selinux: fix setting of security labels on NFS

commit 9fc2b4b4 upstream.

Before calling into the filesystem, vfs_setxattr calls
security_inode_setxattr, which ends up calling selinux_inode_setxattr in
our case.  That returns -EOPNOTSUPP whenever SBLABEL_MNT is not set.
SBLABEL_MNT was supposed to be set by sb_finish_set_opts, which sets it
only if selinux_is_sblabel_mnt returns true.

The selinux_is_sblabel_mnt logic was broken by eadcabc6 "SELinux: do
all flags twiddling in one place", which didn't take into the account
the SECURITY_FS_USE_NATIVE behavior that had been introduced for nfs
with eb9ae686 "SELinux: Add new labeling type native labels".

This caused setxattr's of security labels over NFSv4.2 to fail.

Cc: Eric Paris <eparis@redhat.com>
Cc: David Quigley <dpquigl@davequigley.com>
Reported-by: Richard Chan <rc556677@outlook.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: added the stable dependency]
Signed-off-by: Paul Moore <pmoore@redhat.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

security/selinux/hooks.c

@@ -394,7 +394,8 @@ static int selinux_is_sblabel_mnt(struct super_block *sb)
 
 	if (sbsec->behavior == SECURITY_FS_USE_XATTR ||
 	    sbsec->behavior == SECURITY_FS_USE_TRANS ||
-	    sbsec->behavior == SECURITY_FS_USE_TASK)
+	    sbsec->behavior == SECURITY_FS_USE_TASK ||
+	    sbsec->behavior == SECURITY_FS_USE_NATIVE)
 		return 1;
 
 	/* Special handling for sysfs. Is genfs but also has setxattr handler*/