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

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Date:   Wed, 05 Jul 2017 13:19:40 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     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>,
        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, 2017-07-04 at 16:31 -0700, Linus Torvalds wrote:
> On Tue, Jul 4, 2017 at 4:01 PM, Ben Hutchings <ben@...adent.org.uk>
> wrote:
> > 
> > We have:
> > 
> > bottom = 0xff803fff
> > sp =     0xffffb178
> > 
> > The relevant mappings are:
> > 
> > ff7fc000-ff7fd000 rwxp 00000000 00:00 0
> > fffdd000-ffffe000 rw-p 00000000 00:00
> > 0                                  [stack]
> 
> Ugh. So that stack is actually 8MB in size, but the alloca() is about
> to use up almost all of it, and there's only about 28kB left between
> "bottom" and that 'rwx' mapping.
> 
> Still, that rwx mapping is interesting: it is a single page, and it
> really is almost exactly 8MB below the stack.
> 
> In fact, the top of stack (at 0xffffe000) is *exactly* 8MB+4kB from
> the top of that odd one-page allocation (0xff7fd000).
> 
> Can you find out where that is allocated? Perhaps a breakpoint on
> mmap, with a condition to catch that particular one?
[...]

Found it, and it's now clear why only i386 is affected:
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/os/linux/vm/os_linux.cpp#l4852
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/os_cpu/linux_x86/vm/os_linux_x86.cpp#l881

Ben.

-- 
Ben Hutchings
Anthony's Law of Force: Don't force it, get a larger hammer.

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