[PATCH] selinux: Fix SBLABEL_MNT for NFS mounts

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:

quoted

On 29 March 2017 at 23:34, J. Bruce Fields [off-list ref]
wrote:

quoted

On Wed, Mar 29, 2017 at 05:27:23PM +0200, Tomeu Vizoso wrote:

quoted

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.
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