Re: [PATCH] selinux: Fix SBLABEL_MNT for NFS mounts

[J. Bruce Fields]

On Thu, Mar 30, 2017 at 01:27:07PM -0400, Stephen Smalley wrote:
> On Thu, 2017-03-30 at 09:49 +0200, Tomeu Vizoso wrote:
> > On 29 March 2017 at 23:34, J. Bruce Fields <bfields@xxxxxxxxxx>
> > wrote:
> > > On Wed, Mar 29, 2017 at 05:27:23PM +0200, Tomeu Vizoso wrote:
> > > > Labelling of files in a NFSv4.2 currently fails with ENOTSUPP
> > > > because
> > > > the mount point doesn't have SBLABEL_MNT.
> > > > 
> > > > Add specific condition for NFS4 filesystems so it gets correctly
> > > > labeled.
> > > 
> > > Huh.  Looking at the code, I think this is meant to be handled by
> > > the
> > > SECURITY_FS_USE_NATIVE case--there was a similar failure fixed some
> > > time
> > > ago by 9fc2b4b436cf.  What kernel are you seeing this on?  Is it a
> > > recent regression (in which case, what's the latest kernel that
> > > worked
> > > for you)?
> > 
> > I have seen this on 4.11-rc4, but I never tried to get this working
> > before.
> > 
> > I will try to find time to see why SECURITY_FS_USE_NATIVE isn't
> > working here.
> 
> Does your exports file specify the "security_label" option, e.g.
> /path/to/dir example.com(rw,security_label)

Oops, right, that should have been the first thing I asked about....

> It appears that with recent kernels that is now required; otherwise,
> the mount defaults to not enabling native labeling and all of the files
> are treated as having a single, fixed label defined by the client
> policy (and hence setxattr is not supported).  This was kernel commit
> 32ddd944a056c786f6acdd95ed29e994adc613a2.  I don't recall seeing any
> discussion of this on selinux list.  I understand the rationale, but it
> seems like a user-visible regression

It is.  I also want to keep new protocol upgrades free of user
regressions, which the 4.1->4.2 upgrade is in most cases if we turn on
security labeling by default.  So I was stuck choosing between two
regresisons, and figured 4.2 user depending on security labeling was
still the much rarer case.

So I'd like to keep security labeling off by default, but if there's
anything I can do to smooth the transition obviously that's good.

> and at the very least, it seems odd that they didn't just use
> "seclabel" as the kernel does in /proc/mounts to signify a filesystem
> that supports security labeling by userspace.

I see logic in sb_finish_set_opts() that sets SBLABEL_MNT in the
selinux_is_sblabel_mnt() case.  Doesn't that mean "seclabel" shows up in
/proc/mounts when we nfs sets SECURITY_LSM_NATIVE_LABELS?

I may not understand your comment, I'm pretty unfamiliar with this area.

--b.