selinux: fix bug in conditional rules handling

commit fa1aa14 ("selinux: extended permissions for ioctls")
introduced a bug into the handling of conditional rules, skipping the
processing entirely when the caller does not provide an extended
permissions (xperms) structure.  Access checks from userspace using
/sys/fs/selinux/access do not include such a structure since that
interface does not presently expose extended permission information.
As a result, conditional rules were being ignored entirely on userspace
access requests, producing denials when access was allowed by
conditional rules in the policy.  Fix the bug by only skipping
computation of extended permissions in this situation, not the entire
conditional rules processing.

Reported-by: Laurent Bigonville <bigon@debian.org>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: fixed long lines in patch description]
Cc: stable@vger.kernel.org # 4.3
Signed-off-by: Paul Moore <pmoore@redhat.com>

Author: stephensmalley

security/selinux/ss/conditional.c

@@ -638,7 +638,7 @@ void cond_compute_av(struct avtab *ctab, struct avtab_key *key,
 {
 	struct avtab_node *node;
 
-	if (!ctab || !key || !avd || !xperms)
+	if (!ctab || !key || !avd)
 		return;
 
 	for (node = avtab_search_node(ctab, key); node;
@@ -657,7 +657,7 @@ void cond_compute_av(struct avtab *ctab, struct avtab_key *key,
 		if ((u16)(AVTAB_AUDITALLOW|AVTAB_ENABLED) ==
 		    (node->key.specified & (AVTAB_AUDITALLOW|AVTAB_ENABLED)))
 			avd->auditallow |= node->datum.u.data;
-		if ((node->key.specified & AVTAB_ENABLED) &&
+		if (xperms && (node->key.specified & AVTAB_ENABLED) &&
 				(node->key.specified & AVTAB_XPERMS))
 			services_compute_xperms_drivers(xperms, node);
 	}