The following NULL pointer dereference issue occurred:

BUG: kernel NULL pointer dereference, address: 0000000000000000
<...>
RIP: 0010:ccid_hc_tx_send_packet net/dccp/ccid.h:166 [inline]
RIP: 0010:dccp_write_xmit+0x49/0x140 net/dccp/output.c:356
<...>
Call Trace:
 <TASK>
 dccp_sendmsg+0x642/0x7e0 net/dccp/proto.c:801
 inet_sendmsg+0x63/0x90 net/ipv4/af_inet.c:846
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg+0x83/0xe0 net/socket.c:745
 ____sys_sendmsg+0x443/0x510 net/socket.c:2558
 ___sys_sendmsg+0xe5/0x150 net/socket.c:2612
 __sys_sendmsg+0xa6/0x120 net/socket.c:2641
 __do_sys_sendmsg net/socket.c:2650 [inline]
 __se_sys_sendmsg net/socket.c:2648 [inline]
 __x64_sys_sendmsg+0x45/0x50 net/socket.c:2648
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x43/0x110 arch/x86/entry/common.c:82
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

sk_wait_event() returns an error (-EPIPE) if disconnect() is called on the
socket waiting for the event. However, sk_stream_wait_connect() returns
success, i.e. zero, even if sk_wait_event() returns -EPIPE, so a function
that waits for a connection with sk_stream_wait_connect() may misbehave.

In the case of the above DCCP issue, dccp_sendmsg() is waiting for the
connection. If disconnect() is called in concurrently, the above issue
occurs.

This patch fixes the issue by returning error from sk_stream_wait_connect()
if sk_wait_event() fails.

Fixes: 419ce133 ("tcp: allow again tcp_disconnect() when threads are waiting")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reported-by: syzbot+c71bc336c5061153b502@syzkaller.appspotmail.com
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>