linux-kernel - Re: [PATCH] mm: larger stack guard gap, between vmas

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Date:   Thu, 06 Jul 2017 00:50:42 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, Willy Tarreau <w@....eu>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Tony Luck <tony.luck@...el.com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Diller <deller@....de>,
        James Hogan <james.hogan@...tec.com>,
        Laura Abbott <labbott@...hat.com>, Greg KH <greg@...ah.com>,
        "security@...nel.org" <security@...nel.org>,
        Qualys Security Advisory <qsa@...lys.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ximin Luo <infinity0@...ian.org>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas

On Wed, 2017-07-05 at 13:53 -0700, Andy Lutomirski wrote:
> On Jul 5, 2017, at 12:32 PM, Ben Hutchings <ben@...adent.org.uk> wrote:
> > On Wed, 2017-07-05 at 10:23 -0700, Andy Lutomirski wrote:
[...]
> > >  - As a hardening feature, if the stack would expand within 64k or
> > > whatever of a non-MAP_FIXED mapping, refuse to expand it.  (This might
> > > have to be a non-hinted mapping, not just a non-MAP_FIXED mapping.)
> > > The idea being that, if you deliberately place a mapping under the
> > > stack, you know what you're doing.  If you're like LibreOffice and do
> > > something daft and are thus exploitable, you're on your own.
> > >  - As a hardening measure, don't let mmap without MAP_FIXED position
> > > something within 64k or whatever of the bottom of the stack unless a
> > > MAP_FIXED mapping is between them.
> > 
> > Having tested patches along these lines, I think the above would avoid
> > the reported regressions.
> > 
> 
> FWIW, even this last part may be problematic.  It'll break anything
> that tries to allocate many small MAP_GROWSDOWN stacks on 32-
> bit.  Hopefully nothing does this, but maybe Java does.

glibc (NPTL) does not.  Java (at least Hotspot in OpenJDK 6,7, 8) does
not.  LinuxThreads *does* and is used by uclibc.  dietlibc *does*.  I
would be surprised if either was used for applications with very many
threads, but then this issue has thrown up a lot of surprises.

Ben.

-- 
Ben Hutchings
Man invented language to satisfy his deep need to complain. - Lily
Tomlin


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