Re: reactive audit proposal

> On May 12, 2020, at 7:22 PM, Steve Grubb <sgrubb(a)redhat.com&gt; 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.