Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

Sami Tolvanen
2019-10-18T17:56:35Z

On Fri, Oct 18, 2019 at 10:42 AM Jann Horn <jannh@google.com> wrote:
> (As I mentioned in the other thread, the security documentation there
> doesn't fit the kernel usecase.)

True. I'll add a note about it here too.

> Without CONFIG_SHADOW_CALL_STACK_VMAP, after 128 small stack frames,
> you overflow into random physmap memory even if the main stack is
> vmapped... I guess you can't get around that without making the SCS
> instrumentation more verbose. :/

That's correct. In our testing, 128 stack frames is nearly twice the
maximum amount that's been used (on an arm64 device), and for many use
cases, allocating a full page is simply too costly despite the
advantages.

> Could you maybe change things so that independent of whether you have
> vmapped SCS or slab-allocated SCS, the scs_corrupted() check looks at
> offset 1024-8 (where it currently is for the slab-allocated case)?
> That way, code won't suddenly stop working when you disable
> CONFIG_SHADOW_CALL_STACK_VMAP; and especially if you use
> CONFIG_SHADOW_CALL_STACK_VMAP for development and testing but disable
> it in production, that would be annoying.

Yes, that's a great idea. I'll change this in v2.

Sami