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

Jann Horn
2019-10-18T17:42:53Z

On Fri, Oct 18, 2019 at 6:14 PM Sami Tolvanen <samitolvanen@google.com> wrote:
> This change adds generic support for Clang's Shadow Call Stack, which
> uses a shadow stack to protect return addresses from being overwritten
> by an attacker. Details are available here:
>
> https://clang.llvm.org/docs/ShadowCallStack.html

(As I mentioned in the other thread, the security documentation there
doesn't fit the kernel usecase.)

[...]
> +config SHADOW_CALL_STACK_VMAP
> + def_bool n
> + depends on SHADOW_CALL_STACK
> + help
> + Use virtually mapped shadow call stacks. Selecting this option
> + provides better stack exhaustion protection, but increases per-thread
> + memory consumption as a full page is allocated for each shadow stack.

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. :/

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.