Re: Playing with virtually mapped stacks (with guard pages!)

[Kees Cook]

On Wed, Jun 15, 2016 at 10:23 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Jun 15, 2016 at 10:05 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Tue, Jun 14, 2016 at 11:01 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> Hi all-
>>>
>>> If you want to play with virtually mapped stacks, I have it more or
>>> less working on x86 in a branch here:
>>>
>>> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/vmap_stack
>>>
>>> The core bit (virtually map the stack and fix the accounting) is just
>>> a config option, but it needs the arch to opt-in.  I suspect that
>>> every arch will have its own set of silly issues to address to make it
>>> work well.  For x86, the silly issues are getting the OOPS to work
>>> right and handling some vmalloc_fault oddities to avoid panicing at
>>> random.
>>
>> Awesome! Some notes/questions:
>>
>> - there are a number of typos in commit messages and comments, just FYI
>
> Not surprising.  I'll try to find and fix them.

x86/cpa: In populate_pgd, don't set the pgd entry until it's
populated: "anyther CPU propages"

x86/cpa: Warn if kernel_unmap_pages_in_pgd is used inappropriately:
"kenrnel entries"

There was another repeated word, but I can't find it now. :P

>>
>> - where is the guard page added? I don't see anything leaving a hole at the end?
>
> Magic!  The vmap code does this for us.

Heh, can you point me to where? Does it have guards at both ends?

>> - where is thread_info? I understand there to be two benefits from
>> vmalloc stack: 1) thread_info can live elsewhere, 2) guard page can
>> exist easily
>
> I think that thread_info is a separate issue except insofar as it's
> needed for full exploit protection.  Moving / eliminating it has
> nothing to do with where the stack lives AFAIK.  I'll get to it.

Okay, cool.

> I suspect that the hardest part will be eliminating the (mostly
> pointless) thread_info::task field.

IIUC, grsecurity puts a thread_info pointer in the percpu area, if
that's any help.

>> - this seems like it should Oops not warn:
>> WARN_ON_ONCE(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
>> that being wrong seems like a very bad state to continue from
>
> I'll change that.
>
>>
>> - bikeshed: I think the CONFIG should live in arch/Kconfig (with a
>> description of what an arch needs to support for it) and be called
>> HAVE_ARCH_VMAP_STACK so that archs can select it instead of having
>> multiple definitions of CONFIG_VMAP_STACK in each arch.
>
> I'll change that, too.

Awesome! :)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security