From: Paul Moore <paul@paul-moore.com>
To: Greg KH <gregkh@linuxfoundation.org>
CC: <linux-audit@redhat.com>, wmealing <wmealing@redhat.com>,
        <linux-kernel@vger.kernel.org>, <linux-usb@vger.kernel.org>
Date: Mon, 04 Apr 2016 22:54:56 -0400
Message-ID: <153e4582800.2832.85c95baa4474aabc7814e68940a78392@paul-moore.com>
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Subject: Re: [RFC] Create an audit record of USB specific details
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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 :)

>> 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