[PATCH 5.19 053/155] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

Greg Kroah-Hartman
2022-09-06T14:05:10Z

From: Waiman Long <longman@redhat.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@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>
Signed-off-by: Sasha Levin <sashal@kernel.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