Re: [RFC PATCH v2 3/3] tun: fix LSM/SELinux labeling of tun/tap devices

[Paul Moore <pmoore@redhat.com>]

On Thursday, December 06, 2012 06:29:54 PM Jason Wang wrote:
> On Wednesday, December 05, 2012 03:26:19 PM Paul Moore wrote:
> > This patch corrects some problems with LSM/SELinux that were introduced
> > with the multiqueue patchset.  The problem stems from the fact that the
> > multiqueue work changed the relationship between the tun device and its
> > associated socket; before the socket persisted for the life of the
> > device, however after the multiqueue changes the socket only persisted
> > for the life of the userspace connection (fd open).  For non-persistent
> > devices this is not an issue, but for persistent devices this can cause
> > the tun device to lose its SELinux label.
> > 
> > We correct this problem by adding an opaque LSM security blob to the
> > tun device struct which allows us to have the LSM security state, e.g.
> > SELinux labeling information, persist for the lifetime of the tun
> > device.  In the process we tweak the LSM hooks to work with this new
> > approach to TUN device/socket labeling and introduce a new LSM hook,
> > security_tun_dev_create_queue(), to approve requests to create a new
> > TUN queue via TUNSETQUEUE.
> > 
> > The SELinux code has been adjusted to match the new LSM hooks, the
> > other LSMs do not make use of the LSM TUN controls.  This patch makes
> > use of the recently added "tun_socket:create_queue" permission to
> > restrict access to the TUNSETQUEUE operation.  On older SELinux
> > policies which do not define the "tun_socket:create_queue" permission
> > the access control decision for TUNSETQUEUE will be handled according
> > to the SELinux policy's unknown permission setting.
> > 
> > Signed-off-by: Paul Moore <pmoore@redhat.com>

...

> > @@ -4425,20 +4452,19 @@ static void selinux_tun_dev_post_create(struct
> > sock
> > *sk) * cause confusion to the TUN user that had no idea network labeling *
> > protocols were being used */
> > 
> > -	/* see the comments in selinux_tun_dev_create() about why we ...
> > -
> > -	sksec->sid = current_sid();
> > +	sksec->sid = tunsec->sid;
> 
> Since both tun_set_iff() and tun_set_queue() would call this. I wonder when
> it is called by tun_set_queue() we need some checking just like what we
> done in v1, otherwise it's unconditionally in TUNSETQUEUE. Or we can add
> them in selinux_tun_dev_create_queue()?

In all the cases that call tun_attach() we have a new socket which needs to be 
labeled based on the tun->security label, yes?  That is what the 
security_tun_dev_attach() code does, there is no need for access control at 
this point as the operation has already been authorized by either 
security_tun_dev_create() (new device), security_tun_dev_create_queue() (new 
queue), or security_tun_dev_open() (opening persistent device).

I think we are all set, or am I missing something?
 
> >  	sksec->sclass = SECCLASS_TUN_SOCKET;
> > 
> > +
> > +	return 0;
> > 
> >  }

-- 
paul moore
security and virtualization @ redhat


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