Re: [PATCH v7 02/26] x86/fpu/xstate: Refine CET user xstate bit enabling

[Peter Zijlstra]

On Fri, Nov 24, 2023 at 12:53:06AM -0500, Yang Weijiang wrote:
> Remove XFEATURE_CET_USER entry from dependency array as the entry doesn't
> reflect true dependency between CET features and the user xstate bit.
> Enable the bit in fpu_kernel_cfg.max_features when either SHSTK or IBT is
> available.
> 
> Both user mode shadow stack and indirect branch tracking features depend
> on XFEATURE_CET_USER bit in XSS to automatically save/restore user mode
> xstate registers, i.e., IA32_U_CET and IA32_PL3_SSP whenever necessary.
> 
> Note, the issue, i.e., CPUID only enumerates IBT but no SHSTK is resulted
> from CET KVM series which synthesizes guest CPUIDs based on userspace
> settings,in real world the case is rare. In other words, the exitings
> dependency check is correct when only user mode SHSTK is available.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  arch/x86/kernel/fpu/xstate.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 73f6bc00d178..6e50a4251e2b 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -73,7 +73,6 @@ static unsigned short xsave_cpuid_features[] __initdata = {
>  	[XFEATURE_PT_UNIMPLEMENTED_SO_FAR]	= X86_FEATURE_INTEL_PT,
>  	[XFEATURE_PKRU]				= X86_FEATURE_OSPKE,
>  	[XFEATURE_PASID]			= X86_FEATURE_ENQCMD,
> -	[XFEATURE_CET_USER]			= X86_FEATURE_SHSTK,
>  	[XFEATURE_XTILE_CFG]			= X86_FEATURE_AMX_TILE,
>  	[XFEATURE_XTILE_DATA]			= X86_FEATURE_AMX_TILE,
>  };
> @@ -798,6 +797,14 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
>  			fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
>  	}
>  
> +	/*
> +	 * CET user mode xstate bit has been cleared by above sanity check.
> +	 * Now pick it up if either SHSTK or IBT is available. Either feature
> +	 * depends on the xstate bit to save/restore user mode states.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_SHSTK) || boot_cpu_has(X86_FEATURE_IBT))
> +		fpu_kernel_cfg.max_features |= BIT_ULL(XFEATURE_CET_USER);

So booting a host with "ibt=off" will clear the FEATURE_IBT, this was
fine before this patch-set, but possibly not with.

That kernel argument really only wants to tell the kernel not to use IBT
itself, but not inhibit IBT from being used by guests.