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

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Date:   Thu, 6 Jul 2017 10:29:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Michal Hocko <mhocko@...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>,
        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>,
        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 Thu, Jul 6, 2017 at 9:34 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Jul 5, 2017 at 9:32 PM, Kees Cook <keescook@...omium.org> wrote:
>> In an attempt to provide sensible rlimit defaults for setuid execs, this
>> inherits the namespace's init rlimits:
>
> Yeah, so I have to admit to hating this patch.

Yup! Totally fine. I just wanted to demonstrate the idea with some actual code.

> I'd much rather see something like
>
>  (a) minimal: just use our existing default stack (and stack _only_)
> limit value for suid binaries that actually get extra permissions: {
> _STK_LIM, RLIM_INFINITY }.

This would look a lot like the existing patch; it'd just not copy the
init process rlimits.

>  (c) perhaps encourage people to annotate their suid binaries with
> initial resource requirements (and for stack, I mean the existing
> GNU_STACK ELF annotation in particular).
>
> For an example of (a), that existing _STK_LIM define is what the
> kernel defaults to, and it's a 8MB stack. And looking at my Fedora
> install, I see that the default user rlimit is 8MB for the stack.
> [...]
> Is that just coincidence, or is that just a sign of "nobody ever even
> modifies the default value"? So (a) feels like "nobody really cares,
> and 8MB is fine, and nobody even bothers changing it - just do the
> minimal thing".

I would be terrified to see a setuid binary that needed >8MB stack. :P

> And (c) would be the sane option, and what we already do for things
> like GNU_STACK to enable/disable executable stacks. It really feels
> like allowing the GNU_STACK segment to contain stack rlimit override
> information would be the perfect tool for binaries to say "Yeah, I
> need more stack than _STK_LIM".
>
> So I see many different approaches (that could be combined: I like
> combining (a) and (c), for example), and absolutely none of them
> involve the random "take some values from init".
>
> And yes, a large part of this may be that I no longer feel like I can
> trust "init" to do the sane thing. You all presumably know why.

Heh. Yeah, I think a+c could make sense. "if not GNU_STACK program
header or 0 MemSiz GNU_STACK program header, use _STK_LIM, otherwise
use GNU_STACK MemSiz". I'll send a patch...

-Kees

-- 
Kees Cook
Pixel Security