[Dovecot] Automatically Cleaning Kerberos Credential Cache Files

[Maura Dailey]

On 12/11/2012 08:52 AM, Ben Morrow wrote:
> At  7PM -0500 on 10/12/12 you (Maura Dailey) wrote:
>> On 12/10/2012 05:31 PM, Ben Morrow wrote:
>>> At  3PM -0500 on 10/12/12 you (Maura Dailey) wrote:
>>>> I'm in a situation here at work where I'm trying to support a mixed
>>>> network of OS X and RHEL desktop machines with a Postfix/Dovecot
>>>> combination.
>>>>      - user account information is stored in LDAP
>>>>      - user credentials are in MIT Kerberos
>>>>      - server is running RHEL 6/Dovecot 2.0.9/Postfix 2.6.6
>>>>
>>>> I am currently using the PAM passdb module to authenticate my users (I
>>>> began to have trouble with using GSSAPI directly). After I implemented
>>>> it, a few weeks later, I noticed that some users were no longer getting
>>>> their mail if they hadn't logged in during the past day. Postfix's mailq
>>>> showed that hundreds of messages were backing up in the queue. I
>>>> eventually tracked it down to leftover Kerberos credential cache files
>>>> (/tmp/krb5cc_????) sitting in /tmp on the mail server. The presence of
>>>> expired credential files was preventing Postfix from delivering mail to
>>>> those users' mail spools. If I delete the credential files manually,
>>>> Postfix immediately delivers the queued emails.
>>> This is rather odd. Is krb5-authenticated NFS involved here, or does
>>> Postfix's delivery make any other use of Kerberos? The only other thing
>>> I can think of is that so many expired ccaches are accumulating that the
>>> user goes over their inode quota.
>> Each user has one credential cache file in /tmp on the mail server after
>> logging into Dovecot. We aren't using randomized names, so everything is
>> in the standard format /tmp/krb5cc_uid. We do use KRB5 authenticated
>> (and encrypted) NFS, but we don't deliver mail to home directories.
>> Since all users are "real" users, and our office size is small, everyone
>> has a mail spool directory on the mail server.
> Hmm. I don't have much experience with KrbNFS, and none at all on Linux,
> but the implementations I've seen seem to be terribly flaky about
> passing krb5 creds to the kernel. (What they ought to do is implement
> AFS' aklog and setpag; they're irritating, but at least they're
> well-understood...)

Flaky is an understatement. Especially when you have to support Mac OS X 
users. After months of intermittent RPC errors, I had to revert them to 
NFS3. At least the RHEL users can use NFS4 and get the speed bump.

> In any case, it's likely that the delivery process looks in the user's
> home directory even if delivery is to a separate mail spool, unless
> you've taken steps to prevent this. For instance, Postfix's local(8)
> checks for ~/.forward by default, LDAs like procmail or maildrop look
> for similar per-user RC files, and Dovecot's LDA looks for (at least)
> ~/.dovecot.sieve. Is it possible that the NFS code returns a different
> error for 'no ccache present' vs 'ccache present but the creds have
> expired', such that Postfix will carry on delvering if it gets the first
> error but not the second?

That is a very good point. We're using Postfix's local, which probably 
doesn't even know it needs credentials. It looks like I can change the 
forward_path, or set allow_mail_to_commands and allow_mail_to_files to 
disallow forwarding. As I said, we're a small office, so I doubt anyone 
will complain. I've set up forwarding for users on travel before in 
/etc/aliases.

> <snip>
>>> Well, I don't use RH (I use FreeBSD), but I use and would recommend Russ
>>> Allbery's pam_krb5.so, which may or may not be the same as the normal
>>> pam_krb5.so provided by your system. It has options to control whether
>>> and where ccaches are created; assuming Dovecot doesn't need krb5 creds
>>> (say, for NFS), you would probably be better off telling it not to
>>> create a permanent ccache at all.
>>>
>>> http://www.eyrie.org/~eagle/software/pam-krb5/
>> I haven't had to configure pam_krb5.so directly before (we use the Red
>> Hat/Fedora configured default, pam_sss.so, which claims to be a one stop
>> shop for LDAP/Kerberos/NIS,etc.), but it does seem to have more options.
>> We certainly don't need credential caches to stick around for email
>> users. All the mail spools are stored on locally mounted storage on the
>> mail server.
> Where do users' private IMAP folders live? Are they in the mail spool as
> well, or are they in the user's home directory? Once a user has logged
> in Dovecot will change directory to their home directory (as returned by
> the userdb), so you may find you *do* need ccaches if they are on KrbNFS.

