[PATCH] mm/slab_common: fix possiable double free of kmem_cache

[Feng Tang]

When doing slub_debug test, kfence's 'test_memcache_typesafe_by_rcu'
kunit test case cause a use-after-free error:

  BUG: KASAN: use-after-free in kobject_del+0x14/0x30
  Read of size 8 at addr ffff888007679090 by task kunit_try_catch/261

  CPU: 1 PID: 261 Comm: kunit_try_catch Tainted: G    B            N 6.0.0-rc5-next-20220916 #17
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x34/0x48
   print_address_description.constprop.0+0x87/0x2a5
   print_report+0x103/0x1ed
   kasan_report+0xb7/0x140
   kobject_del+0x14/0x30
   kmem_cache_destroy+0x130/0x170
   test_exit+0x1a/0x30
   kunit_try_run_case+0xad/0xc0
   kunit_generic_run_threadfn_adapter+0x26/0x50
   kthread+0x17b/0x1b0
   </TASK>

The cause is inside kmem_cache_destroy():

kmem_cache_destroy
    acquire lock/mutex
    shutdown_cache
        schedule_work(kmem_cache_release) (if RCU flag set)
    release lock/mutex
    kmem_cache_release (if RCU flag set)

in some certain timing, the scheduled work could be run before
the next RCU flag checking which will get a wrong state.

Fix it by caching the RCU flag inside protected area, just like 'refcnt'

Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---

note:

The error only happens on linux-next tree, and not in Linus' tree,
which already has Waiman's commit:
0495e337b703 ("mm/slab_common: Deleting kobject in kmem_cache_destroy()
without holding slab_mutex/cpu_hotplug_lock")

 mm/slab_common.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 07b948288f84..ccc02573588f 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -475,6 +475,7 @@ void slab_kmem_cache_release(struct kmem_cache *s)
 void kmem_cache_destroy(struct kmem_cache *s)
 {
 	int refcnt;
+	bool rcu_set;
 
 	if (unlikely(!s) || !kasan_check_byte(s))
 		return;
@@ -482,6 +483,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	cpus_read_lock();
 	mutex_lock(&slab_mutex);
 
+	rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;
+
 	refcnt = --s->refcount;
 	if (refcnt)
 		goto out_unlock;
@@ -492,7 +495,7 @@ 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))
+	if (!refcnt && !rcu_set)
 		kmem_cache_release(s);
 }
 EXPORT_SYMBOL(kmem_cache_destroy);
-- 
2.34.1