Maximum Value for q_depth

[Amjad Gabbar]

Hey Steve,

Reaching out regarding the same issue of syslog containing *"auditd
dispatch error (pipe full) event lost messages". *

Post excluding the default events(LOGIN, USER_START etc) mentioned in our
previous chat, there has been a significant drop in the log volume and
hence I was expecting these error messages to be resolved.

But unfortunately, even after increasing the dispatcher queue size(q_depth)
and changing disp_qos to become lossless , I am still seeing mentions of
these pipe full errors in my syslog.
The surprising thing is if I try to take a look at the events/keys causing
this issue, there doesn't seem to be a lot of events for messages to be
dropped.

Ex- Using the command *"aureport --summary -ts <start time of dropped
messages start reported in syslog > -te < end time  of dropped messages
end reported in syslog > -i (-x/-u/--key)"*, the total events are around
2000 during this time period. The dispatcher queue size is close to 25,000,
So I am not really sure why the dispatcher is unable to handle these
messages. The queue size is sufficient enough to handle 10x the total
events being seen.

Some other theoretical questions I had surrounding this are:


   - The audit daemon picks events from the kernel buffer and sends it to
   the dispatcher buffer. Who writes these logs to /var/log/audit.log - is it
   the daemon or the dispatcher? And also, are the total events reported in
   /var/log/audit.log inclusive of the dropped events reported in syslog or
   exclusive? i.e is it possible that all the events have been recorded in
   audit.log but syslog has an issue in keeping up with the events as it is
   the only plugin that is being used by the dispatcher.
   - Is there a way to find out what is the total number of events dropped
   by the dispatcher?
   - In auditd v3+, the daemon itself handles dispatching capabilities. So,
   what does q_depth refer to in this scenario?
   - In the man pages for different distros for disp_qos the following
   statement is common - " There is a 128k buffer between the audit daemon
   and dispatcher." But different distros seem to have different default
   values for q_depth ranging from 80 to 1200. How is it possible that these
   numbers vary but the size of the buffer remains 128k.




On Tue, Dec 21, 2021 at 2:39 PM Steve Grubb <sgrubb at redhat.com> wrote:

> Hello,
>
> On Tuesday, December 21, 2021 12:55:47 AM EST Amjad Gabbar wrote:
> > Based on our discussion above, I performed some analysis as to why we
> were
> > seeing so many events. The reason seems to be due to the default rules
> > being triggered every time a cron job runs. We have numerous cron jobs
> > running per minute as a result of which multiple different events(LOGIN,
> > USER_END,CRED_DISP etc) are generated each time a cron job runs. As we do
> > not enable SELinux, disabling these thing use subj_type=crond_t is not a
> > viable option.
> >
> > 1. I have tried the following way to exclude using msg_type and exe
> > together and it seems to work.
> >
> > -a exclude,always -F msgtype=MAC_IPSEC_EVENT -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=USER_AUTH -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=USER_ACCT -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=CRED_REFR -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=CRED_DISP -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=CRED_ACQ -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=USER_START -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=USER_END -F exe=/usr/sbin/cron
> > -a exclude,always -F msgtype=SERVICE_START -F exe=/usr/sbin/cron
> >
> > Just want to make sure there is nothing I am missing here and that this
> > only excludes the msg types for the cron executable.
>
> I think so. But it's easy enough to test. Just login and see if you get
> any
> USER_START events from something other than cron.
>
> > 2. Apart from these messages, there is a LOGIN message that gets
> generated
> > each time a cron runs. Eventhough, the LOGIN message in auditd does not
> > have an exe field, the following statement surprisingly seems to be
> > working.
> >
> > -a exclude,always -F msgtype=LOGIN -F exe=/usr/sbin/cron
> >
> > I can still see LOGIN messages for other users but the cron LOGIN
> messages
> > seem to be suppressed. Could you provide some detail as to how this is
> > happening and is the expected result.
>
> It doesn't match against the text in the event. It matches against the
> process's attributes.
>
> > 3. Is there a better way to suppress these cron messages that I am not
> > considering apart from the SELinux option mentioned.
>
> I think you found the best way for a non-selinux system. Back when it was
> documented that it could be supressed by selinux type, audit by executable
> did not exist. But as you found, that is an effective way to get rid of
> the
> events.
>
> I also think the cronie program might be a little more audit friendly. It
> does not call PAM for the system crontabs run under the root user. PAM is
> run
> only for the local crontab (i.e. the one edited by the crontab command)
> and
> in case of the system crontabs only for jobs that are run under non-root
> user.
>
> -Steve
>
>
>
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