lxc-attach: User namespaces: Use init's user & group id when attaching

src/lxc/attach.c

@@ -428,3 +428,50 @@ char *lxc_attach_getpwshell(uid_t uid)
 		exit(-1);
 	}
 }
+
+void lxc_attach_get_init_uidgid(uid_t* init_uid, gid_t* init_gid)
+{
+	FILE *proc_file;
+	char proc_fn[MAXPATHLEN];
+	char *line = NULL;
+	size_t line_bufsz = 0;
+	int ret;
+	long value = -1;
+	uid_t uid = (uid_t)-1;
+	gid_t gid = (gid_t)-1;
+
+	/* read capabilities */
+	snprintf(proc_fn, MAXPATHLEN, "/proc/%d/status", 1);
+
+	proc_file = fopen(proc_fn, "r");
+	if (!proc_file)
+		return;
+
+	while (getline(&line, &line_bufsz, proc_file) != -1) {
+		/* format is: real, effective, saved set user, fs
+		 * we only care about real uid
+		 */
+		ret = sscanf(line, "Uid: %ld", &value);
+		if (ret != EOF && ret > 0) {
+			uid = (uid_t) value;
+		} else {
+			ret = sscanf(line, "Gid: %ld", &value);
+			if (ret != EOF && ret > 0)
+				gid = (gid_t) value;
+		}
+		if (uid != (uid_t)-1 && gid != (gid_t)-1)
+			break;
+	}
+
+	fclose(proc_file);
+	free(line);
+
+	/* only override arguments if we found something */
+	if (uid != (uid_t)-1)
+		*init_uid = uid;
+	if (gid != (gid_t)-1)
+		*init_gid = gid;
+
+	/* TODO: we should also parse supplementary groups and use
+	 * setgroups() to set them */
+}

src/lxc/attach.h

@@ -40,4 +40,6 @@ extern int lxc_attach_drop_privs(struct lxc_proc_context_info *ctx);
 
 extern char *lxc_attach_getpwshell(uid_t uid);
 
+extern void lxc_attach_get_init_uidgid(uid_t* init_uid, gid_t* init_gid);
+
 #endif

src/lxc/lxc_attach.c

@@ -418,13 +418,20 @@ int main(int argc, char *argv[])
 		lxc_sync_fini(handler);
 
 		if (namespace_flags & CLONE_NEWUSER) {
-			/* XXX FIXME this should get the uid of the container init and setuid to that */
-			/* XXX FIXME or perhaps try to map in the lxc-attach caller's uid? */
-			if (setgid(0)) {
+			uid_t init_uid = 0;
+			gid_t init_gid = 0;
+
+			/* ignore errors, we will fall back to root in that case
+			 * (/proc was not mounted etc.)
+			 */
+			lxc_attach_get_init_uidgid(&init_uid, &init_gid);
+
+			/* try to set the uid/gid combination */
+			if (setgid(init_gid)) {
 				SYSERROR("switching to container gid");
 				return -1;
 			}
-			if (setuid(0)) {
+			if (setuid(init_uid)) {
 				SYSERROR("switching to container uid");
 				return -1;
 			}