Re: [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET_DYN segment

From: Michal Hocko <mhocko@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jiri Kosina <jkosina@suse.cz>, Al Viro <viro@zeniv.linux.org.uk>,
	Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	Baoquan He <bhe@redhat.com>
Subject: Re: [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET_DYN segment
Date: Thu, 19 Oct 2017 13:20:01 +0200	[thread overview]
Message-ID: <20171019112001.fjqxazjb6sargrqa@dhcp22.suse.cz> (raw)
In-Reply-To: <CAGXu5jLZHJoBrNK-SUdih181R+vC=TO9jm-4xeJQC8=UGxzQ4Q@mail.gmail.com>

On Tue 17-10-17 13:01:04, Kees Cook wrote:
> On Tue, Oct 17, 2017 at 2:04 AM, Michal Hocko <mhocko@kernel.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.

-- 
Michal Hocko
SUSE Labs