KVM: x86 emulator: Forbid modifying CS segment register by mov instruction

commit 8b9f4414 upstream.

Inject #UD if guest attempts to do so. This is in accordance to Intel
SDM.
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/emulate.c

@@ -1875,6 +1875,12 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 		int err;
 
 		sel = c->src.val;
+
+		if (c->modrm_reg == VCPU_SREG_CS) {
+			kvm_queue_exception(ctxt->vcpu, UD_VECTOR);
+			goto done;
+		}
+
 		if (c->modrm_reg == VCPU_SREG_SS)
 			toggle_interruptibility(ctxt, X86_SHADOW_INT_MOV_SS);