KVM: VMX: Check cpl before emulating debug register access

Debug registers may only be accessed from cpl 0.  Unfortunately, vmx will
code to emulate the instruction even though it was issued from guest
userspace, possibly leading to an unexpected trap later.

Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -620,6 +620,7 @@ void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr);
 void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code);
 void kvm_inject_page_fault(struct kvm_vcpu *vcpu, unsigned long cr2,
 			   u32 error_code);
+bool kvm_require_cpl(struct kvm_vcpu *vcpu, int required_cpl);
 
 int kvm_pic_set_irq(void *opaque, int irq, int level);
 

arch/x86/kvm/vmx.c

@@ -2934,6 +2934,8 @@ static int handle_dr(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	unsigned long val;
 	int dr, reg;
 
+	if (!kvm_require_cpl(vcpu, 0))
+		return 1;
 	dr = vmcs_readl(GUEST_DR7);
 	if (dr & DR7_GD) {
 		/*

arch/x86/kvm/x86.c

@@ -222,6 +222,19 @@ void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code)
 }
 EXPORT_SYMBOL_GPL(kvm_queue_exception_e);
 
+/*
+ * Checks if cpl <= required_cpl; if true, return true.  Otherwise queue
+ * a #GP and return false.
+ */
+bool kvm_require_cpl(struct kvm_vcpu *vcpu, int required_cpl)
+{
+	if (kvm_x86_ops->get_cpl(vcpu) <= required_cpl)
+		return true;
+	kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
+	return false;
+}
+EXPORT_SYMBOL_GPL(kvm_require_cpl);
+
 /*
  * Load the pae pdptrs.  Return true is they are all valid.
  */