linux-kernel - Re: [v3] mm: Add SLUB free list pointer obfuscation

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Date:   Fri, 28 Jul 2017 01:48:56 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Christopher Lameter <cl@...ux.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Tejun Heo <tj@...nel.org>, Daniel Mack <daniel@...que.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Helge Deller <deller@....de>, Rik van Riel <riel@...hat.com>,
        Linux-MM <linux-mm@...ck.org>, Tycho Andersen <tycho@...ker.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>, alex.popov@...ux.com
Subject: Re: [v3] mm: Add SLUB free list pointer obfuscation

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