Messages in this thread | | | From | Kees Cook <> | Date | Fri, 20 Oct 2017 07:12:29 -0700 | Subject | Re: [PATCH 2/2] fs, elf: drop MAP_FIXED from initial ET_DYN segment |
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On Fri, Oct 20, 2017 at 1:45 AM, Michal Hocko <mhocko@kernel.org> wrote: > On Thu 19-10-17 10:19:40, Kees Cook wrote: >> On Thu, Oct 19, 2017 at 4:20 AM, Michal Hocko <mhocko@kernel.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@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. >> >> 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.
I'm probably not explaining it well enough! :(
> 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.
Yup, I really like patch 1: it protects us from "impossible" situations, which we know rarely stay impossible. :)
-Kees
-- Kees Cook Pixel Security
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