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

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Message-ID: <CAGXu5jK5m1pVSon=uGy=dLgVi89k33+Lm-XHZ4xiNY-QdUxtng@mail.gmail.com>
Date:   Mon, 10 Jul 2017 09:12:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Michal Hocko <mhocko@...nel.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>,
        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 Mon, Jul 10, 2017 at 1:44 AM, Michal Hocko <mhocko@...nel.org> wrote:
> On Thu 06-07-17 12:12:55, Kees Cook wrote:
>> On Thu, Jul 6, 2017 at 10:52 AM, Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>> > On Thu, Jul 6, 2017 at 10:29 AM, Kees Cook <keescook@...omium.org> wrote:
>> >>>
>> >>>  (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.
>> >
>> > Can't we just do the final rlimit setting so late in execve that we
>> > don't need that whole "saved_rlimit" thing?
>>
>> The stack rlimit defines the mmap layout too:
>>
>> do_execveat_common() ->
>> exec_binprm() ->
>> search_binary_handler() ->
>> fmt->load_binary (load_elf_binary()) ->
>> setup_new_exec() ->
>> arch_pick_mmap_layout() ->
>> mmap_is_legacy() ->
>> rlimit(RLIMIT_STACK) == RLIM_INFINITY
>
> FWIW this is gone in tip tree. See
> lkml.kernel.org/r/20170614082218.12450-1-mhocko@...nel.org

Sounds good to me, but won't large-memory users in 32-bit get annoyed?

A setuid/setgid exec will also get ADDR_COMPAT_LAYOUT cleared in
bprm_fill_uid() (but not for fs-caps), so that'll keep things from
getting layout-controlled. Thanks!

(ADDR_COMPAT_LAYOUT isn't cleared for capability elevations, though, I think.)

-Kees

-- 
Kees Cook
Pixel Security