Re: [RFC PATCH 2/2] tun: fix LSM/SELinux labeling of tun/tap devices

Paul Moore
2012-12-03T16:22:29Z

On Monday, December 03, 2012 06:15:42 PM Jason Wang wrote:
> On 11/30/2012 06:06 AM, 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.

...

> > -static int selinux_tun_dev_attach(struct sock *sk)
> > +static int selinux_tun_dev_attach(struct sock *sk, void *security)
> >
> > {
> >
> > + struct tun_security_struct *tunsec = security;
> >
> > struct sk_security_struct *sksec = sk->sk_security;
> > u32 sid = current_sid();
> > int err;
> >
> > + /* we don't currently perform any NetLabel based labeling here ...
> >
> > err = avc_has_perm(sid, sksec->sid, SECCLASS_TUN_SOCKET,
> >
> > TUN_SOCKET__RELABELFROM, NULL);
> >
> > if (err)
> >
> > return err;
> >
> > - err = avc_has_perm(sid, sid, SECCLASS_TUN_SOCKET,
> > + err = avc_has_perm(sid, tunsec->sid, SECCLASS_TUN_SOCKET,
> >
> > TUN_SOCKET__RELABELTO, NULL);
> >
> > if (err)
> >
> > return err;
> >
> > - sksec->sid = sid;
> > + sksec->sid = tunsec->sid;
> > + sksec->sclass = SECCLASS_TUN_SOCKET;
>
> I'm not sure whether this is correct, looks like we need to differ between
> TUNSETQUEUE and TUNSETIFF. When userspace call TUNSETIFF for persistent
> device, looks like we need change the sid of tunsec like in the past.

It may be that I'm misunderstanding TUNSETQUEUE and/or TUNSETIFF. Can you
elaborate as to why they should be different?

One thing that I think we probably should change is the relabelto/from
permissions in the function above (selinux_tun_dev_attach()); in the case
where the socket does not yet have a label, e.g. 'sksec->sid == 0', we should
probably skip the relabel permissions since we want to assign the TUN device
label regardless in this case.

--
paul moore
security and virtualization @ redhat