Query regarding the lib audit-userspace

[Manojkiran Eda]

Hi Steve,

Thanks for the reply.

I am actually trying to enable audit events support for an embedded webserver (bmcweb<https://github.com/openbmc/bmcweb>) which implements the DMTF Redfish specification. The idea is to use libaudit to generate server operations related audit events. For example

  *   Authentication related events
  *   Power on related events
  *   Code update related events

And the design principle is that all the apps in LF OpenBMC<https://github.com/openbmc/openbmc> are single threaded & use dbus as our communication model & we use event loops for processing events from dbus. I agree that probably it makes sense to disallow a particular operation If we fail to send an event. But can’t we do it without blocking using the event loops ? is there some example of doing that for sending audit events ? As it is a webserver we would want it to as much responsive as possible.

And also , how much time does the daemon have to wait for a ACK reply ? does it have a time out of some sort ? I understand that the API that we discussed above does the send as well as receive part, are there specific API’s for just send & receive separately ? so that I could probably add the file descriptor to the event loop and invoke the receive API whenever we get a reply ACK from the kernel.

Thank,
Manoj


From: Steve Grubb <sgrubb at redhat.com>
Date: Tuesday, 13 September 2022 at 10:55 PM
To: linux-audit at redhat.com <linux-audit at redhat.com>
Cc: ManojKiran Eda <manojkiran.eda at gmail.com>
Subject: Re: Query regarding the lib audit-userspace
Hello,

On Tuesday, September 13, 2022 12:53:32 PM EDT Manojkiran Eda wrote:
> I was working on leveraging the libaudit shared library to generate audit
> events from a user space daemon. I have been using the audit_open as well
> as audit_log_acct_message() API’s to send message to the kernel audit
> subsystem.

audit_log_acct_message()  is meant to send events related to a user's
account. For example, pam and shadow-utils uses it.

> From the man pages I understand that every message to the
> kernel audit subsystem would get an ACK back.

Yes. This is to let you know if there was a problem such as lack of
permissions.

> Now my question is does the daemon[single threaded] consuming this libaudit
> for sending events using audit_log_acct_message() API be blocked until it
> gets an ACK back from the kernel ?

Yes. In general, sending audit events should be rare.

> If yes , is there a way to not have the application blocked during this
> period ?

The requirements that we normally need to adhere to is that if the audit
event fails, then whatever the user was going to do needs to be prevented. If
you really need to keep moving, send the event on it's own thread so that
your application is not waiting. For example, sshd forks the user session so
that it can keep processing other logins. But on that fork, it waits for the
responses to make it easier to tear down the session if sending an audit
event fails.

Or, another approach would be to write the whole thing down to calling sendto
and then you can wait however you want. I think putting on it's own thread is
simplest. But as I said in the beginning, should there be a problem logging
the event, can you undo whatever the event was for if you keep going?

-Steve

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