Date: Tue, 4 Jul 2017 12:46:52 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ben Hutchings <ben@decadent.org.uk>, Hugh Dickins <hughd@google.com>,
        Willy Tarreau <w@1wt.eu>, 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>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas
Message-ID: <20170704104652.GH14722@dhcp22.suse.cz>
References: <alpine.LSU.2.11.1706190355140.2626@eggly.anvils>
 <CA+55aFx6j4na3BVRC2aQuf-kNp1jzGahN8To_SFpNu+H=gopJA@mail.gmail.com>
 <20170619142358.GA32654@1wt.eu>
 <1498009101.2655.6.camel@decadent.org.uk>
 <20170621092419.GA22051@dhcp22.suse.cz>
 <1498042057.2655.8.camel@decadent.org.uk>
 <1499126133.2707.20.camel@decadent.org.uk>
 <CA+55aFzMX72+Kb=zNgjCf6UfPt+C+e7WDp_rpbSLuOVx1k7iqg@mail.gmail.com>
 <20170704084122.GC14722@dhcp22.suse.cz>
 <20170704093538.GF14722@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170704093538.GF14722@dhcp22.suse.cz>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1855
Lines: 40

On Tue 04-07-17 11:35:38, Michal Hocko wrote:
> On Tue 04-07-17 10:41:22, Michal Hocko wrote:
> > On Mon 03-07-17 17:05:27, Linus Torvalds wrote:
> > > On Mon, Jul 3, 2017 at 4:55 PM, Ben Hutchings <ben@decadent.org.uk> wrote:
> > > >
> > > > Firstly, some Rust programs are crashing on ppc64el with 64 KiB pages.
> > > > Apparently Rust maps its own guard page at the lower limit of the stack
> > > > (determined using pthread_getattr_np() and pthread_attr_getstack()).  I
> > > > don't think this ever actually worked for the main thread stack, but it
> > > > now also blocks expansion as the default stack size of 8 MiB is smaller
> > > > than the stack gap of 16 MiB.  Would it make sense to skip over
> > > > PROT_NONE mappings when checking whether it's safe to expand?
> > 
> > This is what my workaround for the older patch was doing, actually. We
> > have deployed that as a follow up fix on our older code bases. And this
> > has fixed verious issues with Java which was doing the similar thing.
> 
> Here is a forward port (on top of the current Linus tree) of my earlier
> patch. I have dropped a note about java stack trace because this would
> most likely be not the case with the Hugh's patch. The problem is the
> same in principle though. Note I didn't get to test this properly yet
> but it should be pretty much obvious.

Tested with the attached program.
root@test1:~# ./stack_crash 
Stack top:0x7fffcdb605ec mmap:0x7fffcc760000
address:0x7fffcc760ff8 aligned:0x7fffcc760000 mapped:[7fffcc760000,7fffcc761000] diff:-8
[...]

so we faulted on the PROT_NONE while with
#define MAPING_PROT PROT_READ
root@test1:~# ./stack_crash 
Stack top:0x7ffe73dde6fc mmap:0x7ffe729de000
address:0x7ffe72adefd8 aligned:0x7ffe72ade000 mapped:[7ffe729de000,7ffe729df000] diff:1048536
[...]

we failed 1MB ahead of the mapping.
-- 
Michal Hocko
SUSE Labs