CVE-2024-41006 netrom: Fix a memory leak in nr_heartbeat_expiry()

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.