KVM: VMX: fix vmx null pointer dereference on debug register access

There is a bug in KVM that can be used to crash a host on Intel
machines. If emulator is tricked into emulating mov to/from DR instruction
it causes NULL pointer dereference on VMX since kvm_x86_ops->(set|get)_dr
are not initialized. Recently this is not exploitable from guest
userspace, but malicious guest kernel can trigger it easily.

CVE-2010-0435

On upstream bug was fixed differently around 2.6.34.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

arch/x86/kvm/x86.c

@@ -2782,6 +2782,9 @@ int emulator_get_dr(struct x86_emulate_ctxt *ctxt, int dr, unsigned long *dest)
 {
 	struct kvm_vcpu *vcpu = ctxt->vcpu;
 
+	if (!kvm_x86_ops->get_dr)
+		return X86EMUL_UNHANDLEABLE;
+
 	switch (dr) {
 	case 0 ... 3:
 		*dest = kvm_x86_ops->get_dr(vcpu, dr);
@@ -2797,6 +2800,9 @@ int emulator_set_dr(struct x86_emulate_ctxt *ctxt, int dr, unsigned long value)
 	unsigned long mask = (ctxt->mode == X86EMUL_MODE_PROT64) ? ~0ULL : ~0U;
 	int exception;
 
+	if (!kvm_x86_ops->set_dr)
+		return X86EMUL_UNHANDLEABLE;
+
 	kvm_x86_ops->set_dr(ctxt->vcpu, dr, value & mask, &exception);
 	if (exception) {
 		/* FIXME: better handling */