selinux: make security_sb_clone_mnt_opts return an error on context mismatch

selinux: make security_sb_clone_mnt_opts return an error on context mismatch

I had the following problem reported a while back. If you mount the
same filesystem twice using NFSv4 with different contexts, then the
second context= option is ignored. For instance:

    # mount server:/export /mnt/test1
    # mount server:/export /mnt/test2 -o context=system_u:object_r:tmp_t:s0
    # ls -dZ /mnt/test1
    drwxrwxrwt. root root system_u:object_r:nfs_t:s0       /mnt/test1
    # ls -dZ /mnt/test2
    drwxrwxrwt. root root system_u:object_r:nfs_t:s0       /mnt/test2

When we call into SELinux to set the context of a "cloned" superblock,
it will currently just bail out when it notices that we're reusing an
existing superblock. Since the existing superblock is already set up and
presumably in use, we can't go overwriting its context with the one from
the "original" sb. Because of this, the second context= option in this
case cannot take effect.

This patch fixes this by turning security_sb_clone_mnt_opts into an int
return operation. When it finds that the "new" superblock that it has
been handed is already set up, it checks to see whether the contexts on
the old superblock match it. If it does, then it will just return
success, otherwise it'll return -EBUSY and emit a printk to tell the
admin why the second mount failed.

Note that this patch may cause casualties. The NFSv4 code relies on
being able to walk down to an export from the pseudoroot. If you mount
filesystems that are nested within one another with different contexts,
then this patch will make those mounts fail in new and "exciting" ways.

For instance, suppose that /export is a separate filesystem on the
server:

    # mount server:/ /mnt/test1
    # mount salusa:/export /mnt/test2 -o context=system_u:object_r:tmp_t:s0
    mount.nfs: an incorrect mount option was specified

...with the printk in the ring buffer. Because we *might* eventually
walk down to /mnt/test1/export, the mount is denied due to this patch.
The second mount needs the pseudoroot superblock, but that's already
present with the wrong context.

OTOH, if we mount these in the reverse order, then both mounts work,
because the pseudoroot superblock created when mounting /export is
discarded once that mount is done. If we then however try to walk into
that directory, the automount fails for the similar reasons:

    # cd /mnt/test1/scratch/
    -bash: cd: /mnt/test1/scratch: Device or resource busy

The story I've gotten from the SELinux folks that I've talked to is that
this is desirable behavior. In SELinux-land, mounting the same data
under different contexts is wrong -- there can be only one.

Cc: Steve Dickson <steved@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>