Re: [RFC] Create an audit record of USB specific details

[Paul Moore]

On April 4, 2016 6:17:23 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Apr 04, 2016 at 05:37:58PM -0400, Paul Moore wrote:
> > On Monday, April 04, 2016 05:56:26 AM Greg KH wrote:
> > > On Mon, Apr 04, 2016 at 12:02:42AM -0400, wmealing wrote:
> > > > From: Wade Mealing <wmealing@xxxxxxxxxx>
> > > >
> > > > Gday,
> > > >
> > > > I'm looking to create an audit trail for when devices are added or removed
> > > > from the system.
> > >
> > > Then please do it in userspace, as I suggested before, that way you
> > > catch all types of devices, not just USB ones.
> >
> > Audit has some odd requirements placed on it by some of its users.  I think
> > most notable in this particular case is the need to take specific actions,
> > including panicking the system, when audit records can't be sent to userspace
> > and are "lost".  Granted, it's an odd requirement, definitely not the
> > norm/default configuration, but supporting weird stuff like this has allowed
> > Linux to be used on some pretty interesting systems that wouldn't have been
> > possible otherwise.  Looking quickly at some of the kobject/uvent code, it
> > doesn't appear that the uevent/netlink channel has this capability.
>
> Are you sure you can loose netlink messages?  If you do, you know you
> lost them, so isn't that good enough?

Last I checked netlink didn't have a provision for panicking the system, so 
no :)

> > It also just noticed that it looks like userspace can send fake uevent
> > messages;
>
> That's how your machine boots properly :)

Yes, it looks like that is how the initial devices are handled, right?  
Allowing something like that is probably okay for a variety of reasons, but 
I expect users would want to restrict access beyond this single trusted 
process.  The good news is that I think you should be able to do that with 
a combination of DAC and MAC.

> > I haven't looked at it closely enough yet, but that may be a concern
> > for users which restrict/subdivide root using a LSM ... although it is
> > possible that the LSM policy could help here.  I'm thinking aloud a bit right
> > now, but for SELinux the netlink controls aren't very granular and sysfs can
> > be tricky so I can't say for certain about blocking fake events from userspace
> > using LSMs/SELinux.
>
> uevents are not tied into LSMs from what I can tell, so I don't
> understand wht you are talking about here, sorry.

Perhaps I'm mistaken, but uevents are sent to userspace via netlink which 
does have LSM controls.  There also appears to be a file I/O mechanism via 
sysfs which also has LSM controls.

--
paul moore
www.paul-moore.com