Re: [kernel-hardening] [PATCH 3/6] x86/mmap: properly account for stack randomization in mmap_base

[Daniel Micay]

On Fri, 2017-06-02 at 21:46 -0700, Kees Cook wrote:
> On Fri, Jun 2, 2017 at 8:20 AM,  <riel@xxxxxxxxxx> wrote:
> > From: Rik van Riel <riel@xxxxxxxxxx>
> > 
> > When RLIMIT_STACK is, for example, 256MB, the current code results
> > in
> > a gap between the top of the task and mmap_base of 256MB, failing to
> > take into account the amount by which the stack address was
> > randomized.
> > In other words, the stack gets less than RLIMIT_STACK space.
> 
> Is this entirely accurate? The top of the task would be task_size, but
> this code is using task_size / 6 * 5 as the bottom of stack / top of
> mmap gap_max. Is there a reason for this?

MIN_GAP / MAX_GAP are only the boundaries that this gap is clamped to.

If it's not smaller than MIN_GAP, MIN_GAP isn't used. If it's not larger
than MAX_GAP, MAX_GAP isn't used. The stack randomization is currently
only taken into account for MIN_GAP. This only fixes that bug by always
taking it into account. It's not a subjective design change.

The MAX_GAP value is 5/6 of the address space which is overly large but
that's a separate bug.