linux-kernel - [PATCH 5.19 053/155] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug

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Date:   Tue,  6 Sep 2022 15:30:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Waiman Long <longman@...hat.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.19 053/155] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

From: Waiman Long <longman@...hat.com>

[ Upstream commit 0495e337b7039191dfce6e03f5f830454b1fae6b ]

A circular locking problem is reported by lockdep due to the following
circular locking dependency.

  +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
  |                                                     |
  +-----------------------------------------------------+

The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
happens in

  kmem_cache_destroy():	cpus_read_lock(); mutex_lock(&slab_mutex);
  ==> sysfs_slab_unlink()
      ==> kobject_del()
          ==> kernfs_remove()
	      ==> __kernfs_remove()
	          ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);

The backward kn->active ==> cpu_hotplug_lock dependency happens in

  kernfs_fop_write_iter(): kernfs_get_active();
  ==> slab_attr_store()
      ==> cpu_partial_store()
          ==> flush_all(): cpus_read_lock()

One way to break this circular locking chain is to avoid holding
cpu_hotplug_lock and slab_mutex while deleting the kobject in
sysfs_slab_unlink() which should be equivalent to doing a write_lock
and write_unlock pair of the kn->active virtual lock.

Since the kobject structures are not protected by slab_mutex or the
cpu_hotplug_lock, we can certainly release those locks before doing
the delete operation.

Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
created kmem_cache_release() and call it outside the slab_mutex &
cpu_hotplug_lock critical sections. There will be a slight delay
in the deletion of sysfs files if kmem_cache_release() is called
indirectly from a work function.

Fixes: 5a836bf6b09f ("mm: slub: move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context")
Signed-off-by: Waiman Long <longman@...hat.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>
Acked-by: David Rientjes <rientjes@...gle.com>
Link: https://lore.kernel.org/all/YwOImVd+nRUsSAga@hyeyoo/
Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
 1 file changed, 29 insertions(+), 16 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 77c3adf40e504..dbd4b6f9b0e79 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -420,6 +420,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
 }
 EXPORT_SYMBOL(kmem_cache_create);
 
+#ifdef SLAB_SUPPORTS_SYSFS
+/*
+ * For a given kmem_cache, kmem_cache_destroy() should only be called
+ * once or there will be a use-after-free problem. The actual deletion
+ * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
+ * protection. So they are now done without holding those locks.
+ *
+ * Note that there will be a slight delay in the deletion of sysfs files
+ * if kmem_cache_release() is called indrectly from a work function.
+ */
+static void kmem_cache_release(struct kmem_cache *s)
+{
+	sysfs_slab_unlink(s);
+	sysfs_slab_release(s);
+}
+#else
+static void kmem_cache_release(struct kmem_cache *s)
+{
+	slab_kmem_cache_release(s);
+}
+#endif
+
 static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
 {
 	LIST_HEAD(to_destroy);
@@ -446,11 +468,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
 	list_for_each_entry_safe(s, s2, &to_destroy, list) {
 		debugfs_slab_release(s);
 		kfence_shutdown_cache(s);
-#ifdef SLAB_SUPPORTS_SYSFS
-		sysfs_slab_release(s);
-#else
-		slab_kmem_cache_release(s);
-#endif
+		kmem_cache_release(s);
 	}
 }
 
@@ -465,20 +483,11 @@ static int shutdown_cache(struct kmem_cache *s)
 	list_del(&s->list);
 
 	if (s->flags & SLAB_TYPESAFE_BY_RCU) {
-#ifdef SLAB_SUPPORTS_SYSFS
-		sysfs_slab_unlink(s);
-#endif
 		list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
 		schedule_work(&slab_caches_to_rcu_destroy_work);
 	} else {
 		kfence_shutdown_cache(s);
 		debugfs_slab_release(s);
-#ifdef SLAB_SUPPORTS_SYSFS
-		sysfs_slab_unlink(s);
-		sysfs_slab_release(s);
-#else
-		slab_kmem_cache_release(s);
-#endif
 	}
 
 	return 0;
@@ -493,14 +502,16 @@ void slab_kmem_cache_release(struct kmem_cache *s)
 
 void kmem_cache_destroy(struct kmem_cache *s)
 {
+	int refcnt;
+
 	if (unlikely(!s) || !kasan_check_byte(s))
 		return;
 
 	cpus_read_lock();
 	mutex_lock(&slab_mutex);
 
-	s->refcount--;
-	if (s->refcount)
+	refcnt = --s->refcount;
+	if (refcnt)
 		goto out_unlock;
 
 	WARN(shutdown_cache(s),
@@ -509,6 +520,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
 out_unlock:
 	mutex_unlock(&slab_mutex);
 	cpus_read_unlock();
+	if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
+		kmem_cache_release(s);
 }
 EXPORT_SYMBOL(kmem_cache_destroy);
 
-- 
2.35.1