linux-kernel - Re: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP

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Message-Id: <20170705141849.2e0e4721d975277183eb178f@linux-foundation.org>
Date:   Wed, 5 Jul 2017 14:18:49 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>,
        Oleg Nesterov <oleg@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes
 in the stack

On Wed, 5 Jul 2017 20:28:49 +0200 Michal Hocko <mhocko@...nel.org> 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@...e.cz>
> Signed-off-by: Michal Hocko <mhocko@...e.com>