fix non-root user cannot write /dev/stdout

src/lxc/attach.c

@@ -875,6 +875,9 @@ static int attach_child_main(struct attach_clone_payload *payload)
 
 	if (new_gid == ns_root_gid)
 		new_gid = LXC_INVALID_GID;
+	
+	/* Make sure that the processes STDIO is correctly owned by the user that we are switching to */
+	fix_stdio_permissions(new_uid);
 
 	if (!lxc_switch_uid_gid(new_uid, new_gid))
 		goto on_error;

src/lxc/start.c

@@ -1365,6 +1365,9 @@ static int do_start(void *data)
 
 	if (new_gid == nsgid)
 		new_gid = LXC_INVALID_GID;
+	
+	/* Make sure that the processes STDIO is correctly owned by the user that we are switching to */
+	fix_stdio_permissions(new_uid);
 
 	/* If we are in a new user namespace we already dropped all groups when
 	 * we switched to root in the new user namespace further above. Only

src/lxc/utils.c

@@ -1860,3 +1860,48 @@ bool lxc_can_use_pidfd(int pidfd)
 
 	return log_trace(true, "Kernel supports pidfds");
 }
+
+void fix_stdio_permissions(uid_t uid)
+{
+	int std_fds[3] = {STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO};
+	int devnull_fd = -1;
+	int ret;
+	int i = 0;
+	struct stat st;
+	struct stat null_st;
+
+	devnull_fd = open_devnull();
+	if (devnull_fd < 0) {
+		ERROR("Open /dev/null failed");
+		goto out;
+	}
+	
+	ret = fstat(devnull_fd, &null_st);
+
+	for (; i < 3; i++) {
+		ret = fstat(std_fds[i], &st);
+		if (ret != 0) {
+			ERROR("Failed to get fd %d stat", std_fds[i]);
+			continue;
+		}
+
+		if (st.st_rdev == null_st.st_rdev) {
+			continue;
+		}
+
+		ret = fchown(std_fds[i], uid, st.st_gid);
+		if (ret != 0) {
+			ERROR("Failed to change fd %d owner", std_fds[i]);
+		}
+
+		ret = fchmod(std_fds[i], 0700);
+		if (ret != 0) {
+			ERROR("Failed to change fd %d mode", std_fds[i]);
+		}
+	}
+
+out:
+	if (devnull_fd >= 0) {
+		close(devnull_fd);
+	}
+}

src/lxc/utils.h

@@ -239,4 +239,7 @@ extern int lxc_rm_rf(const char *dirname);
 extern int lxc_setup_keyring(char *keyring_label);
 extern bool lxc_can_use_pidfd(int pidfd);
 
+/* Fix the permissions of init PID's STDIO within the container to the specified user */
+extern void fix_stdio_permissions(uid_t uid);
+
 #endif /* __LXC_UTILS_H */