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

From: Paul Moore <[email-masked]>
Date: Tue, 04 Dec 2012 11:18:57 -0500

On Tuesday, December 04, 2012 09:24:43 PM Jason Wang wrote:
> On Monday, December 03, 2012 11:22:29 AM Paul Moore wrote:
> > It may be that I'm misunderstanding TUNSETQUEUE and/or TUNSETIFF.  Can you
> > elaborate as to why they should be different?
> 
> If I understand correctly, before multiqueue patchset, TUNSETIFF is used to:
> 
> 1) Create the tun/tap network device
> 2) For persistent device, re-attach the fd to the network device / socket.
> In this case, we call selinux_tun_dev_attch() to relabel the socket sid (in
> fact also the device's since the socket were persistent also) to the sid of
> process that calls TUNSETIFF.
> 
> So, after the changes of multiqueue, we need try to preserve those policy.
> The interesting part is the introducing of TUNSETQUEUE, it's used to attach
> more file descriptors/sockets to a tun/tap device after at least one file
> descriptor were attached to the tun/tap device through TUNSETIFF. So I
> think maybe we need differ those two ioctls. This patch looks fine for
> TUNSETQUEUE, but for TUNSETIFF, we need relabel the tunsec to the process
> that calling TUNSETIFF for persistent device?

Okay, based on your explanation of TUNSETQUEUE, the steps below are what I 
believe we need to do ... if you disagree speak up quickly please.

A. TUNSETIFF (new, non-persistent device)

[Allocate and initialize the tun_struct LSM state based on the calling 
process, use this state to label the TUN socket.]

1. Call security_tun_dev_create() which authorizes the action.
2. Call security_tun_dev_alloc_security() which allocates the tun_struct LSM 
blob and SELinux sets some internal blob state to record the label of the 
calling process.
3. Call security_tun_dev_attach() which sets the label of the TUN socket to 
match the label stored in the tun_struct LSM blob during A2.  No authorization 
is done at this point since the socket is new/unlabeled.

B. TUNSETIFF (existing, persistent device)

[Relabel the existing tun_struct LSM state based on the calling process, use
this state to label the TUN socket.]

1. Attempt to relabel/reset the tun_struct LSM blob from the currently stored 
value, set during A2, to the label of the current calling process. *** THIS IS 
NOT CURRENTLY DONE IN THE RFC PATCH ***
2. Call security_tun_dev_attach() which sets the label of the TUN socket to
match the label stored in the tun_struct LSM blob during B1. No authorization 
is done at this point since the socket is new/unlabeled.

C. TUNSETQUEUE

[Use the existing tun_struct LSM state to label the new TUN socket.]

1. Call security_tun_dev_attach() which sets the label of the TUN socket to 
match the label stored in the tun_struct LSM blob set during either A2 or B1.  
No authorization is done at this point since the socket is new/unlabeled.

> btw. Current code does allow calling TUNSETQUEUE to a persistent tun/tap
> device with no file attached. It should be a bug and need to be fixed.

Since you wrote that code will you be submitting a patch to fix that problem?

> > 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.
> 
> I'm not familiar with the selinux, have a quick glance of the code, looks
> like the label has been initialized to SECINITSID_KERNEL in
> selinux_socket_post_create().

Unless I've missed something in your changes, the multiqueue code never calls 
any socket code which ends up calling {security,selinux}_socket_post_create(); 
I believe you only call sk_alloc() which ends up calling 
{security,selinux}_sk_alloc() which sets SECINITSID_UNLABELED (I mistakenly 
wrote 0 instead in my earlier email which is techincally SECSID_NULL).  Either 
way, I still think the logic I originally described above is correct.

-- 
paul moore
security and virtualization @ redhat