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

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

[Imre Kaloz]

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
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>
---
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
-- 
2.47.3

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

[Harry Yoo]

On Wed, Oct 14, 2026 at 05:08:43PM +0200, 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
> 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>
> ---

Looks good to me,
Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>

I guess it's late to queue this to slab/for-next-fixes.
Let's queue this to slab/for-next for 7.4-rc1.

-- 
Cheers,
Harry / Hyeonggon

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

[Harry Yoo (Meta)]

On Wed, 14 Oct 2026 17:08:43 +0200, 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:
> 
> [...]

Applied to mm/slab.git slab/for-next, thanks!

[1/1] mm/slab: sample slab_state once in kmem_cache_destroy()
      commit: 6d9bc3d860e64e2c5dbfd7531954df9ea8c388aa

-- 
Cheers,
Harry / Hyeonggon

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

[Harry Yoo]

On Wed, Oct 14, 2026 at 08:46:56PM +0100, Harry Yoo wrote:
> On Wed, Oct 14, 2026 at 05:08:43PM +0200, Imre Kaloz wrote:
> 
> Looks good to me,
> Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>
> 
> I guess it's late to queue this to slab/for-next-fixes.
> Let's queue this to slab/for-next for 7.4-rc1.

I can already see this won't be a straightforward backport as
the code has changed a lot over the history.

Let's try best to minimize divergence from the mainline commit,
and let's backport to older kernels only when it can be reproduced
fairly easily.

-- 
Cheers,
Harry / Hyeonggon