Bug#659000: sshd logs spurious errors with publickey authentication

[Sergio Gelato <Sergio.Gelato@astro.su.se>]

Package: openssh-server
Version: 1:5.5p1-6+squeeze1

Consider an authorized_keys file of the form:
# begin
from="1.2.3.4,::ffff:1.2.3.4" ssh-rsa key1234 comment
from="5.6.7.8,::ffff:5.6.7.8" ssh-rsa key5678 comment
# end

Logging in from host 5.6.7.8 with RSA key key5678 succeeds but logs the
following two messages on the server:

Authentication tried for <username> with correct key but not from a permitted host (host=5.6.7.8, ip=5.6.7.8)
Authentication tried for <username> with correct key but not from a permitted host (host=5.6.7.8, ip=5.6.7.8)

Running the server with -dd reveals that these messages are emitted just after
debug2: user_key_allowed: check options: 'from="1.2.3.4,::ffff:1.2.3.4" ssh-rsa key1234 comment'
and before
debug2: key_type_from_name: unknown key type 'from="5.6.7.8,::ffff:5.6.7.8"'
debug2: user_key_allowed: check options: 'from="5.6.7.8,::ffff:5.6.7.8" ssh-rsa key5678 comment'

On the client side, I see the following:
debug1: Remote: Your host '5.6.7.8' is not permitted to use this key for login.
debug1: Remote: Your host '5.6.7.8' is not permitted to use this key for login.

It's clear from the source code (auth2-pubkey.c) that the "with correct key"
part of the log message is just obfuscation: the validity of the key is still
untested at the point where the message is emitted.

This seems to have mild security implications: the client is able to tell how
many other keys there are ahead of its own in the authorized keys file, an
information leak.