Reply-To: alex.popov@linux.com
Subject: Re: [v3] mm: Add SLUB free list pointer obfuscation
To: Christopher Lameter <cl@linux.com>, Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Josh Triplett <josh@joshtriplett.org>,
        Andy Lutomirski <luto@kernel.org>,
        Nicolas Pitre <nicolas.pitre@linaro.org>, Tejun Heo <tj@kernel.org>,
        Daniel Mack <daniel@zonque.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Helge Deller <deller@gmx.de>, Rik van Riel <riel@redhat.com>,
        Linux-MM <linux-mm@kvack.org>, Tycho Andersen <tycho@docker.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>,
        alex.popov@linux.com
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From: Alexander Popov <alex.popov@linux.com>
Message-ID: <515333f5-1815-8591-503e-c0cf6941670e@linux.com>
Date: Fri, 28 Jul 2017 01:48:56 +0300
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Hello Christopher and Kees,

On 26.07.2017 19:55, Christopher Lameter wrote:
> On Wed, 26 Jul 2017, Kees Cook wrote:
> 
>>>> What happens if, instead of BUG_ON, we do:
>>>>
>>>> if (unlikely(WARN_RATELIMIT(object == fp, "double-free detected"))
>>>>         return;
>>>
>>> This may work for the free fastpath but the set_freepointer function is
>>> use in multiple other locations. Maybe just add this to the fastpath
>>> instead of to this fucnction?
>>
>> Do you mean do_slab_free()?
> 
> Yes inserting these lines into do_slab_free() would simple ignore the
> double free operation in the fast path and that would be safe.

I don't really like ignoring double-free. I think, that:
  - it will hide dangerous bugs in the kernel,
  - it can make some kernel exploits more stable.
I would rather add BUG_ON to set_freepointer() behind SLAB_FREELIST_HARDENED. Is
it fine?

At the same time avoiding the consequences of some double-free errors is better
than not doing that. It may be considered as kernel "self-healing", I don't
know. I can prepare a second patch for do_slab_free(), as you described. Would
you like it?

Best regards,
Alexander