tcp: Disallow re-connection of a connected socket

[markj]

Summary

soconnectat() tries to ensure that one cannot connect a connected
socket. However, the check is racy and does not really prevent two
threads from attempting to connect the same TCP socket.

Modify tcp_connect() and tcp6_connect() to perform the check again, this
time synchronized by the inpcb lock, under which we call
soisconnecting().

Reported by: syzkaller

[markj]

• Undo the modification to tcp6_usr_connect().

[glebius]

May I ask two additions to the existing patch?

1. Your discovery of the racyness of soconnectat() is very important and it deserves a comment in the function code with reference to the fact that TCP solves the problem for itself.
2. I'd suggest to brace the new check info __predict_false().

[markj]

• Add a comment to soconnectat().
• Add __predict_false annotations.

[markj]

In D38507#877427, @glebius wrote:

May I ask two additions to the existing patch?

1. Your discovery of the racyness of soconnectat() is very important and it deserves a comment in the function code with reference to the fact that TCP solves the problem for itself.
2. I'd suggest to brace the new check info __predict_false().

BTW, this is already checked implicitly by UDP when it checks inp->inp_faddr.