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

[Andy Lutomirski]

On Thu, Jul 6, 2017 at 10:45 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Jul 6, 2017 at 10:36 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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.