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

[Kees Cook]

On Thu, Jul 6, 2017 at 10:36 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Thu, Jul 6, 2017 at 10:15 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Thu, Jul 6, 2017 at 10:10 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> On Thu, Jul 6, 2017 at 9:48 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>> How about a much simpler solution: don't read rlimit at all in
>>>> copy_strings(), let alone try to enforce it.  Instead, just before the
>>>> point of no return, check how much stack space is already used and, if
>>>> it's more than an appropriate threshold (e.g. 1/4 of the rlimit),
>>>> abort.  Sure, this adds overhead if we're going to abort, but does
>>>> that really matter?
>>>
>>> We should avoid using up tons of memory and then failing. Better to
>>> cap it as we use it. Plumbing a sane value into this shouldn't be hard
>>> at all. Just making this a hardcoded 2MB seems sane (1/4 of 8MB).
>
> 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;

>>> IIUC, this is a big deal on 32-bit. Unlimited stack triggers top-down
>>> mmap instead of bottom-up. I mean, I'd be delighted to get rid of
>>> this, but I thought it was relied on by userspace.
>>
>> 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):
>
> Uhh, crikey!  Where's the code that does that?

That was the call path I quoted earlier:

> 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

i.e. arch_pick_mmap_layout().

-Kees

-- 
Kees Cook
Pixel Security