linux-kernel - Re: [PATCH v2] mm: Add SLUB free list pointer obfuscation

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Date:   Sun, 25 Jun 2017 12:56:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Laura Abbott <labbott@...hat.com>,
        Daniel Micay <danielmicay@...il.com>,
        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>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2] mm: Add SLUB free list pointer obfuscation

On Thu, Jun 22, 2017 at 6:50 PM, Kees Cook <keescook@...omium.org> wrote:
> This SLUB free list pointer obfuscation code is modified from Brad
> Spengler/PaX Team's code in the last public patch of grsecurity/PaX based
> on my understanding of the code. Changes or omissions from the original
> code are mine and don't reflect the original grsecurity/PaX code.
>
> This adds a per-cache random value to SLUB caches that is XORed with
> their freelist pointers. This adds nearly zero overhead and frustrates the
> very common heap overflow exploitation method of overwriting freelist
> pointers. A recent example of the attack is written up here:
> http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit

BTW, to quantify "nearly zero overhead", I ran multiple 200-run cycles
of "hackbench -g 20 -l 1000", and saw:

before:
mean 10.11882499999999999995
variance .03320378329145728642
stdev .18221905304181911048

after:
mean 10.12654000000000000014
variance .04700556623115577889
stdev .21680767106160192064

The difference gets lost in the noise, but if the above is sensible,
it's 0.07% slower. ;)

-Kees

-- 
Kees Cook
Pixel Security