reactive audit proposal

[Joe Nall]

> On May 12, 2020, at 7:22 PM, Steve Grubb <sgrubb at redhat.com> wrote:
> 
> Hello,
> 
> I wanted to run this by the crowd to see what people's reaction might be.
> 
> The audit system sometimes needs to have rules applied when something
> happens. For example, if someone plugs in a USB flash drive, the system
> creates the device in /dev and then automatically mounts it under some
> circumstances.
> 
> I would propose 2 new additions to the audit rule syntax: on-mount and
> on-login.These rules would be in a separate file from the main audit rules.
> When a file system is mounted, /proc/mounts changes and the mount table can
> be scanned to see if something new is there. In this way we can reliably
> detect newly mounted filesystems. We can then match against a specifier to
> see if this is a file system in which we want to apply new rules. If so, we
> send the new rules to the kernel. When the device is unmounted, the kernel
> drops all watches on that file system. So, we only need to worry about when
> a device is mounted.
> 
> This works good for anything that gets mounted. But it is also possible for
> a USB flash drive to be accessed as a block device, such as the dd utility.
> If we had to detect device discovery, there is a netlink group,
> NETLINK_KOBJECT_UEVENT which we could monitor for events. The only thing is
> that we could only detect open/read/write/close/ioctl/lseek. And we probably
> do not want to monitor anything except block devices.
> 
> It may also be possible to poll /sys/block to watch for changes. This might
> be easier as the names are more friendly. This would take some research to
> see if its even possible.
> 
> The rule syntax could look something like:
> on=mount mount=/run/user/1000 : -a exit,always ...
> on=device device=/dev/sdd : -a exit,always ...
> 
> The on-login event would simply watch the audit trail for any AUDIT_LOGIN
> events. That event can be parsed to get the new auid. If the auid matches
> any rules, then it will load them into the kernel. To remove the rules, we
> could watch for the AUDIT_USER_END event. The only issue is that we would
> need to track how many sessions the user has open and remove the rules only
> when the last session closes out.
> 
> The rules for this might look something like this:
> on=login auid=1000 : -a exit,always ...
> 
> The question is whether or not this should be done as part of the audit
> daemon or as a plugin for the audit daemon. One advantage of doing this as
> a plugin is that it will keep the audit daemon focused on getting events
> and distributing them. Any programming mistake in the plugin will crash it
> and not the daemon. The tradeoff is that it will get the event slightly
> after auditd sees it. This only matters for the on-login functionality. The
> device and mount events come from an entirely different source. And I'm sure 
> that in every case, the program will react faster than a user possibily can 
> winning the race evry time.
> 
> Thoughts?

Would bind mounts trigger these rules? I'm sitting next to a box with 10k polyinstantiated bind mounts right now.

joe