pidfd: prevent creation of pidfds for kthreads

It's currently possible to create pidfds for kthreads but it is unclear
what that is supposed to mean. Until we have use-cases for it and we
figured out what behavior we want block the creation of pidfds for
kthreads.

Link: https://lore.kernel.org/r/20240731-gleis-mehreinnahmen-6bbadd128383@brauner
Fixes: 32fcb426 ("pid: add pidfd_open()")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

kernel/fork.c

@@ -2053,11 +2053,24 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
  */
 int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
 {
-	bool thread = flags & PIDFD_THREAD;
+	if (!pid)
+		return -EINVAL;
+
+	scoped_guard(rcu) {
+		struct task_struct *tsk;
 
-	if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
+		if (flags & PIDFD_THREAD)
+			tsk = pid_task(pid, PIDTYPE_PID);
+		else
+			tsk = pid_task(pid, PIDTYPE_TGID);
+		if (!tsk)
 			return -EINVAL;
 
+		/* Don't create pidfds for kernel threads for now. */
+		if (tsk->flags & PF_KTHREAD)
+			return -EINVAL;
+	}
+
 	return __pidfd_prepare(pid, flags, ret);
 }
 
@@ -2403,6 +2416,12 @@ __latent_entropy struct task_struct *copy_process(
 	if (clone_flags & CLONE_PIDFD) {
 		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;
 
+		/* Don't create pidfds for kernel threads for now. */
+		if (args->kthread) {
+			retval = -EINVAL;
+			goto bad_fork_free_pid;
+		}
+
 		/* Note that no task has been attached to @pid yet. */
 		retval = __pidfd_prepare(pid, flags, &pidfile);
 		if (retval < 0)