[3.13-stable only] KVM: x86: Fix far-jump to non-canonical check

commit 7e46dddd upstream.

[3.13-stable's first backport (f9bffe04) of this commit accidentally omitted
part of the upstream patch (the WARN_ON fixes), supplied here.]

Commit d1442d85 ("KVM: x86: Handle errors when RIP is set during far
jumps") introduced a bug that caused the fix to be incomplete.  Due to
incorrect evaluation, far jump to segment with L bit cleared (i.e., 32-bit
segment) and RIP with any of the high bits set (i.e, RIP[63:32] != 0) set may
not trigger #GP.  As we know, this imposes a security problem.

In addition, the condition for two warnings was incorrect.

Fixes: d1442d85Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
[Add #ifdef CONFIG_X86_64 to avoid complaints of undefined behavior. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Vinson Lee <vlee@twopensource.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

arch/x86/kvm/emulate.c

@@ -1994,7 +1994,7 @@ static int em_jmp_far(struct x86_emulate_ctxt *ctxt)
 
 	rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l);
 	if (rc != X86EMUL_CONTINUE) {
-		WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64);
+		WARN_ON(ctxt->mode != X86EMUL_MODE_PROT64);
 		/* assigning eip failed; restore the old cs */
 		ops->set_segment(ctxt, old_sel, &old_desc, 0, VCPU_SREG_CS);
 		return rc;
@@ -2088,7 +2088,7 @@ static int em_ret_far(struct x86_emulate_ctxt *ctxt)
 		return rc;
 	rc = assign_eip_far(ctxt, eip, new_desc.l);
 	if (rc != X86EMUL_CONTINUE) {
-		WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64);
+		WARN_ON(ctxt->mode != X86EMUL_MODE_PROT64);
 		ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
 	}
 	return rc;