Re: [PATCH v4 31/39] x86/shstk: Support wrss for userspace

Kees Cook
2022-12-03T02:52:40Z

On Fri, Dec 02, 2022 at 04:35:58PM -0800, Rick Edgecombe wrote:
> For the current shadow stack implementation, shadow stacks contents can't
> easily be provisioned with arbitrary data. This property helps apps
> protect themselves better, but also restricts any potential apps that may
> want to do exotic things at the expense of a little security.
>
> The x86 shadow stack feature introduces a new instruction, wrss, which
> can be enabled to write directly to shadow stack permissioned memory from
> userspace. Allow it to get enabled via the prctl interface.
>
> Only enable the userspace wrss instruction, which allows writes to
> userspace shadow stacks from userspace. Do not allow it to be enabled
> independently of shadow stack, as HW does not support using WRSS when
> shadow stack is disabled.
>
> From a fault handler perspective, WRSS will behave very similar to WRUSS,
> which is treated like a user access from a #PF err code perspective.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook