On Thu, Jan 19, 2023 at 01:22:41PM -0800, Rick Edgecombe wrote:

From: Yu-cheng Yu <redacted>

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 user shadow stack bit should depend on the HW "shstk"
capability and that logic will be implemented in future patches.

Tested-by: Pengfei Xu <redacted>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Yu-cheng Yu <redacted>

Reviewed-by: Kees Cook <redacted>

-- 
Kees Cook