Re: [PATCH v1 0/1] ioctl to disallow detaching kernel USB drivers

[Krzysztof Opasiak]

On 11/30/2015 05:16 PM, Alan Stern wrote:
> On Fri, 27 Nov 2015, Krzysztof Opasiak wrote:
>
> > > > I run through your code and as far as I understand above is not exactly
> > > > true. Your patch allows only to prevent userspace from accessing interfaces
> > > > which has kernel drivers, there is no way to stop an application from taking
> > > > control over all free interfaces.
> > > >
> > > > Let's say that your device has 3 interfaces. First of them has a kernel
> > > > driver but second and third doesn't. You have 2 apps. One should communicate
> > > > using second interface and another one third. But first app is malicious and
> > > > it claims all free interfaces of received device (your patch doesn't prevent
> > > > this). And when second app starts it is unable to do anything with the
> > > > device because all interfaces are taken. How would you like to handle this?
> > >
> > > You can't, and why would you ever want to, as you can't tell what an app
> > > "should" or "should not" do.  If you really care about this, then use a
> > > LSM policy to prevent this.
> >
> > Well, an app can declare what it does and what it needs in it's manifest
> > file (or some equivalent of this) and the platform should ensure that
> > app can do only what it has declared.
> >
> > I would really like to use LSM policy in here but currently it is
> > impossible as one device node represents whole device. Permissions (even
> > those from LSM) are being checked only on open() not on each ioctl() so
> > as far as I know there is nothing which prevents any owner of opened fd
> > to claim all available (not taken by someone else) interfaces and LSM
> > policy is unable to filter those calls (unless we add some LSM hooks
> > over there).
>
> How about this approach?  Once a process has dropped its usbfs
> privileges, it's not allowed to claim any interfaces (either explicitly
> or implicitly).  Instead, it or some manager program must claim the
> appropriate interfaces before dropping privileges.

I agree that restricting interface claiming only to privileged process 
is a good idea. Unfortunately this generates a problem when program 
needs more than one interface (like in cdc - data + control for 
example). We need to declare both of them in first call to "usb-manager" 
or reopen the dev node at second call and claim all interfaces claimed 
using this fd till now and claim one more and then drop privileges and 
send a new fd.

Maybe better option would be to add optional argument to claim interface 
ioctl() and allow to claim interface for other fd than the current one? 
So "usb-manager" could have fd with full control and claim interfaces 
for apps which have fds with restricted privileges.

Best regards,
--
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
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