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

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Date:   Thu, 6 Jul 2017 22:36:53 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.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 10:15 PM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 6, 2017 at 10:10 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Thu, Jul 6, 2017 at 9:48 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>> How about a much simpler solution: don't read rlimit at all in
>>> copy_strings(), let alone try to enforce it.  Instead, just before the
>>> point of no return, check how much stack space is already used and, if
>>> it's more than an appropriate threshold (e.g. 1/4 of the rlimit),
>>> abort.  Sure, this adds overhead if we're going to abort, but does
>>> that really matter?
>>
>> We should avoid using up tons of memory and then failing. Better to
>> cap it as we use it. Plumbing a sane value into this shouldn't be hard
>> at all. Just making this a hardcoded 2MB seems sane (1/4 of 8MB).

Aren't there real use cases that use many megs of arguments?

We could probably get away with saying max(rlimit(RLIMIT_STACK), 2MB)
as long as we make sure later on that we don't screw up if we've
overallocated?

>>
>>> I don't see why using rlimit for layout control makes any sense
>>> whatsoever.  Is there some historical reason we need that?  As far as
>>> I can see (on insufficient inspection) is that the kernel is trying to
>>> guarantee that, if we have so much arg crap that our remaining stack
>>> is less than 128k, then we don't exceed our limit by a little bit.
>>
>> IIUC, this is a big deal on 32-bit. Unlimited stack triggers top-down
>> mmap instead of bottom-up. I mean, I'd be delighted to get rid of
>> this, but I thought it was relied on by userspace.
>
> I always say this backwards. :P Default is top-down (allocate at high
> addresses and work down toward low). With unlimited stack, allocations
> start at low addresses and work up. Here's the results (shown with
> randomize_va_space sysctl set to 0):

Uhh, crikey!  Where's the code that does that?