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

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Date:   Fri, 7 Jul 2017 07:48:31 -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 10:45 PM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 6, 2017 at 10:36 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> Aren't there real use cases that use many megs of arguments?
>
> They'd be relatively new since the args were pretty limited before.
> I'd be curious to see them.
>
>> 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?
>
> min, not max, but yeah. Here's part of what I have for get_arg_page():
>
>                 rlim = current->signal->rlim;
> -               if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
> +               arg_stack = READ_ONCE(rlim[RLIMIT_STACK].rlim_cur);
> +               arg_stack = min_t(unsigned long, arg_stack, _STK_LIM) / 4;
> +               if (size > arg_stack)
>                         goto fail;

I really did mean max, the idea being that, if we're going to increase
rlim_cur, it's a bit odd to fail the exec if it would have worked
under the higher value.  That being said, I see no real exploit vector
here if just rlimit(RLIMIT_STACK) is used.

(Can you just use rlimit()?  The open-coding seems entirely useless.)

I thought of another approach, though: change the rlimit macros so
that a secureexec program always gets at least 8MB stack.  Might be
less regression-prone.