KVM: x86: Mitigate the cross-thread return address predictions bug

By default, KVM/SVM will intercept attempts by the guest to transition
out of C0. However, the KVM_CAP_X86_DISABLE_EXITS capability can be used
by a VMM to change this behavior. To mitigate the cross-thread return
address predictions bug (X86_BUG_SMT_RSB), a VMM must not be allowed to
override the default behavior to intercept C0 transitions.

Use a module parameter to control the mitigation on processors that are
vulnerable to X86_BUG_SMT_RSB. If the processor is vulnerable to the
X86_BUG_SMT_RSB bug and the module parameter is set to mitigate the bug,
KVM will not allow the disabling of the HLT, MWAIT and CSTATE exits.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <4019348b5e07148eb4d593380a5f6713b93c9a16.1675956146.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -191,6 +191,10 @@ module_param(enable_pmu, bool, 0444);
 bool __read_mostly eager_page_split = true;
 module_param(eager_page_split, bool, 0644);
 
+/* Enable/disable SMT_RSB bug mitigation */
+bool __read_mostly mitigate_smt_rsb;
+module_param(mitigate_smt_rsb, bool, 0444);
+
 /*
  * Restoring the host value for MSRs that are only consumed when running in
  * usermode, e.g. SYSCALL MSRs and TSC_AUX, can be deferred until the CPU
@@ -4448,10 +4452,15 @@ 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_HLT | KVM_X86_DISABLE_EXITS_PAUSE |
+		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())
+
+			if (kvm_can_mwait_in_guest())
 				r |= KVM_X86_DISABLE_EXITS_MWAIT;
+		}
 		break;
 	case KVM_CAP_X86_SMM:
 		if (!IS_ENABLED(CONFIG_KVM_SMM))
@@ -6227,15 +6236,26 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
 			break;
 
+		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_PAUSE)
-			kvm->arch.pause_in_guest = true;
 			if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
 				kvm->arch.cstate_in_guest = true;
+		}
+
 		r = 0;
 		break;
 	case KVM_CAP_MSR_PLATFORM_INFO:
@@ -13456,6 +13476,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_vmgexit_msr_protocol_exit);
 static int __init kvm_x86_init(void)
 {
 	kvm_mmu_x86_module_init();
+	mitigate_smt_rsb &= boot_cpu_has_bug(X86_BUG_SMT_RSB) && cpu_smt_possible();
 	return 0;
 }
 module_init(kvm_x86_init);