linux-kernel - Re: [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET

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Date:   Fri, 20 Oct 2017 10:45:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET_DYN segment

On Thu 19-10-17 10:19:40, Kees Cook wrote:
> On Thu, Oct 19, 2017 at 4:20 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Tue 17-10-17 13:01:04, Kees Cook wrote:
> >> On Tue, Oct 17, 2017 at 2:04 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > [...]
> >> > I am not insisting on this patch but it seems to me is just makes a
> >> > recoverable state a failure.
> >>
> >> Right, I understand you're trying to make it recoverable. I'm
> >> suggesting that making it recoverable provides a way for an attack to
> >> abuse it, and that what we'd be recovering from is a case we should
> >> never ever see.
> >>
> >> Consider the case where through some future bug/feature, it's possible
> >> to put the stack at an arbitrary location during an exec. (We've
> >> worked to fix that already, but who knows what the future holds either
> >> through misfeatures or bugs.) If an attacker maps the stack over a
> >> large portion of the PIE exec range, patch 2 will result in vmmap
> >> searching out a location that isn't already allocated. This means that
> >> instead of the PIE ASLR choosing from the entire possible range, it
> >> will get limited to only the area where something isn't already
> >> overlapping. This would give an attacker the ability to control the
> >> PIE ASLR, possibly forcing it into a fixed location.
> >
> > Yes, I guess I understand that part. What is not clear to me exactly is
> > why this matters as we have the mmap_base randomized and not under the
> > control of the attacker.
> 
> mmap_base is separate from the PIE base, so patch 2 would allow for a
> reduction of the PIE ASLR entropy in the case of a novel overlap
> attack.

OK, it seems that I am just too dull see through your concerns here.
Anyway, are you willing to ack the patch 1 (when metag fix is included)?
I would resubmit in that case and ask for merging without patch 2.
-- 
Michal Hocko
SUSE Labs