User's IMAP folders live in their mail spools. I've been watching the 
maillogs all morning, and I've noticed that my test users (who are using 
GSSAPI without leaving credential files behind) are getting the error 
message "Error: chdir(/home/user/) failed: Permission denied 
(euid=1000(user) egid=2002(group) missing +x perm: /home, euid is not 
dir owner)," so your premise that the credential files might have been 
reused for Kerberos seems to be correct. However, I'd much rather 
prevent Dovecot AND Postfix from looking in /home. I will try your tip 
that you gave at the end of your email, overriding the user's home 
directory.

>>> This certainly does all sound related. What are the ccache files called:
>>> are they just /tmp/krb5cc_UID or is there a random portion as well? Are
>>> they being created with the correct permissions, and are there any
>>> security policies (SELinux or ACLs of some kind) set up which might
>>> interfere with their creation or destruction?
>> No random portion (the only application we have configured with random
>> ccache names is SSH), just the usual /tmp/krb5cc_uid.
> (I assume you're aware of the potential DoS here, given that /tmp is
> world-writable and sticky? I'm not sure if there's anything you can do
> about it if you're using KrbNFS, though.)
Oh yeah, I'm painfully aware of how vulnerable we are to this now. Even 
if I find a fix, I'm pretty sure the cron job stays. If I can get direct 
GSSAPI logins (which don't create credential cache files) to work on the 
Macs, then I can proceed to lock down /tmp some more. Right now, if I 
create an empty file /tmp/krb5cc_myuid and chown it to root, sudo 
breaks. I assume the same is true even if I just create it as a 
different user.

>> Permissions appear
>> correct, they belong to the appropriate user and group accounts. The
>> SELinux permissions are set to: system_u:object_r:user_tmp_t:s0. This is
>> different from the machine's credential cache
>> (system_u:object_r:gssd_tmp_t:s0), but I've disabled and re-enabled
>> SELinux during different parts of my testing and didn't notice any errors.
> I know nothing whatever about SELinux, but this might be relevant. gssd
> handles client-side credentials for NFS, so if it ends up unable to
> snoop on a user's ccache you will have problems. (This is why you can't
> rename them to something secure, and is the problem aklog solves.)
The targeted policy in RHEL really simplifies SELinux deployment. I've 
only ever had a few problems with it (usually whenever they start 
covering a service for the first time). In this case, PAM can clobber 
the files without a problem, so it's probably the case that user_tmp_t 
is generous enough.

>> Using GSSAPI directly doesn't create any cached credential files on the
>> server. However, I just verified that I'm still sporadically getting the
>> duplicate tickets (two lines for imap/hostname.server.com at SERVER.COM,
>> with identical expiration dates).
> That's odd, but it shouldn't be a problem. A valid ticket is a valid
> ticket, regardless of whatever other tickets might exist.
I suspect it's the kind of thing you notice when you're debugging things 
and not otherwise. It did make me wonder if I'd somehow screwed up my 
keytab file, but it looks clean and all the key numbers match.

>> It comes and goes. I have a few users
>> testing it for me, and they'll let me know if their logins break
>> tomorrow like they have in the past once the normal log in period
>> elapses. Since the "fix" for that problem is to log into the mail
>> server, I have a hard time testing for the problem myself (being logged
>> into the mail server nearly continuously just now).
> It sounds to me as though clearing out dead ccaches, maybe even with an
> hourly-or-more cronjob that only deletes them if they've expired, will
> fix the problem for the moment. A more fundamental fix will require
> understanding when your PAM modules delete ccaches, and possibly turning
> off Postfix features like ~/.forward if you're not using them.
>
> AFAICS the only piece of Dovecot configuration that might be relevant at
> this point is that a user's Dovecot home directory does not have to be
> the same as their 'real' home directory, so you can (if necessary) move
> all Dovecot-related files onto a local disk. See
> http://wiki2.dovecot.org/AuthDatabase/Passwd .
That does sound like a useful feature. In the past, I've seen Dovecot 
spit out spurious errors about accessing user home directories. I'll 
have to check if I'm still getting those, because this sounds like a fix 
for that. I should be getting some good data from my GSSAPI test users 
later today or tomorrow. One woman claimed she has duplicate emails, but 
I suspect that's because I migrated her to IMAP.

> Ben
>

I really do appreciate all the help.

- Maura Dailey
maura at eclipse.ncsc.mil