Date: Mon, 4 Apr 2016 20:39:05 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Paul Moore <paul@paul-moore.com>
Cc: linux-audit@redhat.com, wmealing <wmealing@redhat.com>,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org
Subject: Re: [RFC] Create an audit record of USB specific details
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On Mon, Apr 04, 2016 at 10:54:56PM -0400, Paul Moore wrote:
> On April 4, 2016 6:17:23 PM Greg KH <gregkh@linuxfoundation.org> 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@redhat.com>
> > > > >
> > > > > 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 :)

Userspace can panic the system if it detects this, so why not just do
that?

> > > 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.

Again, please step back.  What exactly are you trying to do here?  What
is the requirement?

> > > 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.

And do any of them look at uevents through these mechanisms?

I doubt they care...

thanks,

greg k-h