From: Eric Biggers Subject: Re: Locking rules for fscrypt_operations->set_context() Date: Wed, 21 Sep 2016 17:14:02 -0700 Message-ID: <20160922001402.GA124993@google.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-ext4@vger.kernel.org, linux-fsdevel , "linux-kernel@vger.kernel.org" , Theodore Ts'o , David Gstir To: Richard Weinberger Return-path: Received: from mail-pf0-f170.google.com ([209.85.192.170]:36051 "EHLO mail-pf0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753383AbcIVAOK (ORCPT ); Wed, 21 Sep 2016 20:14:10 -0400 Received: by mail-pf0-f170.google.com with SMTP id q2so24114542pfj.3 for ; Wed, 21 Sep 2016 17:14:09 -0700 (PDT) Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, Sep 20, 2016 at 04:30:06PM +0200, Richard Weinberger wrote: > Hi! > > To my understanding ->setxattr() is always being called with i_mutex held. > ->set_context() in ext4 stores the security context using ext4_xattr_set(), > but the fs crypto framework does not lock the inode itself. > So, depending on the call path, ext4_xattr_set() is sometimes being > called with i_mutex held and some times not. > > What are the locking rules for fscrypt_operations and especially ->set_context()? Hi Richard, this is a great question. I would like to document somewhere the semantics of each of the fscrypt_operations, but I am still figuring them out myself. With regards to ->set_context(), it is called in two distinct situtations: (1) when a user process uses FS_IOC_SET_ENCRYPTION_POLICY to set the encryption policy on an empty directory (2) when an encryption policy is inherited by a newly created file in an encrypted directory In case (1), I think there needs to be an inode_lock() added. For ext4 and f2fs, it looks like setting an xattr without inode_lock() isn't problematic by itself. Instead, the problem I see is that fscrypt_process_policy() does several operations, including the ->empty_dir() check, which aren't guaranteed to be atomic if the directory inode is not locked with inode_lock(). In case (2), I don't think it matters whether inode_lock() is held, since the inode is still being initialized and is still "locked" in a different way, in the I_NEW state. There are also other xattrs being set in __ext4_new_inode(), seemingly without inode_lock(), which I *think* is fine. So I am currently thinking that fscrypt_process_policy() should be fixed to do inode_lock(), and ->set_context() should be documented as a filesystem internal operation (not necessarily related to ->setxattr()) that is called on either an inode_lock()-ed inode or on an I_NEW inode. Eric