Improve error handling in tcp channels

[bcook-r7]

This makes some minor idiomatic changes to channel handling code, but also fixes some areas where Meterpreter payloads actually send failure responses to opening sockets, but Metasploit does not handle these as expected. Rather than raising an exception or returning a nil socket, often a nil object is instead dereferenced, throwing an entirely different exception, and as a result, a lot of things sort of work only by luck.

Verification

Run various exploits and modules that use TCP sockets through a pivot. For example, I have been running this RC script for testing mettle's network channel support:

use exploit/multi/handler
set payload linux/x86/mettle_reverse_tcp
set lhost 127.0.0.1
run -j
sleep 4
route add 192.168.1.0 255.255.255.0 1
use auxiliary/scanner/portscan/tcp
set RHOSTS 192.168.1.0/24
set SESSION -1
run

Your mileage may vary. This is certainly up for discussion, and I suspect I may have gotten intent vs what the code was actually doing mixed up. It does seem to help a bit with handling sudden unexpected socket closes.

[bcook-r7]

Thanks @acammack-r7 !

[acammack-r7]

Release Notes

Previously, Metasploit TCP channels would crash in certain race conditions, and cause the whole subsystem to hang. This fix improves the reliability of TCP channels.