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

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Date:   Tue, 4 Jul 2017 12:46:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>, Willy Tarreau <w@....eu>,
        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>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas

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@...adent.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@...t1:~# ./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@...t1:~# ./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