Reply-To: alex.popov@linux.com
Subject: Re: [PATCH 1/1] mm/slub.c: add a naive detection of double free or
 corruption
To: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        keescook@chromium.org
References: <1500309907-9357-1-git-send-email-alex.popov@linux.com>
 <20170717175459.GC14983@bombadil.infradead.org>
From: Alexander Popov <alex.popov@linux.com>
Message-ID: <46e2d4b9-94a4-76e3-be25-144f26f74fb6@linux.com>
Date: Mon, 17 Jul 2017 21:23:44 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.1
MIME-Version: 1.0
In-Reply-To: <20170717175459.GC14983@bombadil.infradead.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 941
Lines: 24

On 17.07.2017 20:54, Matthew Wilcox wrote:
> On Mon, Jul 17, 2017 at 07:45:07PM +0300, Alexander Popov wrote:
>> Add an assertion similar to "fasttop" check in GNU C Library allocator:
>> an object added to a singly linked freelist should not point to itself.
>> That helps to detect some double free errors (e.g. CVE-2017-2636) without
>> slub_debug and KASAN. Testing with hackbench doesn't show any noticeable
>> performance penalty.
> 
>>  {
>> +	BUG_ON(object == fp); /* naive detection of double free or corruption */
>>  	*(void **)(object + s->offset) = fp;
>>  }
> 
> Is BUG() the best response to this situation?  If it's a corruption, then
> yes, but if we spot a double-free, then surely we should WARN() and return
> without doing anything?

Hello Matthew,

Double-free leads to the memory corruption too, since the next two kmalloc()
calls return the same address to their callers. And we can spot it early here.

--
Alexander