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

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Date:   Thu, 6 Jul 2017 08:43:19 -0500 (CDT)
From:   Christoph Lameter <cl@...ux.com>
To:     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@...ck.org, Tycho Andersen <tycho@...ker.com>,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v3] mm: Add SLUB free list pointer obfuscation

On Wed, 5 Jul 2017, Kees Cook wrote:

> @@ -3536,6 +3565,9 @@ static int kmem_cache_open(struct kmem_cache *s, unsigned long flags)
>  {
>  	s->flags = kmem_cache_flags(s->size, flags, s->name, s->ctor);
>  	s->reserved = 0;
> +#ifdef CONFIG_SLAB_FREELIST_HARDENED
> +	s->random = get_random_long();
> +#endif
>
>  	if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
>  		s->reserved = sizeof(struct rcu_head);
>

So if an attacker knows the internal structure of data then he can simply
dereference page->kmem_cache->random to decode the freepointer.

Assuming someone is already targeting a freelist pointer (which indicates
detailed knowledge of the internal structure) then I would think that
someone like that will also figure out how to follow the pointer links to
get to the random value.

Not seeing the point of all of this.