Commit f9fa1d91 authored Feb 24, 2017 by Greg Thelen Committed by Linus Torvalds Feb 24, 2017

kasan: drain quarantine of memcg slab objects

Per memcg slab accounting and kasan have a problem with kmem_cache
destruction.
 - kmem_cache_create() allocates a kmem_cache, which is used for
   allocations from processes running in root (top) memcg.
 - Processes running in non root memcg and allocating with either
   __GFP_ACCOUNT or from a SLAB_ACCOUNT cache use a per memcg
   kmem_cache.
 - Kasan catches use-after-free by having kfree() and kmem_cache_free()
   defer freeing of objects. Objects are placed in a quarantine.
 - kmem_cache_destroy() destroys root and non root kmem_caches. It takes
   care to drain the quarantine of objects from the root memcg's
   kmem_cache, but ignores objects associated with non root memcg. This
   causes leaks because quarantined per memcg objects refer to per memcg
   kmem cache being destroyed.

To see the problem:

 1) create a slab cache with kmem_cache_create(,,,SLAB_ACCOUNT,)
 2) from non root memcg, allocate and free a few objects from cache
 3) dispose of the cache with kmem_cache_destroy() kmem_cache_destroy()
    will trigger a "Slab cache still has objects" warning indicating
    that the per memcg kmem_cache structure was leaked.

Fix the leak by draining kasan quarantined objects allocated from non
root memcg.

Racing memcg deletion is tricky, but handled.  kmem_cache_destroy() =>
shutdown_memcg_caches() => __shutdown_memcg_cache() => shutdown_cache()
flushes per memcg quarantined objects, even if that memcg has been
rmdir'd and gone through memcg_deactivate_kmem_caches().

This leak only affects destroyed SLAB_ACCOUNT kmem caches when kasan is
enabled.  So I don't think it's worth patching stable kernels.

Link: http://lkml.kernel.org/r/1482257462-36948-1-git-send-email-gthelen@google.com


Signed-off-by: Greg Thelen <gthelen@google.com>
Reviewed-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent dc18d706

include/linux/kasan.h

+2 −2

Original line number	Diff line number	Diff line
		@@ -52,7 +52,7 @@ void kasan_free_pages(struct page *page, unsigned int order);
		void kasan_cache_create(struct kmem_cache cache, size_t size,
		unsigned long *flags);
		void kasan_cache_shrink(struct kmem_cache *cache);
		void kasan_cache_destroy(struct kmem_cache *cache);
		void kasan_cache_shutdown(struct kmem_cache *cache);

		void kasan_poison_slab(struct page *page);
		void kasan_unpoison_object_data(struct kmem_cache cache, void object);
		@@ -98,7 +98,7 @@ static inline void kasan_cache_create(struct kmem_cache *cache,
		size_t *size,
		unsigned long *flags) {}
		static inline void kasan_cache_shrink(struct kmem_cache *cache) {}
		static inline void kasan_cache_destroy(struct kmem_cache *cache) {}
		static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}

		static inline void kasan_poison_slab(struct page *page) {}
		static inline void kasan_unpoison_object_data(struct kmem_cache *cache,

mm/kasan/kasan.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -435,7 +435,7 @@ void kasan_cache_shrink(struct kmem_cache *cache)
		quarantine_remove_cache(cache);
		}

		void kasan_cache_destroy(struct kmem_cache *cache)
		void kasan_cache_shutdown(struct kmem_cache *cache)
		{
		quarantine_remove_cache(cache);
		}

mm/kasan/quarantine.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -274,6 +274,7 @@ static void per_cpu_remove_cache(void *arg)
		qlist_free_all(&to_free, cache);
		}

		/* Free all quarantined objects belonging to cache. */
		void quarantine_remove_cache(struct kmem_cache *cache)
		{
		unsigned long flags, i;

mm/slab_common.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -528,6 +528,9 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)

		static int shutdown_cache(struct kmem_cache *s)
		{
		/* free asan quarantined objects */
		kasan_cache_shutdown(s);

		if (__kmem_cache_shutdown(s) != 0)
		return -EBUSY;

		@@ -816,7 +819,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
		get_online_cpus();
		get_online_mems();

		kasan_cache_destroy(s);
		mutex_lock(&slab_mutex);

		s->refcount--;

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