linux-kernel - Re: [PATCH v8 02/26] x86/fpu/xstate: Refine CET user xstate bit enabling

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Date: Wed, 03 Jan 2024 00:24:44 +0200
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Yang Weijiang <weijiang.yang@...el.com>, seanjc@...gle.com, 
	pbonzini@...hat.com, dave.hansen@...el.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, chao.gao@...el.com, rick.p.edgecombe@...el.com, 
	john.allen@....com
Subject: Re: [PATCH v8 02/26] x86/fpu/xstate: Refine CET user xstate bit
 enabling

On Thu, 2023-12-21 at 09:02 -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 existing
> dependency check is correct when only user mode SHSTK is available.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
>  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 07911532b108..f6b98693da59 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);
> +

I am still not convinced that this is not a workaround for a bug in the sanity check code,
and I don't really like this, but whatever, as long as the code works, 
I don't intend to fight over this. Let it be.

Best regards,
	Maxim Levitsky


>  	if (!cpu_feature_enabled(X86_FEATURE_XFD))
>  		fpu_kernel_cfg.max_features &= ~XFEATURE_MASK_USER_DYNAMIC;
>