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

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Message-ID: <1499209315.2707.29.camel@decadent.org.uk>
Date:   Wed, 05 Jul 2017 00:01:55 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Michal Hocko <mhocko@...nel.org>, Willy Tarreau <w@....eu>
Cc:     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, 2017-07-04 at 12:36 +0100, Ben Hutchings wrote:
[...]
> This *doesn't* fix the LibreOffice regression on i386.

gdb shows me that the crash is at the last statement in this function:

static void _expand_stack_to(address bottom) {
  address sp;
  size_t size;
  volatile char *p;

  // Adjust bottom to point to the largest address within the same page, it
  // gives us a one-page buffer if alloca() allocates slightly more memory.
  bottom = (address)align_size_down((uintptr_t)bottom, os::Linux::page_size());
  bottom += os::Linux::page_size() - 1;

  // sp might be slightly above current stack pointer; if that's the case, we
  // will alloca() a little more space than necessary, which is OK. Don't use
  // os::current_stack_pointer(), as its result can be slightly below current
  // stack pointer, causing us to not alloca enough to reach "bottom".
  sp = (address)&sp;

  if (sp > bottom) {
    size = sp - bottom;
    p = (volatile char *)alloca(size);
    assert(p != NULL && p <= (volatile char *)bottom, "alloca problem?");
    p[0] = '\0';
  }
}

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]

So instead of a useless guard page, we have a dangerous WX page
underneath the stack!  I suppose I should find out where and why that's
being allocated.

Ben.

-- 
Ben Hutchings
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