mail_crypt crashing randomly during LMTP

[Andi Krischer]

Hello everybody,

hope you’re all well!

We have a really strange bug with our dovecot setup. After weeks and month of debugging we’re finally reached the point when we ask you for help.

Our basic setup is quite simple: a single dovecot 2.3.10 (0da0eff44) install on Debian 10 with LDAP as user/passdb and sieve, imap_sieve and mail_crypt as mail_plugins.

For the mail_crypt we have a special rule which allows us to enable crypting for each user individually.

See this line from dovecot-ldap.conf.ext:

pass_attrs = userPassword=password,=userdb_mail_plugins=%{if;%{real_lport};eq;993;imap_sieve;} %{if;%{real_lport};eq;143;imap_sieve;} notify mail_log %{if;%{ldap:encryptMail};eq;TRUE;mail_crypt; },=userdb_mail_crypt_private_password=%w

As you can see for IMAP connections we enable imap_sieve and if the LDAP attribute “encryptMail” is set to TRUE we enable “mail_crypt” as mail_plugin.

This works fine except that around 5-10 times a day the dovecot server crashes during a LMTP connection. It crashes in the last steps of LMTP which result that the mail get delivered to Maildir but Postfix doesn’t get a positive response and delivers the mail again after a few minutes. So all of our users get some mails twice or even more times.

This is what the log says in the moment of crashing:
dovecot: lmtp(53852): Panic: Module context mail_crypt_user_module missing
*** backtrace *** (see: https://pastebin.com/YCiFtxmy)
dovecot: lmtp(53852): Fatal: master: service(lmtp): child 53852 killed with signal 6 (core dumps disabled - https://dovecot.org/bugreport.html#coredumps)

Here I’ve pasted the LTMP session in mail.log: https://pastebin.com/A4ughNUF

Obviously it looks like it has something to do with our mail_crypt setup. But the thing is that although we’re monitoring this for months we cannot find a condition which cause this. It seems to happen more frequent for mails with lot of people in To or CC (large headers), but definitively not all of them…

Some more things we’ve already thought on:
First my guess was that the mail_crypt plugin isn’t loaded in some case but in the backtrace are things from “lib10_mail_crypt_plugin.so”, so it should be loaded, right?
On the other hand a non-crashing LTMP session looks exactly the same in log except it has three additional lines in mail.log:
Debug: Loading modules from directory: /usr/lib/dovecot/modules
Debug: Module loaded: /usr/lib/dovecot/modules/lib15_notify_plugin.so
Debug: Module loaded: /usr/lib/dovecot/modules/lib20_mail_log_plugin.so

My college already had a look in the source code, but this didn’t make us much smarter. Here his references:
mail context is defined here: https://github.com/dovecot/core/blob/81b5b188c478ec36bea8bda8fcad1e5f32ac612b/src/plugins/mail-crypt/mail-crypt-plugin.c#L453
and removed again here:
https://github.com/dovecot/core/blob/81b5b188c478ec36bea8bda8fcad1e5f32ac612b/src/plugins/mail-crypt/mail-crypt-plugin.c#L397

Does any of the experts on this list have an idea what could cause these crashes and how to fix them?

Thanks in advance!

Best,
Andi