attach: improve id switching

src/lxc/attach.c

@@ -726,6 +726,8 @@ static int attach_child_main(struct attach_clone_payload *payload)
 	int fd, lsm_fd, ret;
 	uid_t new_uid;
 	gid_t new_gid;
+	uid_t ns_root_uid = 0;
+	gid_t ns_root_gid = 0;
 	lxc_attach_options_t* options = payload->options;
 	struct lxc_proc_context_info* init_ctx = payload->init_ctx;
 	bool needs_lsm = (options->namespaces & CLONE_NEWNS) &&
@@ -809,25 +811,40 @@ static int attach_child_main(struct attach_clone_payload *payload)
 	/* Ignore errors, we will fall back to root in that case (/proc was not
 	 * mounted etc.).
 	 */
-	if (options->namespaces & CLONE_NEWUSER)
-		lxc_attach_get_init_uidgid(&new_uid, &new_gid);
+	if (options->namespaces & CLONE_NEWUSER) {
+		/* Check whether nsuid 0 has a mapping. */
+		ns_root_uid = get_ns_uid(0);
 
-	if (options->uid != (uid_t)-1)
-		new_uid = options->uid;
-	if (options->gid != (gid_t)-1)
-		new_gid = options->gid;
+		/* Check whether nsgid 0 has a mapping. */
+		ns_root_gid = get_ns_gid(0);
 
-	/* Try to set the {u,g}id combination. */
-	if (new_uid != 0 || new_gid != 0 || options->namespaces & CLONE_NEWUSER) {
-		ret = lxc_switch_uid_gid(new_uid, new_gid);
-		if (ret < 0)
+		/* If there's no mapping for nsuid 0 try to retrieve the nsuid
+		 * init was started with.
+		 */
+		if (ns_root_uid == LXC_INVALID_UID)
+			lxc_attach_get_init_uidgid(&ns_root_uid, &ns_root_gid);
+
+		if (ns_root_uid == LXC_INVALID_UID)
 			goto on_error;
 
-		ret = lxc_setgroups(0, NULL);
-		if (ret < 0)
+		if (!lxc_switch_uid_gid(ns_root_uid, ns_root_gid))
 			goto on_error;
 	}
 
+	if (!lxc_setgroups(0, NULL) && errno != EPERM)
+		goto on_error;
+
+	/* Set {u,g}id. */
+	if (options->uid != LXC_INVALID_UID)
+		new_uid = options->uid;
+	else
+		new_uid = ns_root_uid;
+
+	if (options->gid != LXC_INVALID_GID)
+		new_gid = options->gid;
+	else
+		new_gid = ns_root_gid;
+
 	if (needs_lsm) {
 		bool on_exec;
 
@@ -909,6 +926,16 @@ static int attach_child_main(struct attach_clone_payload *payload)
 		TRACE("Prepared pty file descriptor %d", payload->pty_fd);
 	}
 
+	/* Avoid unnecessary syscalls. */
+	if (new_uid == ns_root_uid)
+		new_uid = LXC_INVALID_UID;
+
+	if (new_gid == ns_root_gid)
+		new_gid = LXC_INVALID_GID;
+
+	if (!lxc_switch_uid_gid(new_uid, new_gid))
+		goto on_error;
+
 	/* We're done, so we can now do whatever the user intended us to do. */
 	rexit(payload->exec_function(payload->exec_payload));
 

src/lxc/utils.c

@@ -1001,17 +1001,18 @@ void **lxc_append_null_to_array(void **array, size_t count)
 
 int randseed(bool srand_it)
 {
+	FILE *f;
 	/*
-	   srand pre-seed function based on /dev/urandom
-	   */
+	 * srand pre-seed function based on /dev/urandom
+	 */
 	unsigned int seed = time(NULL) + getpid();
 
-	FILE *f;
 	f = fopen("/dev/urandom", "r");
 	if (f) {
 		int ret = fread(&seed, sizeof(seed), 1, f);
 		if (ret != 1)
-			DEBUG("unable to fread /dev/urandom, %s, fallback to time+pid rand seed", strerror(errno));
+			SYSDEBUG("Unable to fread /dev/urandom, fallback to time+pid rand seed");
+
 		fclose(f);
 	}
 
@@ -1026,26 +1027,62 @@ uid_t get_ns_uid(uid_t orig)
 	char *line = NULL;
 	size_t sz = 0;
 	uid_t nsid, hostid, range;
-	FILE *f = fopen("/proc/self/uid_map", "r");
-	if (!f)
+	FILE *f;
+
+	f = fopen("/proc/self/uid_map", "r");
+	if (!f) {
+		SYSERROR("Failed to open uid_map");
 		return 0;
+	}
 
 	while (getline(&line, &sz, f) != -1) {
 		if (sscanf(line, "%u %u %u", &nsid, &hostid, &range) != 3)
 			continue;
+
 		if (hostid <= orig && hostid + range > orig) {
 			nsid += orig - hostid;
 			goto found;
 		}
 	}
 
-	nsid = 0;
+	nsid = LXC_INVALID_UID;
+
 found:
 	fclose(f);
 	free(line);
 	return nsid;
 }
 
+gid_t get_ns_gid(gid_t orig)
+{
+	char *line = NULL;
+	size_t sz = 0;
+	gid_t nsid, hostid, range;
+	FILE *f;
+
+	f = fopen("/proc/self/gid_map", "r");
+	if (!f) {
+		SYSERROR("Failed to open gid_map");
+		return 0;
+	}
+
+	while (getline(&line, &sz, f) != -1) {
+		if (sscanf(line, "%u %u %u", &nsid, &hostid, &range) != 3)
+			continue;
+
+		if (hostid <= orig && hostid + range > orig) {
+			nsid += orig - hostid;
+			goto found;
+		}
+	}
+
+	nsid = LXC_INVALID_GID;
+
+found:
+	fclose(f);
+	free(line);
+	return nsid;
+}
 bool dir_exists(const char *path)
 {
 	struct stat sb;

src/lxc/utils.h

@@ -415,6 +415,7 @@ inline static bool am_host_unpriv(void)
  * parse /proc/self/uid_map to find what @orig maps to
  */
 extern uid_t get_ns_uid(uid_t orig);
+extern gid_t get_ns_gid(gid_t orig);
 
 extern bool dir_exists(const char *path);