KVM: unbreak userspace that does not sets tss address

Commit 6440e5967bc broke old userspaces that do not set tss address
before entering vcpu. Unbreak it by setting tss address to a safe
value on the first vcpu entry. New userspaces should set tss address,
so print warning in case it doesn't.
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/vmx.c

@@ -180,6 +180,7 @@ static u64 construct_eptp(unsigned long root_hpa);
 static void kvm_cpu_vmxon(u64 addr);
 static void kvm_cpu_vmxoff(void);
 static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3);
+static int vmx_set_tss_addr(struct kvm *kvm, unsigned int addr);
 
 static DEFINE_PER_CPU(struct vmcs *, vmxarea);
 static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
@@ -1762,6 +1763,18 @@ static void enter_rmode(struct kvm_vcpu *vcpu)
 	vmx->emulation_required = 1;
 	vmx->rmode.vm86_active = 1;
 
+	/*
+	 * Very old userspace does not call KVM_SET_TSS_ADDR before entering
+	 * vcpu. Call it here with phys address pointing 16M below 4G.
+	 */
+	if (!vcpu->kvm->arch.tss_addr) {
+		printk_once(KERN_WARNING "kvm: KVM_SET_TSS_ADDR need to be "
+			     "called before entering vcpu\n");
+		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+		vmx_set_tss_addr(vcpu->kvm, 0xfeffd000);
+		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+	}
+
 	vmx->rmode.tr.selector = vmcs_read16(GUEST_TR_SELECTOR);
 	vmx->rmode.tr.base = vmcs_readl(GUEST_TR_BASE);
 	vmcs_writel(GUEST_TR_BASE, rmode_tss_base(vcpu->kvm));