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

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Message-ID: <alpine.DEB.2.20.1707061052380.26079@east.gentwo.org>
Date:   Thu, 6 Jul 2017 10:55:39 -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 <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: [PATCH v3] mm: Add SLUB free list pointer obfuscation

On Thu, 6 Jul 2017, Kees Cook wrote:

> On Thu, Jul 6, 2017 at 6:43 AM, Christoph Lameter <cl@...ux.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.