Commit f9fa1d91 authored by Greg Thelen's avatar Greg Thelen Committed by Linus Torvalds
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kasan: drain quarantine of memcg slab objects

Per memcg slab accounting and kasan have a problem with kmem_cache
 - 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
 - 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.


Signed-off-by: default avatarGreg Thelen <>
Reviewed-by: default avatarVladimir Davydov <>
Acked-by: default avatarAndrey Ryabinin <>
Cc: Alexander Potapenko <>
Cc: Dmitry Vyukov <>
Cc: Christoph Lameter <>
Cc: Pekka Enberg <>
Cc: David Rientjes <>
Cc: Joonsoo Kim <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent dc18d706
......@@ -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,
......@@ -435,7 +435,7 @@ void kasan_cache_shrink(struct kmem_cache *cache)
void kasan_cache_destroy(struct kmem_cache *cache)
void kasan_cache_shutdown(struct kmem_cache *cache)
......@@ -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;
......@@ -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 */
if (__kmem_cache_shutdown(s) != 0)
return -EBUSY;
......@@ -816,7 +819,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
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