Don't crash if a connection closes early

https://bugzilla.redhat.com/show_bug.cgi?id=1047577

When a client closes its connection to libvirtd early during
virConnectOpen, more specifically just after making
REMOTE_PROC_CONNECT_SUPPORTS_FEATURE call to check if
VIR_DRV_FEATURE_PROGRAM_KEEPALIVE is supported without even waiting for
the result, libvirtd may crash due to a race in keep-alive
initialization. Once receiving the REMOTE_PROC_CONNECT_SUPPORTS_FEATURE
call, the daemon's event loop delegates it to a worker thread. In case
the event loop detects EOF on the connection and calls
virNetServerClientClose before the worker thread starts to handle
REMOTE_PROC_CONNECT_SUPPORTS_FEATURE call, client->keepalive will be
disposed by the time virNetServerClientStartKeepAlive gets called from
remoteDispatchConnectSupportsFeature. Because the flow is common for
both authenticated and read-only connections, even unprivileged clients
may cause the daemon to crash.

To avoid the crash, virNetServerClientStartKeepAlive needs to check if
the connection is still open before starting keep-alive protocol.

Every libvirt release since 0.9.8 is affected by this bug.

src/rpc/virnetserverclient.c

@@ -1539,9 +1539,22 @@ cleanup:
 int
 virNetServerClientStartKeepAlive(virNetServerClientPtr client)
 {
-    int ret;
+    int ret = -1;
+
     virObjectLock(client);
+
+    /* The connection might have been closed before we got here and thus the
+     * keepalive object could have been removed too.
+     */
+    if (!client->sock) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("connection not open"));
+        goto cleanup;
+    }
+
     ret = virKeepAliveStart(client->keepalive, 0, 0);
+
+cleanup:
     virObjectUnlock(client);
     return ret;
 }