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

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Message-ID: <CAGXu5j+AXx+CCzMcX1LSginDS9E6OhJrU2hjGCB1SVyMf14CZQ@mail.gmail.com>
Date:   Fri, 7 Jul 2017 11:28:55 -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 Fri, Jul 7, 2017 at 9:06 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jul 6, 2017 at 11:10 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Thu, Jul 6, 2017 at 11:02 PM, Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>>> So 2+MB is still definitely something people can do (and probably *do* do).
>>
>> With the default 8MB stack, most people are already limited to 2MB
>> here. I guess the question is, do people raise their stack rlimit to
>> gain more arguments? Should I pick a different value for the args?
>
> So I would not be at all surprised if people just made the stack limit
> higher when they hit the E2BIG issue in some script.
>
> So yes, I'd make the max args cutoff be higher than 2MB.
>
> I'd suggest we make the code do:
>
>  (a) keep the existing rlimit/4 check (so *most* people will see the
> exact same behavior)
>
>  (b) add a static max arg check for something that is closer to 8MB
> but leaves a somewhat reasonable stack size even if the stack size get
> reset to 8MB
>
> I'd suggest that (b) case just be 6MB or something. Maybe make it
> (_STK_LIM/4*3) or whatever, in case we ever end up changing that
> value.

Sounds good. I'll send a patch...

-Kees

-- 
Kees Cook
Pixel Security