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

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Date:   Thu, 6 Jul 2017 22:49:18 -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 10:39 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jul 6, 2017 at 10:15 PM, Kees Cook <keescook@...omium.org> wrote:
>>
>> 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):
>
> But this doesn't affect the stack layout itself.
>
> So we could do the stack copying without much caring, because that
> happens first, right?
>
> So I think we can do all the envp/argv copying first, and then - as we
> change the credentials, change the rlimit. And the string copies
> wouldn't need to care much - although I guess they are also fine
> checking against a possible *smaller* stack rlimit, which is actually
> what we'd want.

Yup, agreed.

> And I think the credentials switch (which is the point of no return
> anyway) happens before we start mmap'ing the executable etc. We used
> to have some odd code there and do it in the completely wrong order
> (checking that the binary was executable for the *old* user, which
> makes no sense, iirc)

Yeah, it all happens in setup_new_exec(). The first thing is layout
selection, then switching credentials. It could be made to take a hint
from GNU_STACK (which was parsed before setup_new_exec() is called),
check security_bprm_secureexec() and then make the rlimit changes, all
before the layout selection.

> So I'm getting the sense that none of this should be a problem.
>
> But it's entirely possible that I missed something, and am just full
> of shit. Our execve() path has traditionally been very hard to read.
> It's actually gotten a bit better, but the whole "jump back and forth
> between the generic fs/exec.c code and the binfmt code" is certainly
> still there.

Yeah, there might be something else lurking here, but so far, I'm
satisfied this will work too.

-Kees

-- 
Kees Cook
Pixel Security