Re: Using attachment_dir with plugin zlib corrupt mails

22 Oct 2019 · *Patrick Cernko*

      at least for me it didn't fix. still see in logs:
Error: lzma.read(/var/spool/mail/telsch/storage/m.11800): corrupted
data at 110015
before i upgrade from 2.3.4.1 to 2.3.8 + your patch i noticed i can
access the mails with alpine and roundcube but not with thunderbird.
On 10.10.19 11:38, Timo Sirainen via dovecot wrote:
...
Can you test if
https://github.com/dovecot/core/commit/5068b11e594ad7cc1f7cedf2bd9280520e0e5... fixes
it for you?
...
On 10 Oct 2019, at 11.34, MAREN ZUBIZARRETA via dovecot
mailto:dovecot@dovecot.org> wrote:
Hello:
I have found the same problem reported above by Patrick Cernko
affecting our system and corrupting our messages. Even worse, Outlook
2016 will no synchronize and the clients cannot see any message, even
if there is only one corrupted mail per mailbox.
I cannot figure out a feasible workaround for our system, and I can
see that in new version 2.38 the bug is not fixed.
Will this issue be treated soon?
Thanks a lot
Maren Zubizarreta
WARNING: using attachment_dir with plugin zlib can corrupt mails
*Patrick Cernko*pcernko at mpi-klsb.mpg.de
mailto:dovecot%40dovecot.org?Subject=Re:%20Re%3A%20WARNING%3A%20using%20attachment_dir%20with%20plugin%20zlib%20can%20corrupt%20mails&In-Reply-To=%3C1ab8edcc-bef4-3b41-d09a-c523674a5031%40mpi-klsb.mpg.de%3E
/Fri Jul 19 17:52:37 EEST 2019/

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...
/imap(USERNAME)<4767>: Error: Corrupted record in
index cache file
/IMAP/mail/mailboxes/USERNAME/mdbox/mailboxes/Trash/dbox-Mails/dovecot.index.cache:
UID 489113: Broken fields in mailbox Trash:
read(attachments-connector(zlib(/IMAP/mail/mailboxes/USERNAME/mdbox/storage/m.813))):
FETCH BODY[HEADER.FIELDS (RETURN-PATH SUBJECT)] got too little data: 2
vs 122/
In our case that finally grabbed my attention, the client was the users
iphone that did not display any new messages but his Thunderbird did.
The bug seems to be triggered by a bad "interaction" of attachment_dir
Hello list, hello Dovecot developers,
this week, I discovered a serious bug in Dovecot, that lead to several
broken mails on our servers. The bug corrupts the first few characters
of the mail header during saving. On our setup, it was almost always
only the very first line of text, that was corrupted.
Depending on the IMAP client (they seem to request different header
fields, ... during mail access), the bug causes the imap process to hang
up the TCP connection and log errors like this:
option and zlib plugin. If you use both, you most likely are affected,
too, except you only use zlib plugin for reading previously compressed
stored mails. That's also the workaround we use now: zlib plugin only
enabled in mail_plugins but no plugin/zlib_save set.
The bug occurs on very specific mails. Due to privacy reasons I could
not provide sample mails here. Storing such mails seems to trigger the
bug reproducible.
I attached a very minimal doveconf -n config, that can be used to
trigger the bug. If one of the developers is interested, I can try to
generate an "anonymized" version of such a specific mail that still
causes the issue. I discovered the bug on our productive systems,
running latest Dovecot 2.2 release, but the latest 2.3 I used during
debugging is affected, too.
During debugging, I also found one hint, that might help find the bug:
If you store a problematic mail with zlib_save=gz (or zlib_save=bz2) and
then disable the zlib plugin in mail_plugins, you can call
doveadm fetch -u test hdr all | grep -v ^hdr: | gzip --decompress
on test's mailbox with only that one broken mail.
This will display the beginning of the rfc822 mail text until gzip
terminates with "gzip: stdin: unexpected end of file", approximately
after twice the length of the mail HEADER. This might indicate, that
dovecot stores the uncompressed size of the header in it's data
structures although the mail is stored compressed.
I also found a very efficient way to find all affected mails in our setup:
doveadm -f flow fetch -A 'user guid mailbox uid seq flags hdr' all | 

grep -a "^[^ ]+ user=" | 

grep -avF ' hdr=Return-path: ' | 

grep -av '.* hdr=[[:print:][:space:]]*$'
(runtime for ~6M mails on our servers was 20-30min)
This can be even more optimized if you have a powerful storage system
with GNU parallel:
...
/doveadm user '*' | parallel "doveadm -f flow fetch -u '{}' 'user guid
mailbox uid seq flags hdr' all | grep -a '^user=' | grep -avF '
hdr=Return-path: ' | grep -av '.* hdr=[[:print:][:space:]]*$' || true"/
(runtime for ~6M mails on our servers was ~4min)
The command will give you a list of mails that possibly are affected,
check the full output of
doveadm fetch -u USERNAME hdr guid GUID | less
to verify that the header is really broken.
On our systems I found 39 mails within ~12M mails.
I was able to recover these mails "manually" by reconstructing the
Return-Path header line, importing the fixed mails and expunging the
corrupt ones. Before importing, I had to disable zlib_save option
obviously.
Best regards,
Patrick Cernko https://dovecot.org/mailman/listinfo/dovecot> +49 681 9325 5815
Joint Administration: Information Services and Technology
Max-Planck-Institute fuer Informatik & Softwaresysteme