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

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Date:   Tue, 4 Jul 2017 14:33:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Ben Hutchings <ben@...adent.org.uk>
Cc:     Willy Tarreau <w@....eu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        linux-distros@...openwall.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 Tue 04-07-17 13:21:02, Ben Hutchings wrote:
> On Tue, 2017-07-04 at 14:00 +0200, Michal Hocko wrote:
> > 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?
> 
> Yes it's their bug, but you know the rule - don't break user-space.

Absolutely, that is why I belive we should consider the prev VMA but
doing anything more just risks for new regressions. Or why do you think
that not-checking them would cause a regression?

> > 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.
> 
> I don't want odd behaviour to weaken the stack guard.
> 
> > [...]
> > 
> > > This *doesn't* fix the LibreOffice regression on i386.
> > 
> > Are there any details about this regression?
> 
> Here:
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=865303#170
> 
> I haven't reproduced it in Writer, but if I use Base to create a new
> HSQLDB database it reliably crashes (HSQLDB is implemented in Java).

I haven't read through previous 169 comments but I do not see any stack
trace. Ideally with info proc mapping that would tell us the memory
layout.

-- 
Michal Hocko
SUSE Labs