daemon: Fix core dumps if unix_sock_group is set

Setting unix_sock_group to something else than default "root" in
/etc/libvirt/libvirtd.conf prevents system libvirtd from dumping core on
crash. This is because we used setgid(unix_sock_group) before binding to
/var/run/libvirt/libvirt-sock* and setgid() back to original group.
However, if a process changes its effective or filesystem group ID, it
will be forbidden from leaving core dumps unless fs.suid_dumpable sysctl
is set to something else then 0 (and it is 0 by default).

Changing socket's group ownership after bind works better. And we can do
so without introducing a race condition since we loosen access rights by
changing the group from root to something else.

diff --git a/daemon/libvirtd.c b/daemon/libvirtd.c
index 2df93372791441ed5aa010a2d0beec31eadfe5b6..b1539b10d37e94542af952efb7e036a98c43cafb 100644 (file)
--- a/daemon/libvirtd.c
+++ b/daemon/libvirtd.c
@@ -542,7 +542,6 @@ static int qemudListenUnix(struct qemud_server *server,
                            char *path, int readonly, int auth) {
     struct qemud_socket *sock;
     mode_t oldmask;
-    gid_t oldgrp;
     char ebuf[1024];
 
     if (VIR_ALLOC(sock) < 0) {
@@ -579,21 +578,21 @@ static int qemudListenUnix(struct qemud_server *server,
     if (sock->addr.data.un.sun_path[0] == '@')
         sock->addr.data.un.sun_path[0] = '\0';
 
-    oldgrp = getgid();
     oldmask = umask(readonly ? ~unix_sock_ro_mask : ~unix_sock_rw_mask);
-    if (server->privileged && setgid(unix_sock_gid)) {
-        VIR_ERROR(_("Failed to set group ID to %d"), unix_sock_gid);
-        goto cleanup;
-    }
-
     if (bind(sock->fd, &sock->addr.data.sa, sock->addr.len) < 0) {
         VIR_ERROR(_("Failed to bind socket to '%s': %s"),
                   path, virStrerror(errno, ebuf, sizeof ebuf));
         goto cleanup;
     }
     umask(oldmask);
-    if (server->privileged && setgid(oldgrp)) {
-        VIR_ERROR(_("Failed to restore group ID to %d"), oldgrp);
+
+    /* chown() doesn't work for abstract sockets but we use them only
+     * if libvirtd runs unprivileged
+     */
+    if (server->privileged && chown(path, -1, unix_sock_gid)) {
+        VIR_ERROR(_("Failed to change group ID of '%s' to %d: %s"),
+                  path, unix_sock_gid,
+                  virStrerror(errno, ebuf, sizeof ebuf));
         goto cleanup;
     }