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

Borislav Petkov
2022-12-07T11:02:36Z

On Fri, Dec 02, 2022 at 04:35:30PM -0800, Rick Edgecombe wrote:
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 11a0e06362e4..aab7fa4104d7 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -307,6 +307,7 @@
> #define X86_FEATURE_SGX_EDECCSSA (11*32+18) /* "" SGX EDECCSSA user leaf function */
> #define X86_FEATURE_CALL_DEPTH (11*32+19) /* "" Call depth tracking for RSB stuffing */
> #define X86_FEATURE_MSR_TSX_CTRL (11*32+20) /* "" MSR IA32_TSX_CTRL (Intel) implemented */
> +#define X86_FEATURE_USER_SHSTK (11*32+21) /* Shadow stack support for user mode applications */
>
> /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
> #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */
> @@ -369,6 +370,7 @@
> #define X86_FEATURE_OSPKE (16*32+ 4) /* OS Protection Keys Enable */
> #define X86_FEATURE_WAITPKG (16*32+ 5) /* UMONITOR/UMWAIT/TPAUSE Instructions */
> #define X86_FEATURE_AVX512_VBMI2 (16*32+ 6) /* Additional AVX512 Vector Bit Manipulation Instructions */
> +#define X86_FEATURE_SHSTK (16*32+ 7) /* Shadow Stack */
> #define X86_FEATURE_GFNI (16*32+ 8) /* Galois Field New Instructions */
> #define X86_FEATURE_VAES (16*32+ 9) /* Vector AES */
> #define X86_FEATURE_VPCLMULQDQ (16*32+10) /* Carry-Less Multiplication Double Quadword */

What is the end goal here?

To have X86_FEATURE_KERNEL_SHSTK or so someday too?

If so, then the hardware bit X86_FEATURE_SHSTK should be hidden in
/proc/cpuinfo, i.e.,

#define X86_FEATURE_SHSTK (16*32+ 7) /* "" Shadow Stack hardware support */

note the "", otherwise you'll have people go:

"hey, I have "shstk" in /proc/cpuinfo on my machine. Why doesn't it do
anything?"

Or am I misreading where this is headed?

Thx.

--
Regards/Gruss,
Boris.