Code: void unix_notinflight(struct file *fp)
 {
 	struct sock *s = unix_get_socket(fp);
 
	spin_lock(&unix_gc_lock);

	if (s) {
		struct unix_sock *u = unix_sk(s);

		BUG_ON(list_empty(&u->link));

		if (atomic_long_dec_and_test(&u->inflight))
 			list_del_init(&u->link);
 		unix_tot_inflight--;
 	}
	fp->f_cred->user->unix_inflight--;
 	spin_unlock(&unix_gc_lock);
 }

Commit Message: unix: correctly track in-flight fds in sending process user_struct

The commit referenced in the Fixes tag incorrectly accounted the number
of in-flight fds over a unix domain socket to the original opener
of the file-descriptor. This allows another process to arbitrary
deplete the original file-openers resource limit for the maximum of
open files. Instead the sending processes and its struct cred should
be credited.

To do so, we add a reference counted struct user_struct pointer to the
scm_fp_list and use it to account for the number of inflight unix fds.

Fixes: 712f4aad406bb1 ("unix: properly account for FDs passed over unix sockets")
Reported-by: David Herrmann <dh.herrmann@gmail.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
CWE ID: CWE-399
void unix_notinflight(struct file *fp)
void unix_notinflight(struct user_struct *user, struct file *fp)
 {
 	struct sock *s = unix_get_socket(fp);
 
	spin_lock(&unix_gc_lock);

	if (s) {
		struct unix_sock *u = unix_sk(s);

		BUG_ON(list_empty(&u->link));

		if (atomic_long_dec_and_test(&u->inflight))
 			list_del_init(&u->link);
 		unix_tot_inflight--;
 	}
	user->unix_inflight--;
 	spin_unlock(&unix_gc_lock);
 }
