linux-kernel - Re: [RFC][PATCH] exec: Use init rlimits for setuid exec

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Date:   Mon, 10 Jul 2017 14:29:45 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>, Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.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>,
        Qualys Security Advisory <qsa@...lys.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ximin Luo <infinity0@...ian.org>
Subject: Re: [RFC][PATCH] exec: Use init rlimits for setuid exec

On Mon, 2017-07-10 at 20:16 +0200, Michal Hocko wrote:

> OK, I misread the code. 32b applications on 64b systems do top down
> by
> default and only if they override this by ADDR_COMPAT_LAYOUT
> personality. For some reason I thought that 32b userspace goes a
> different path and makes sure that they are always doing bottom up.
> 
> Anyway even if somebody really needs to grow stack really large we
> have
> the personality to give them the legacy layout.

I think what will happen when rlimit_stack is RLIMIT_INFINITY
is that mmap_base will end up placing mm->mmap_base at 512MB
(task_size / 6 * 5 below the top of address space) for 32 bit
kernels, and we eventually fall back to a bottom-up search
if the space below mmap_base is exhausted (if it ever is).