KVM: prevent spurious exit to userspace during task switch emulation.

If kvm_task_switch() fails code exits to userspace without specifying
exit reason, so the previous exit reason is reused by userspace. Fix
this by specifying exit reason correctly.
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/svm.c

@@ -2266,8 +2266,14 @@ static int task_switch_interception(struct vcpu_svm *svm)
 	     (int_vec == OF_VECTOR || int_vec == BP_VECTOR)))
 		skip_emulated_instruction(&svm->vcpu);
 
-	return kvm_task_switch(&svm->vcpu, tss_selector, reason,
-			       has_error_code, error_code);
+	if (kvm_task_switch(&svm->vcpu, tss_selector, reason,
+				has_error_code, error_code) == EMULATE_FAIL) {
+		svm->vcpu.run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		svm->vcpu.run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
+		svm->vcpu.run->internal.ndata = 0;
+		return 0;
+	}
+	return 1;
 }
 
 static int cpuid_interception(struct vcpu_svm *svm)

arch/x86/kvm/vmx.c

@@ -3316,9 +3316,13 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
 		       type != INTR_TYPE_NMI_INTR))
 		skip_emulated_instruction(vcpu);
 
-	if (!kvm_task_switch(vcpu, tss_selector, reason, has_error_code,
-			     error_code))
+	if (kvm_task_switch(vcpu, tss_selector, reason,
+				has_error_code, error_code) == EMULATE_FAIL) {
+		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
+		vcpu->run->internal.ndata = 0;
 		return 0;
+	}
 
 	/* clear all local breakpoint enable flags */
 	vmcs_writel(GUEST_DR7, vmcs_readl(GUEST_DR7) & ~55);