High: libcrmcommon: fix CVE-2016-7035 (improper IPC guarding)

It was discovered that at some not so uncommon circumstances, some
pacemaker daemons could be talked to, via libqb-facilitated IPC, by
unprivileged clients due to flawed authorization decision.  Depending
on the capabilities of affected daemons, this might equip unauthorized
user with local privilege escalation or up to cluster-wide remote
execution of possibly arbitrary commands when such user happens to
reside at standard or remote/guest cluster node, respectively.

The original vulnerability was introduced in an attempt to allow
unprivileged IPC clients to clean up the file system materialized
leftovers in case the server (otherwise responsible for the lifecycle
of these files) crashes.  While the intended part of such behavior is
now effectively voided (along with the unintended one), a best-effort
fix to address this corner case systemically at libqb is coming along
(ClusterLabs/libqb#231).

Affected versions:  1.1.10-rc1 (2013-04-17) - 1.1.15 (2016-06-21)
Impact:             Important
CVSSv3 ranking:     8.8 : AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Credits for independent findings, in chronological order:
  Jan "poki" Pokorný, of Red Hat
  Alain Moulle, of ATOS/BULL

lib/common/ipc.c

@@ -293,7 +293,6 @@ crm_client_disconnect_all(qb_ipcs_service_t *service)
 crm_client_t *
 crm_client_new(qb_ipcs_connection_t * c, uid_t uid_client, gid_t gid_client)
 {
-    static uid_t uid_server = 0;
     static gid_t gid_cluster = 0;
 
     crm_client_t *client = NULL;
@@ -304,7 +303,6 @@ crm_client_new(qb_ipcs_connection_t * c, uid_t uid_client, gid_t gid_client)
     }
 
     if (gid_cluster == 0) {
-        uid_server = getuid();
         if(crm_user_lookup(CRM_DAEMON_USER, NULL, &gid_cluster) < 0) {
             static bool have_error = FALSE;
             if(have_error == FALSE) {
@@ -314,16 +312,10 @@ crm_client_new(qb_ipcs_connection_t * c, uid_t uid_client, gid_t gid_client)
         }
     }
 
-    if(gid_cluster != 0 && gid_client != 0) {
-        uid_t best_uid = -1; /* Passing -1 to chown(2) means don't change */
-
-        if(uid_client == 0 || uid_server == 0) { /* Someone is priveliged, but the other may not be */
-            best_uid = QB_MAX(uid_client, uid_server);
-            crm_trace("Allowing user %u to clean up after disconnect", best_uid);
-        }
-
+    if (uid_client != 0) {
         crm_trace("Giving access to group %u", gid_cluster);
-        qb_ipcs_connection_auth_set(c, best_uid, gid_cluster, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP);
+        /* Passing -1 to chown(2) means don't change */
+        qb_ipcs_connection_auth_set(c, -1, gid_cluster, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP);
     }
 
     crm_client_init();