Date: Tue, 4 Jul 2017 14:00:00 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Ben Hutchings <ben@decadent.org.uk>
Cc: Willy Tarreau <w@1wt.eu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>, Oleg Nesterov <oleg@redhat.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>, Rik van Riel <riel@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Tony Luck <tony.luck@intel.com>,
        "James E.J. Bottomley" <jejb@parisc-linux.org>,
        Helge Diller <deller@gmx.de>, James Hogan <james.hogan@imgtec.com>,
        Laura Abbott <labbott@redhat.com>, Greg KH <greg@kroah.com>,
        "security@kernel.org" <security@kernel.org>,
        linux-distros@vs.openwall.org,
        Qualys Security Advisory <qsa@qualys.com>,
        LKML <linux-kernel@vger.kernel.org>, Ximin Luo <infinity0@debian.org>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas
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On Tue 04-07-17 12:36:11, Ben Hutchings wrote:
> On Tue, 2017-07-04 at 12:42 +0200, Michal Hocko wrote:
> > On Tue 04-07-17 11:47:28, Willy Tarreau wrote:
> > > On Tue, Jul 04, 2017 at 11:35:38AM +0200, Michal Hocko wrote:
> [...]
> > > But wouldn't this completely disable the check in case such a guard page
> > > is installed, and possibly continue to allow the collision when the stack
> > > allocation is large enough to skip this guard page ?
> > 
> > Yes and but a PROT_NONE would fault and as the changelog says, we _hope_
> > that userspace does the right thing.
> 
> It may well not be large enough, because of the same wrong assumptions
> that resulted in the kernel's guard page not being large enough.  We
> should count it as part of the guard gap but not a substitute.

yes, you are right of course. But isn't this a bug on their side
considering they are managing their _own_ stack gap? Our stack gap
management is a best effort thing and two such approaches competing will
always lead to weird cornercases. That was my assumption when saying
that I am not sure this is really _worth_ it. We should definitely try
to workaround clashes but that's about it. If others think that we
should do everything to prevent even those issues I will not oppose
of course. It just adds more cycles to something that is a weird case
already.

[...]

> This *doesn't* fix the LibreOffice regression on i386.

Are there any details about this regression?

-- 
Michal Hocko
SUSE Labs