telnetd: fix two-byte input crash

[brooks]

Summary

(Move initialization of the slc table earlier so it doesn't get
accessed before that happens.)

For details on the issue, see:
https://pierrekim.github.io/blog/2022-08-24-2-byte-dos-freebsd-netbsd-telnetd-netkit-telnetd-inetutils-telnetd-kerberos-telnetd.html

Obtained from: NetBSD via cy@

[cy]

There's also a fix for CVE-2020-10188 (https://github.com/cschuber/freebsd-telnet/commit/e76b51e91f6d9aa7b72ee6624c29b46ddce2a406). Do you want to commit that or should I?

[cy]

Next question. I maintain the extract of the FreeBSD telnet/telnetd on GH, of which telnetd is a port while telnet is in the repo should we choose to remove it as well. I've been toying with the idea of telnet and telnetd ports based on NetBSD. They will probably pay more attention to it than we might. Thoughts?

[brooks]

In D36680#832950, @cy wrote:

There's also a fix for CVE-2020-10188 (https://github.com/cschuber/freebsd-telnet/commit/e76b51e91f6d9aa7b72ee6624c29b46ddce2a406). Do you want to commit that or should I?

Please go ahead and commit that one. I'll wait to delete the build bits (D36620) and sources (D36621) until you do so the changes be be MFCd.

[brooks]

In D36680#832953, @cy wrote:

Next question. I maintain the extract of the FreeBSD telnet/telnetd on GH, of which telnetd is a port while telnet is in the repo should we choose to remove it as well. I've been toying with the idea of telnet and telnetd ports based on NetBSD. They will probably pay more attention to it than we might. Thoughts?

There's probably some sense in making a port of the NetBSD ones. Ours will continue to rot. On the client side, the OpenBSD version might be worth considering.