Re: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes in the stack

From: Michal Hocko
Date: Thu Jul 06 2017 - 02:47:59 EST

On Wed 05-07-17 14:18:49, Andrew Morton wrote:
> On Wed, 5 Jul 2017 20:28:49 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > "mm: enlarge stack guard gap" has introduced a regression in some rust
> > and Java environments which are trying to implement their own stack
> > guard page. They are punching a new MAP_FIXED mapping inside the
> > existing stack Vma.
> >
> > This will confuse expand_{downwards,upwards} into thinking that the stack
> > expansion would in fact get us too close to an existing non-stack vma
> > which is a correct behavior wrt. safety. It is a real regression on
> > the other hand. Let's work around the problem by considering PROT_NONE
> > mapping as a part of the stack. This is a gros hack but overflowing to
> > such a mapping would trap anyway an we only can hope that usespace
> > knows what it is doing and handle it propely.
> >
> > Fixes: d4d2d35e6ef9 ("mm: larger stack guard gap, between vmas")
> That should be 1be7107fbe18, yes?

yes. d4d2d35e6ef9 was a cherry-pick into the mmotm git tree. Sorry about

> > Debugged-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Michal Hocko