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

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Date:   Wed, 26 Jul 2017 10:13:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Alexander Popov <alex.popov@...ux.com>,
        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>
Subject: Re: [v3] mm: Add SLUB free list pointer obfuscation

On Wed, Jul 26, 2017 at 9:55 AM, Christopher Lameter <cl@...ux.com> 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.
>
> Although in either case we are adding code to the fastpath...

While I'd like it unconditionally, I think Alexander's proposal was to
put it behind CONFIG_SLAB_FREELIST_HARDENED.

BTW, while I've got your attention, can you Ack the other patch? I
sent a v4 for the pointer obfuscation, which we really need:
https://lkml.org/lkml/2017/7/26/4

Thanks!

-Kees

-- 
Kees Cook
Pixel Security