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

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Jun 15, 2016 at 10:05 AM, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Jun 14, 2016 at 11:01 PM, Andy Lutomirski <luto@amacapital.net> 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.

>
> - 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.

>
> - 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.

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

>
> - 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.

>
> Thanks for digging into this!
>
> -Kees
>
> --
> Kees Cook
> Chrome OS & Brillo Security



-- 
Andy Lutomirski
AMA Capital Management, LLC

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