Re: reactive audit proposal

[Steve Grubb <sgrubb@redhat.com>]

On Tuesday, May 12, 2020 8:31:45 PM EDT Joe Nall wrote:
> > On May 12, 2020, at 7:22 PM, Steve Grubb <sgrubb@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.

If you do cat /proc/mounts  do you see 10k entries? And do you want them or 
do you think they are harmful?

-Steve



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