Re: [PATCH v3 2/8] KVM: x86: Clear all supported MPX xfeatures if they are not all set

[Mingwei Zhang <mizhang@google.com>]

On Fri, Feb 24, 2023, Aaron Lewis wrote:
> Be a good citizen and don't allow any of the supported MPX xfeatures[1]
> to be set if they can't all be set.  That way userspace or a guest
> doesn't fail if it attempts to set them in XCR0.
> 
> [1] CPUID.(EAX=0DH,ECX=0):EAX.BNDREGS[bit-3]
>     CPUID.(EAX=0DH,ECX=0):EAX.BNDCSR[bit-4]
> 
> Suggested-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Aaron Lewis <aaronlewis@google.com>
> ---
>  arch/x86/kvm/cpuid.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index e1165c196970..b2e7407cd114 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -60,9 +60,22 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
>  	return ret;
>  }
>  
> +static u64 sanitize_xcr0(u64 xcr0)
> +{
> +	u64 mask;
> +
> +	mask = XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR;
> +	if ((xcr0 & mask) != mask)
> +		xcr0 &= ~mask;
> +
> +	return xcr0;
> +}

Is it better to put sanitize_xcr0() into the previous patch? If we do
that, this one will be just adding purely the MPX related logic and thus
cleaner I think.
> +
>  u64 kvm_permitted_xcr0(void)
>  {
> -	return kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
> +	u64 permitted_xcr0 = kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
> +
> +	return sanitize_xcr0(permitted_xcr0);
>  }
>  
>  /*
> -- 
> 2.39.2.637.g21b0678d19-goog
>