mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex...

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@redhat.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: David Rientjes <rientjes@google.com>
Link: https://lore.kernel.org/all/YwOImVd+nRUsSAga@hyeyoo/
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1799664..07b9482 100644 (file)
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -392,6 +392,28 @@ 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);
@@ -418,11 +440,7 @@ 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);
        }
 }
 
@@ -437,20 +455,11 @@ 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;
@@ -465,14 +474,16 @@ 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),
@@ -481,6 +492,8 @@ 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);