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

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Date:   Wed, 05 Jul 2017 20:32:43 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 10:23 -0700, Andy Lutomirski wrote:
[...]
> Looking at it that way, I think a new inherited-on-exec flag is nucking futs.
> 
> I'm starting to think that the right approach is to mostly revert all
> this stuff (the execve fixes are fine).  Then start over and think
> about it as hardening.  I would suggest the following approach:
> 
>  - The stack gap is one page, just like it's been for years.

Given that in the following points you say that something sounding like
a stack gap would be "64k or whatever", what does "the stack gap" mean
in this first point?

>  - 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.

Ben.

> And that's all.  It's not like a 64k gap actually fixes these bugs for
> real -- it just makes them harder to exploit.
> 
> [1] The code that GCC generates for char buf[bug number] and alloca()
> is flat-out wrong.  Everyone who's ever thought about it all all knows
> it and has known about it for years, but no one cared to fix it.
-- 
Ben Hutchings
Anthony's Law of Force: Don't force it, get a larger hammer.


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