Should become root in the namespace after joined user ns

fix: #4466, in containerd, the net and user ns has been created
before start the container, and let runc join these two ns when
starting the init process, it works for normal system, except
the system with selinux enabling and has mount label configed.
We can resolve it with two steps:
1. Join the user ns after joined all other namespaces;
2. If we have joined a user ns path, we should also become root
in the namespace, like what we do in unsharing a new user ns.

Signed-off-by: lifubang <lifubang@acmcoder.com>

libcontainer/nsenter/nsexec.c

@@ -444,9 +444,14 @@ void nl_free(struct nlconfig_t *config)
 	free(config->data);
 }
 
-void join_namespaces(char *nslist)
+/*
+  Join all namespaces except user ns(issue: #4466),
+  we return user ns fd to left it join in the next step
+  if there is.
+*/
+int join_namespaces(char *nslist)
 {
-	int num = 0, i;
+	int num = 0, userns_fd = -1, i;
 	char *saveptr = NULL;
 	char *namespace = strtok_r(nslist, ",", &saveptr);
 	struct namespace_t {
@@ -501,6 +506,11 @@ void join_namespaces(char *nslist)
 		struct namespace_t *ns = &namespaces[i];
 		int flag = nsflag(ns->type);
 
+		if (flag == CLONE_NEWUSER) {
+			userns_fd = ns->fd;
+			continue;
+		}
+
 		write_log(DEBUG, "setns(%#x) into %s namespace (with path %s)", flag, ns->type, ns->path);
 		if (setns(ns->fd, flag) < 0)
 			bail("failed to setns into %s namespace", ns->type);
@@ -509,6 +519,7 @@ void join_namespaces(char *nslist)
 	}
 
 	free(namespaces);
+	return userns_fd;
 }
 
 static inline int sane_kill(pid_t pid, int signum)
@@ -829,6 +840,8 @@ void nsexec(void)
 	case STAGE_CHILD:{
 			pid_t stage2_pid = -1;
 			enum sync_t s;
+			int userns_fd = -1;
+			bool has_userns = false;
 
 			/* For debugging. */
 			current_stage = STAGE_CHILD;
@@ -849,7 +862,7 @@ void nsexec(void)
 			 * using cmsg(3) but that's just annoying.
 			 */
 			if (config.namespaces)
-				join_namespaces(config.namespaces);
+				userns_fd = join_namespaces(config.namespaces);
 
 			/*
 			 * Deal with user namespaces first. They are quite special, as they
@@ -870,6 +883,7 @@ void nsexec(void)
 			 * in some scenarios. This also mirrors how LXC deals with this
 			 * problem.
 			 */
+			has_userns = (config.cloneflags & CLONE_NEWUSER) || userns_fd > -1;
 			if (config.cloneflags & CLONE_NEWUSER) {
 				try_unshare(CLONE_NEWUSER, "user namespace");
 				config.cloneflags &= ~CLONE_NEWUSER;
@@ -907,7 +921,13 @@ void nsexec(void)
 					if (prctl(PR_SET_DUMPABLE, 0, 0, 0, 0) < 0)
 						bail("failed to re-set process as non-dumpable");
 				}
-
+			}
+			if (userns_fd > -1) {
+				if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+					bail("failed to join user namespace");
+				}
+			}
+			if (has_userns) {
 				/* Become root in the namespace proper. */
 				if (setresuid(0, 0, 0) < 0)
 					bail("failed to become root in user namespace");