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

From: Kees Cook
Date: Thu Jul 06 2017 - 13:29:41 EST

On Thu, Jul 6, 2017 at 9:34 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Jul 5, 2017 at 9:32 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> In an attempt to provide sensible rlimit defaults for setuid execs, this
>> inherits the namespace's init rlimits:
> Yeah, so I have to admit to hating this patch.

Yup! Totally fine. I just wanted to demonstrate the idea with some actual code.

> I'd much rather see something like
> (a) minimal: just use our existing default stack (and stack _only_)
> limit value for suid binaries that actually get extra permissions: {

This would look a lot like the existing patch; it'd just not copy the
init process rlimits.

> (c) perhaps encourage people to annotate their suid binaries with
> initial resource requirements (and for stack, I mean the existing
> GNU_STACK ELF annotation in particular).
> For an example of (a), that existing _STK_LIM define is what the
> kernel defaults to, and it's a 8MB stack. And looking at my Fedora
> install, I see that the default user rlimit is 8MB for the stack.
> [...]
> Is that just coincidence, or is that just a sign of "nobody ever even
> modifies the default value"? So (a) feels like "nobody really cares,
> and 8MB is fine, and nobody even bothers changing it - just do the
> minimal thing".

I would be terrified to see a setuid binary that needed >8MB stack. :P

> And (c) would be the sane option, and what we already do for things
> like GNU_STACK to enable/disable executable stacks. It really feels
> like allowing the GNU_STACK segment to contain stack rlimit override
> information would be the perfect tool for binaries to say "Yeah, I
> need more stack than _STK_LIM".
> So I see many different approaches (that could be combined: I like
> combining (a) and (c), for example), and absolutely none of them
> involve the random "take some values from init".
> And yes, a large part of this may be that I no longer feel like I can
> trust "init" to do the sane thing. You all presumably know why.

Heh. Yeah, I think a+c could make sense. "if not GNU_STACK program
header or 0 MemSiz GNU_STACK program header, use _STK_LIM, otherwise
use GNU_STACK MemSiz". I'll send a patch...


Kees Cook
Pixel Security