Fixes the below NULL pointer dereference:

  [...]
  [   14.471200] Call Trace:
  [   14.471562]  <TASK>
  [   14.471882]  lock_acquire+0x245/0x2e0
  [   14.472416]  ? remove_wait_queue+0x12/0x50
  [   14.473014]  ? _raw_spin_lock_irqsave+0x17/0x50
  [   14.473681]  _raw_spin_lock_irqsave+0x3d/0x50
  [   14.474318]  ? remove_wait_queue+0x12/0x50
  [   14.474907]  remove_wait_queue+0x12/0x50
  [   14.475480]  sk_stream_wait_memory+0x20d/0x340
  [   14.476127]  ? do_wait_intr_irq+0x80/0x80
  [   14.476704]  do_tcp_sendpages+0x287/0x600
  [   14.477283]  tcp_bpf_push+0xab/0x260
  [   14.477817]  tcp_bpf_sendmsg_redir+0x297/0x500
  [   14.478461]  ? __local_bh_enable_ip+0x77/0xe0
  [   14.479096]  tcp_bpf_send_verdict+0x105/0x470
  [   14.479729]  tcp_bpf_sendmsg+0x318/0x4f0
  [   14.480311]  sock_sendmsg+0x2d/0x40
  [   14.480822]  ____sys_sendmsg+0x1b4/0x1c0
  [   14.481390]  ? copy_msghdr_from_user+0x62/0x80
  [   14.482048]  ___sys_sendmsg+0x78/0xb0
  [   14.482580]  ? vmf_insert_pfn_prot+0x91/0x150
  [   14.483215]  ? __do_fault+0x2a/0x1a0
  [   14.483738]  ? do_fault+0x15e/0x5d0
  [   14.484246]  ? __handle_mm_fault+0x56b/0x1040
  [   14.484874]  ? lock_is_held_type+0xdf/0x130
  [   14.485474]  ? find_held_lock+0x2d/0x90
  [   14.486046]  ? __sys_sendmsg+0x41/0x70
  [   14.486587]  __sys_sendmsg+0x41/0x70
  [   14.487105]  ? intel_pmu_drain_pebs_core+0x350/0x350
  [   14.487822]  do_syscall_64+0x34/0x80
  [   14.488345]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
  [...]

The test scenario has the following flow:

thread1                               thread2
-----------                           ---------------
 tcp_bpf_sendmsg
  tcp_bpf_send_verdict
   tcp_bpf_sendmsg_redir              sock_close
    tcp_bpf_push_locked                 __sock_release
     tcp_bpf_push                         //inet_release
      do_tcp_sendpages                    sock->ops->release
       sk_stream_wait_memory          	   // tcp_close
          sk_wait_event                      sk->sk_prot->close
           release_sock(__sk);
            ***
                                                lock_sock(sk);
                                                  __tcp_close
                                                    sock_orphan(sk)
                                                      sk->sk_wq  = NULL
                                                release_sock
            ****
           lock_sock(__sk);
          remove_wait_queue(sk_sleep(sk), &wait);
             sk_sleep(sk)
             //NULL pointer dereference
             &rcu_dereference_raw(sk->sk_wq)->wait

While waiting for memory in thread1, the socket is released with its wait
queue because thread2 has closed it. This caused by tcp_bpf_send_verdict
didn't increase the f_count of psock->sk_redir->sk_socket->file in thread1.

We should check if SOCK_DEAD flag is set on wakeup in sk_stream_wait_memory
before accessing the wait queue.
Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/bpf/20220823133755.314697-2-liujian56@huawei.com