pidfd: prevent creation of pidfds for kthreads

commit 3b5bbe798b2451820e74243b738268f51901e7d0 upstream.

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: 32fcb426ec00 ("pid: add pidfd_open()")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/fork.c b/kernel/fork.c
index 99076dbe27d83f93d704882d4f5c09ae103af303..5ac4558d480162a63a97584bafdcec3ba1bd7f97 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2069,11 +2069,24 @@ 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 || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
+       if (!pid)
                return -EINVAL;
 
+       scoped_guard(rcu) {
+               struct task_struct *tsk;
+
+               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);
 }
 
@@ -2419,6 +2432,12 @@ __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)