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

From: Andrew Morton
Date: Wed Jul 05 2017 - 17:18:54 EST

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?

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