Subject: Re: [PATCH 1/2] kasan: drain quarantine of memcg slab objects
To: Greg Thelen <gthelen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "Dmitry Vyukov" <dvyukov@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>
References: <1482257462-36948-1-git-send-email-gthelen@google.com>
CC: Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        "David Rientjes" <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, <kasan-dev@googlegroups.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
From: Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <7a7bdc20-121e-07d7-cb02-bb44902cd797@virtuozzo.com>
Date: Wed, 21 Dec 2016 19:42:22 +0300
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On 12/20/2016 09:11 PM, 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 <gthelen@google.com>
> 

Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>