[Dovecot] vpopmail passdb deadlock if tcprules binary is missing

[Teodor Milkov]

Hello,

I've just found that if vpopmail is compiled with --enable-roaming-users=y, but
tcprules binary is missing there is kind of denial of service situation, which is
not very obvious for debugging.

I know this is result of misconfiguration and I suppose this is more of a
vpopmail() bug, but it is somewhat hard to debug and causes greater harm to
dovecot long running auth process than vpopmail's short living vchkpw process.

Symptoms
--------

Login to dovecot imap takes very long time. There are auth processes in D state:

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root      9873  0.0  0.0   2884  1396 ?        D    14:04   0:00 dovecot/auth -w
root     11292  0.0  0.0   2884  1396 ?        S    14:25   0:00  \_ dovecot/auth -w


Cause
-----

When new user has to be authenticated from vpopmail, the sequence of events
goes something like this:

 * libexec/dovecot/auth -w process is doing the authentication
   (passdb-vpopmail.c)

 * passdb-vpopmail.c: at some point user is authenticated and open_smtp_relay()
   is called

  * vpopmail.c: open_smtp_relay() gets a write lock on
    ~vpopmail/etc/open-smtp.lock adds new IP to ~vpopmail/etc/open-smtp and
    calls update_rules() etc. to rebuild the tcp.cdb file

  * vpopmailc: tcprules_open() is called and it fork()s and tries to execv()
    tcprules

  * if tcprules is not found, execv() fails silently, and we are left with a
    forked libexec/dovecot/auth instance, which runs all over again up to the
    open_smtp_relay() point where it tries to obtain write lock on
    ~vpopmail/etc/open-smtp.lock again (it is already locked by parent)

Hope this helps someone.