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

[Kees Cook]

On Thu, Oct 19, 2017 at 4:20 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Tue 17-10-17 13:01:04, Kees Cook wrote:
>> On Tue, Oct 17, 2017 at 2:04 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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.

-Kees

-- 
Kees Cook
Pixel Security