Re: [PATCH v4 03/39] x86/cpufeatures: Add CPU feature flags for shadow stacks

Kees Cook
2022-12-03T02:22:10Z

On Fri, Dec 02, 2022 at 04:35:30PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> The Control-Flow Enforcement Technology contains two related features,
> one of which is Shadow Stacks. Future patches will utilize this feature
> for shadow stack support in KVM, so add a CPU feature flags for Shadow
> Stacks (CPUID.(EAX=7,ECX=0):ECX[bit 7]).
>
> To protect shadow stack state from malicious modification, the registers
> are only accessible in supervisor mode. This implementation
> context-switches the registers with XSAVES. Make X86_FEATURE_SHSTK depend
> on XSAVES.
>
> The shadow stack feature, enumerated by the CPUID bit described above,
> encompasses both supervisor and userspace support for shadow stack. In
> near future patches, only userspace shadow stack will be enabled. In
> expectation of future supervisor shadow stack support, create a software
> CPU capability to enumerate kernel utilization of userspace shadow stack
> support. This will also allow for userspace shadow stack to be disabled,
> while leaving the shadow stack hardware capability exposed in the cpuinfo
> proc. This user shadow stack bit should depend on the HW "shstk"
> capability and that logic will be implemented in future patches.
>
> 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>

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

--
Kees Cook