KVM: x86: Reject disabling of MWAIT/HLT interception when not allowed

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 always-running APIC timer, or because KVM is
configured to mitigate cross-thread vulnerabilities.

Cc: Kechen Lu <kechenl@nvidia.com>
Fixes: 4d5422c ("KVM: X86: Provide a capability to disable MWAIT intercepts")
Fixes: 6f0f2d5 ("KVM: x86: Mitigate the cross-thread return address predictions bug")
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -4531,6 +4531,20 @@ 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)
@@ -4673,15 +4687,7 @@ 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))
@@ -6528,33 +6534,29 @@ 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())
 			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);