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

From: Michal Hocko <mhocko@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Ben Hutchings <ben@decadent.org.uk>, Willy Tarreau <w@1wt.eu>,
	Oleg Nesterov <oleg@redhat.com>, Rik van Riel <riel@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes in the stack
Date: Thu, 6 Jul 2017 08:47:34 +0200	[thread overview]
Message-ID: <20170706064733.GA29724@dhcp22.suse.cz> (raw)
In-Reply-To: <20170705141849.2e0e4721d975277183eb178f@linux-foundation.org>

On Wed 05-07-17 14:18:49, Andrew Morton wrote:
> On Wed, 5 Jul 2017 20:28:49 +0200 Michal Hocko <mhocko@kernel.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?

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

> > Debugged-by: Vlastimil Babka <vbabka@suse.cz>
> > Signed-off-by: Michal Hocko <mhocko@suse.com>

-- 
Michal Hocko
SUSE Labs