Re: [PATCH v5 07/39] x86: Add user control-protection fault handler

Kees Cook
2023-01-20T00:50:32Z

On Thu, Jan 19, 2023 at 01:22:45PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> A control-protection fault is triggered when a control-flow transfer
> attempt violates Shadow Stack or Indirect Branch Tracking constraints.
> For example, the return address for a RET instruction differs from the copy
> on the shadow stack.
>
> There already exists a control-protection fault handler for handling kernel
> IBT faults. Refactor this fault handler into separate user and kernel
> handlers, like the page fault handler. Add a control-protection handler
> for usermode. To avoid ifdeffery, put them both in a new file cet.c, which
> is compiled in the case of either of the two CET features supported in the
> kernel: kernel IBT or user mode shadow stack. Move some static inline
> functions from traps.c into a header so they can be used in cet.c.
>
> Opportunistically fix a comment in the kernel IBT part of the fault
> handler that is on the end of the line instead of preceding it.
>
> Keep the same behavior for the kernel side of the fault handler, except for
> converting a BUG to a WARN in the case of a #CP happening when the feature
> is missing. This unifies the behavior with the new shadow stack code, and
> also prevents the kernel from crashing under this situation which is
> potentially recoverable.
>
> The control-protection fault handler works in a similar way as the general
> protection fault handler. It provides the si_code SEGV_CPERR to the signal
> handler.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

This diff would have been a bit easier to review if the file move was
separate from the addition of the handler, but regardless:

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

--
Kees Cook