In the Linux kernel, the following vulnerability has been resolved:

netrom: Fix a memory leak in nr_heartbeat_expiry()

syzbot reported a memory leak in nr_create() [0].

Commit 409db27e3a2e ("netrom: Fix use-after-free of a listening socket.")
added sock_hold() to the nr_heartbeat_expiry() function, where
a) a socket has a SOCK_DESTROY flag or
b) a listening socket has a SOCK_DEAD flag.

But in the case "a," when the SOCK_DESTROY flag is set, the file descriptor
has already been closed and the nr_release() function has been called.
So it makes no sense to hold the reference count because no one will
call another nr_destroy_socket() and put it as in the case "b."

nr_connect
  nr_establish_data_link
    nr_start_heartbeat

nr_release
  switch (nr->state)
  case NR_STATE_3
    nr->state = NR_STATE_2
    sock_set_flag(sk, SOCK_DESTROY);

                        nr_rx_frame
                          nr_process_rx_frame
                            switch (nr->state)
                            case NR_STATE_2
                              nr_state2_machine()
                                nr_disconnect()
                                  nr_sk(sk)->state = NR_STATE_0
                                  sock_set_flag(sk, SOCK_DEAD)

                        nr_heartbeat_expiry
                          switch (nr->state)
                          case NR_STATE_0
                            if (sock_flag(sk, SOCK_DESTROY) ||
                               (sk->sk_state == TCP_LISTEN
                                 && sock_flag(sk, SOCK_DEAD)))
                               sock_hold()  // ( !!! )
                               nr_destroy_socket()

To fix the memory leak, let's call sock_hold() only for a listening socket.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with Syzkaller.

[0]: https://syzkaller.appspot.com/bug?extid=d327a1f3b12e1e206c16

kernel.org https://git.kernel.org/stable/c/0b9130247f3b6a1122478471ff0e014ea96bb735 [No types assigned]

kernel.org https://git.kernel.org/stable/c/280cf1173726a7059b628c610c71050d5c0b6937 [No types assigned]

kernel.org https://git.kernel.org/stable/c/5391f9db2cab5ef1cb411be1ab7dbec728078fba [No types assigned]

kernel.org https://git.kernel.org/stable/c/a02fd5d775cf9787ee7698c797e20f2fa13d2e2b [No types assigned]

kernel.org https://git.kernel.org/stable/c/b6ebe4fed73eedeb73f4540f8edc4871945474c8 [No types assigned]

kernel.org https://git.kernel.org/stable/c/d377f5a28332954b19e373d36823e59830ab1712 [No types assigned]

kernel.org https://git.kernel.org/stable/c/d616876256b38ecf9a1a1c7d674192c5346bc69c [No types assigned]

kernel.org https://git.kernel.org/stable/c/e07a9c2a850cdebf625e7a1b8171bd23a8554313 [No types assigned]