Re: [PATCH] KVM: x86: Disallow EFER.LMSLE when EferLmsleUnsupported is set in guest CPUID

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kvm: x86: EFER.LMSLE fix is ineffective due to dropped CPUID bit
--

commit 0daddb3cb6cd07afcd9545a8e1643427a1542393
Author: Jim Mattson <jmattson@google.com>

KVM: x86: Disallow EFER.LMSLE when EferLmsleUnsupported is set in guest CPUID

This commit updates KVM to reject guest writes to EFER that set EFER.LMSLE
when the EferLmsleUnsupported bit is set in the guest's CPUID. It aims to fix
an issue where a guest could enable EFER.LMSLE on hosts that support it, even
if the VMM hid the feature for migration compatibility.

> diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
> index 37d5bed3529f5..c8f894ccfb0f9 100644
> --- a/arch/x86/kvm/msrs.c
> +++ b/arch/x86/kvm/msrs.c
> @@ -598,8 +598,11 @@ static bool __kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
>  	if (efer & EFER_NX && !guest_cpu_cap_has(vcpu, X86_FEATURE_NX))
>  		return false;
>  
> -	return true;
> +	if (efer & EFER_LMSLE &&
> +	    guest_cpu_cap_has(vcpu, X86_FEATURE_EFER_LMSLE_MBZ))

[Severity: High]
Does this check actually work as intended on hosts that support LMSLE?

Because X86_FEATURE_EFER_LMSLE_MBZ is defined as a PASSTHROUGH_F,
kvm_cpu_cap_init() clears this bit in kvm_cpu_caps when the host hardware
CPUID does not set it (which is the case when the host supports LMSLE).

Since the bit is missing from kvm_cpu_caps and is not an EMULATED_F,
kvm_set_cpuid() ANDs the VMM-provided CPUID with 0 for this capability.
Consequently, vcpu->arch.cpu_caps does not retain the bit, and
guest_cpu_cap_has() here will unconditionally return false.

Does this mean the bug remains triggerable by a guest running on a host with
LMSLE support when the VMM attempts to hide the feature?

> +		return false;
>  
> +	return true;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260918154530.4129698-1-jmattson@google.com?part=1