0x2c0, CPU#1: kworker/u8:3/55
>   kobject: '(null)' ((____ptrval____)): is not initialized, yet kobject_put() is being called.
>   refcount_t: underflow; use-after-free.
>   kobject_put+0x64/0x2c0
>   sysfs_slab_release+0xc/0x20
>   kmem_cache_destroy+0x104/0x1e0
>   bioset_exit+0x13c/0x1e0
>   disk_release+0x54/0x140
>   put_disk+0x18/0x40
>   floppy_async_init+0xbec/0xd10
> 
> The release-time test has run outside slab_mutex since at least v4.4 for
> SLAB_TYPESAFE_BY_RCU caches, and since v6.0 for the others. Since v6.12
> the unlink-time test in kmem_cache_destroy() is a separate read of its
> own, also after mutex_unlock(), so the two can disagree. Both tests ask

It would be useful to mention the commits here along with the versions. Also
this suggests 3 commits but there's only 2 in Fixes. I would guess the 6.0
change is not a crucial one?

> whether the cache has a kobject, which cache creation skips while
> slab_state <= UP. UP is the state just below FULL, so a false sample
> cannot skip a kobject_put() that was owed.
> 
> Seen on sparc64 at boot, where the asynchronous floppy init tears down
> its bio slab while the late initcalls are running.
> 
> Sample slab_state once, under slab_mutex which slab_late_init() holds
> when it sets FULL, and use the result for both the unlink and the
> release.
> 
> Fixes: d50d82faa0c9 ("slub: fix failure when we delete and create a slab cache")
> Fixes: 4ec10268ed98 ("mm, slab: unlink slabinfo, sysfs and debugfs immediately")
> Cc: stable@vger.kernel.org
> Signed-off-by: Imre Kaloz <kaloz@kernel.org>
> Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Thanks!

> ---
> v2:
>  - Changelog reworked around the release-time read outside slab_mutex,
>    with the separate unlink-time test as the second read; Fixes:
>    d50d82faa0c9 added, as discussed with Harry.
>  - Correction to what I wrote in the v1 thread: the release-time read is
>    older than d50d82faa0c9 (the RCU work function had it from v4.11
>    until v6.12); d50d82faa0c9 added the unlink-time test under
>    slab_mutex.
>  - Added the explanation Jose asked for; his Reviewed-by from v1 is
>    kept. The code is unchanged.
> 
> v1: https://lore.kernel.org/r/20260930195110.13296-1-kaloz@kernel.org
> 
>  mm/slab_common.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 7223a7596dab..de11edfd1a0b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -515,10 +515,10 @@ EXPORT_SYMBOL(kmem_buckets_create);
>   * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
>   * protection. So they are now done without holding those locks.
>   */
> -static void kmem_cache_release(struct kmem_cache *s)
> +static void kmem_cache_release(struct kmem_cache *s, bool sysfs_ready)
>  {
>  	kfence_shutdown_cache(s);
> -	if (__is_defined(SLAB_SUPPORTS_SYSFS) && slab_state >= FULL)
> +	if (__is_defined(SLAB_SUPPORTS_SYSFS) && sysfs_ready)
>  		sysfs_slab_release(s);
>  	else
>  		slab_kmem_cache_release(s);
> @@ -533,6 +533,7 @@ void slab_kmem_cache_release(struct kmem_cache *s)
>  
>  void kmem_cache_destroy(struct kmem_cache *s)
>  {
> +	bool sysfs_ready;
>  	int err;
>  
>  	if (unlikely(!s) || !kasan_check_byte(s))
> @@ -580,10 +581,18 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  
>  	list_del(&s->list);
>  
> +	/*
> +	 * slab_late_init() sets slab_state to FULL under slab_mutex and adds
> +	 * sysfs entries only for caches still on the list. Sample the state
> +	 * here, so that a cache unlinked before that point is not handed to
> +	 * sysfs_slab_release() with an uninitialized kobject.
> +	 */
> +	sysfs_ready = slab_state >= FULL;
> +
>  	mutex_unlock(&slab_mutex);
>  	cpus_read_unlock();
>  
> -	if (slab_state >= FULL)
> +	if (sysfs_ready)
>  		sysfs_slab_unlink(s);
>  	debugfs_slab_release(s);
>  
> @@ -593,7 +602,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	if (s->flags & SLAB_TYPESAFE_BY_RCU)
>  		rcu_barrier();
>  
> -	kmem_cache_release(s);
> +	kmem_cache_release(s, sysfs_ready);
>  }
>  EXPORT_SYMBOL(kmem_cache_destroy);
>  
> 
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907