ovl: check privs before decoding file handle

CAP_DAC_READ_SEARCH is required by open_by_handle_at(2) so check it in
ovl_decode_real_fh() as well to prevent privilege escalation for
unprivileged overlay mounts.

[Amir] If the mounter is not capable in init ns, ovl_check_origin() and
ovl_verify_index() will not function as expected and this will break index
and nfs export features.  So check capability in ovl_can_decode_fh(), to
auto disable those features.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/namei.c

@@ -156,6 +156,9 @@ struct dentry *ovl_decode_real_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
 	struct dentry *real;
 	int bytes;
 
+	if (!capable(CAP_DAC_READ_SEARCH))
+		return NULL;
+
 	/*
 	 * Make sure that the stored uuid matches the uuid of the lower
 	 * layer where file handle will be decoded.

fs/overlayfs/util.c

@@ -50,6 +50,9 @@ const struct cred *ovl_override_creds(struct super_block *sb)
  */
 int ovl_can_decode_fh(struct super_block *sb)
 {
+	if (!capable(CAP_DAC_READ_SEARCH))
+		return 0;
+
 	if (!sb->s_export_op || !sb->s_export_op->fh_to_dentry)
 		return 0;