Date: Thu, 6 Jul 2017 10:55:39 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
To: 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>
Subject: Re: [PATCH v3] mm: Add SLUB free list pointer obfuscation
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On Thu, 6 Jul 2017, Kees Cook wrote:

> On Thu, Jul 6, 2017 at 6:43 AM, Christoph Lameter <cl@linux.com> wrote:
> > 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.
>
> That requires a series of arbitrary reads. This is protecting against
> attacks that use an adjacent slab object write overflow to write the
> freelist pointer. This internal structure is very reliable, and has
> been the basis of freelist attacks against the kernel for a decade.

These reads are not arbitrary. You can usually calculate the page struct
address easily from the address and then do a couple of loads to get
there.

Ok so you get rid of the old attacks because we did not have that
hardening in effect when they designed their approaches?

> It is a probabilistic defense, but then so is the stack protector.
> This is a similar defense; while not perfect it makes the class of
> attack much more difficult to mount.

Na I am not convinced of the "much more difficult". Maybe they will just
have to upgrade their approaches to fetch the proper values to decode.