pidfs: check for valid pid namespace

When we access a no-current task's pid namespace we need check that the
task hasn't been reaped in the meantime and it's pid namespace isn't
accessible anymore.

The user namespace is fine because it is only released when the last
reference to struct task_struct is put and exit_creds() is called.

Link: https://lore.kernel.org/r/20240926-klebt-altgedienten-0415ad4d273c@brauner
Fixes: 5b08bd40 ("pidfs: allow retrieval of namespace file descriptors")
CC: stable@vger.kernel.org # v6.11
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/pidfs.c

@@ -120,6 +120,7 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	struct nsproxy *nsp __free(put_nsproxy) = NULL;
 	struct pid *pid = pidfd_pid(file);
 	struct ns_common *ns_common = NULL;
+	struct pid_namespace *pid_ns;
 
 	if (arg)
 		return -EINVAL;
@@ -202,7 +203,9 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case PIDFD_GET_PID_NAMESPACE:
 		if (IS_ENABLED(CONFIG_PID_NS)) {
 			rcu_read_lock();
-			ns_common = to_ns_common( get_pid_ns(task_active_pid_ns(task)));
+			pid_ns = task_active_pid_ns(task);
+			if (pid_ns)
+				ns_common = to_ns_common(get_pid_ns(pid_ns));
 			rcu_read_unlock();
 		}
 		break;