FAILED: patch "[PATCH] mm/slab: sample slab_state once in kmem_cache_destroy()" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 088bfdc0d7947d623f4899b9011c866e4445074d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026110452-amendment-easing-e4e5@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 088bfdc0d7947d623f4899b9011c866e4445074d Mon Sep 17 00:00:00 2001
From: Imre Kaloz <kaloz@kernel.org>
Date: Wed, 14 Oct 2026 17:08:43 +0200
Subject: [PATCH] mm/slab: sample slab_state once in kmem_cache_destroy()

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

slab_state >= FULL asks 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.

The history of this problem dates back to v3.2:

  Commit 69cb8e6b7c29 ("slub: free slabs without holding locks") in
  v3.2 moved sysfs_slab_remove() outside slub_lock and introduced
  a race condition between kmem_cache_destroy() and slab_sysfs_init().

  Commit 945cf2b6199be ("mm/sl[aou]b: Extract a common function for
  kmem_cache_destroy") in v3.7 accidentally closed the race while
  extracting a common implementation for kmem_cache_destroy().

  Commit 5413dfba88d6 ("slub: drop mutex before deleting sysfs entry")
  in v3.8 re-opened the race by dropping slab_mutex around
  sysfs_slab_remove() in __kmem_cache_shutdown(), to avoid a lockdep
  splat between slab_mutex and the sysfs lock.

  Commit 657dc2f97220 ("slab: remove synchronous rcu_barrier() call in
  memcg cache release path") in v4.11 shifted from synchronous
  rcu_barrier() to asynchronous RCU work for SLAB_TYPESAFE_BY_RCU caches.
  That commit, again, accidentally closed the race by moving
  sysfs_slab_release() to shutdown_cache(), but only for
  !SLAB_TYPESAFE_BY_RCU caches because destruction of
  SLAB_TYPESAFE_BY_RCU caches is done in an RCU work.

  Commit 0495e337b703 ("mm/slab_common: Deleting kobject in
  kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock") in
  v6.0 re-introduced the race for !SLAB_TYPESAFE_BY_RCU caches.

Fixes: 69cb8e6b7c29 ("slub: free slabs without holding locks")
Fixes: 5413dfba88d6 ("slub: drop mutex before deleting sysfs entry")
Fixes: 0495e337b703 ("mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock")
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: Harry Yoo (Meta) <harry@kernel.org>
Link: https://patch.msgid.link/20261014150844.8536-1-kaloz@kernel.org
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 980ca4cb6c43..e66c0778ee98 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);