Re: [PATCH v2 12/49] KVM: x86: Reject disabling of MWAIT/HLT interception when not allowed

[Xiaoyao Li]

On 5/18/2024 1:38 AM, Sean Christopherson wrote:
> Reject KVM_CAP_X86_DISABLE_EXITS if userspace attempts to disable MWAIT or
> HLT exits and KVM previously reported (via KVM_CHECK_EXTENSION) that
> disabling the exit(s) is not allowed.  E.g. because MWAIT isn't supported
> or the CPU doesn't have an aways-running APIC timer, or because KVM is
> configured to mitigate cross-thread vulnerabilities.
>
> Cc: Kechen Lu <kechenl@xxxxxxxxxx>
> Fixes: 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT intercepts")
> Fixes: 6f0f2d5ef895 ("KVM: x86: Mitigate the cross-thread return address predictions bug")
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Just realize the same issue when reading the MWAIT code then find your 
this fix.

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>

> ---
>   arch/x86/kvm/x86.c | 54 ++++++++++++++++++++++++----------------------
>   1 file changed, 28 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4cb0c150a2f8..c729227c6501 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4590,6 +4590,20 @@ static inline bool kvm_can_mwait_in_guest(void)
>   		boot_cpu_has(X86_FEATURE_ARAT);
>   }
>   
> +static u64 kvm_get_allowed_disable_exits(void)
> +{
> +	u64 r = KVM_X86_DISABLE_EXITS_PAUSE;
> +
> +	if (!mitigate_smt_rsb) {
> +		r |= KVM_X86_DISABLE_EXITS_HLT |
> +			KVM_X86_DISABLE_EXITS_CSTATE;
> +
> +		if (kvm_can_mwait_in_guest())
> +			r |= KVM_X86_DISABLE_EXITS_MWAIT;
> +	}
> +	return r;
> +}
> +
>   #ifdef CONFIG_KVM_HYPERV
>   static int kvm_ioctl_get_supported_hv_cpuid(struct kvm_vcpu *vcpu,
>   					    struct kvm_cpuid2 __user *cpuid_arg)
> @@ -4726,15 +4740,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   		r = KVM_CLOCK_VALID_FLAGS;
>   		break;
>   	case KVM_CAP_X86_DISABLE_EXITS:
> -		r = KVM_X86_DISABLE_EXITS_PAUSE;
> -
> -		if (!mitigate_smt_rsb) {
> -			r |= KVM_X86_DISABLE_EXITS_HLT |
> -			     KVM_X86_DISABLE_EXITS_CSTATE;
> -
> -			if (kvm_can_mwait_in_guest())
> -				r |= KVM_X86_DISABLE_EXITS_MWAIT;
> -		}
> +		r |= kvm_get_allowed_disable_exits();
>   		break;
>   	case KVM_CAP_X86_SMM:
>   		if (!IS_ENABLED(CONFIG_KVM_SMM))
> @@ -6565,33 +6571,29 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>   		break;
>   	case KVM_CAP_X86_DISABLE_EXITS:
>   		r = -EINVAL;
> -		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
> +		if (cap->args[0] & ~kvm_get_allowed_disable_exits())

sigh.

KVM_X86_DISABLE_VALID_EXITS has no user now. But we cannot remove it 
since it's in uapi header, right?

>   			break;
>   
>   		mutex_lock(&kvm->lock);
>   		if (kvm->created_vcpus)
>   			goto disable_exits_unlock;
>   
> -		if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
> -			kvm->arch.pause_in_guest = true;
> -
>   #define SMT_RSB_MSG "This processor is affected by the Cross-Thread Return Predictions vulnerability. " \
>   		    "KVM_CAP_X86_DISABLE_EXITS should only be used with SMT disabled or trusted guests."
>   
> -		if (!mitigate_smt_rsb) {
> -			if (boot_cpu_has_bug(X86_BUG_SMT_RSB) && cpu_smt_possible() &&
> -			    (cap->args[0] & ~KVM_X86_DISABLE_EXITS_PAUSE))
> -				pr_warn_once(SMT_RSB_MSG);
> -
> -			if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
> -			    kvm_can_mwait_in_guest())
> -				kvm->arch.mwait_in_guest = true;
> -			if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
> -				kvm->arch.hlt_in_guest = true;
> -			if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
> -				kvm->arch.cstate_in_guest = true;
> -		}
> +		if (!mitigate_smt_rsb && boot_cpu_has_bug(X86_BUG_SMT_RSB) &&
> +		    cpu_smt_possible() &&
> +		    (cap->args[0] & ~KVM_X86_DISABLE_EXITS_PAUSE))
> +			pr_warn_once(SMT_RSB_MSG);
>   
> +		if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
> +			kvm->arch.pause_in_guest = true;
> +		if (cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT)
> +			kvm->arch.mwait_in_guest = true;
> +		if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
> +			kvm->arch.hlt_in_guest = true;
> +		if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
> +			kvm->arch.cstate_in_guest = true;
>   		r = 0;
>   disable_exits_unlock:
>   		mutex_unlock(&kvm->lock);