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Date:   Thu, 22 Dec 2016 11:54:58 +0300
From:   Vladimir Davydov <>
To:     Greg Thelen <>
Cc:     Andrew Morton <>,
        Andrey Ryabinin <>,
        Alexander Potapenko <>,
        Dmitry Vyukov <>,
        Christoph Lameter <>,
        Pekka Enberg <>,
        David Rientjes <>,
        Joonsoo Kim <>,,,
Subject: Re: [PATCH 1/2] kasan: drain quarantine of memcg slab objects

On Tue, Dec 20, 2016 at 10:11:01AM -0800, Greg Thelen wrote:
> 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.
> Signed-off-by: Greg Thelen <>

Reviewed-by: Vladimir Davydov <>

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