Re: setting up CDB filters in udev (was Re: [PATCH v2 0/3] block: add queue-private command filter, editable via sysfs)

[Tejun Heo <tj@kernel.org>]

Hello, Paolo.

On Thu, Oct 25, 2012 at 02:35:20PM -0400, Paolo Bonzini wrote:
> > Disabling filters if opened by root and tranfering via SCM_RIGHTS
> > would be the simplest interface-wise (there's no new interface at
> > all).  Would that be too dangerous security-wise?
> 
> That would be a change with respect to what we have now.  After
> transferring a root-opened (better: CAP_SYS_RAWIO-opened) file
> descriptor to an unprivileged process your SG_IO commands get
> filtered.  So a ioctl is needed if you want to rely on SCM_RIGHTS.

Yeah, I get that it's a behavior change, but would that be a problem?

> > I guess I just feel quite reluctant to expose another rather obscure
> > userland configurable in-kernel filter and at the same time I'm not
> > sure whether this is flexible enough.  What if a device is shared by
> > multiple virtual machines which are trusted at different levels?
> 
> No, you just don't do that.  If a device is passed through to virtual
> machines, it is between similar virtual machines (for some definition
> of similar).  The only case where you have this sharing is in practice
> if either the device is read-only (my patch does give you a basic
> two-level filtering, with two separate bitmaps for RO and RW) or if you
> allow persistent reservations (which is as close to full trust as you
> can get).

What disturbs me is that it's a completely new interface to userland
and at the same a very limited one at that.  So, yeah, it's
bothersome.  I personally would prefer SCM_RIGHTS behavior change +
hard coded filters per device class.

But, I'd really like to hear what other guys are thinking.  Jens?
Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? :P

Thanks.

-- 
tejun