linux-ext4 - Re: [PATCH] ext4: inherit encryption xattr before other xattrs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170227213637.GA123733@gmail.com>
Date:   Mon, 27 Feb 2017 13:36:37 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] ext4: inherit encryption xattr before other xattrs

On Mon, Feb 27, 2017 at 01:28:28PM -0700, Andreas Dilger wrote:
> On Feb 17, 2017, at 4:33 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> > 
> > From: Eric Biggers <ebiggers@...gle.com>
> > 
> > When using both encryption and SELinux (or another feature that requires
> > an xattr per file) on a filesystem with 256-byte inodes, each file's
> > xattrs usually spill into an external xattr block.  Currently, the
> > xattrs are inherited in the order ACL, security, then encryption.
> > Therefore, if spillage occurs, the encryption xattr will always end up
> > in the external block.  This is not ideal because the encryption xattrs
> > contain a nonce, so they will always be unique and will prevent the
> > external xattr blocks from being deduplicated.
> > 
> > To improve the situation, change the inheritance order to encryption,
> > ACL, then security.  This gives the encryption xattr a better chance to
> > be stored in-inode, allowing the other xattr(s) to be deduplicated.
> > 
> > Note that it may be better for userspace to format the filesystem with
> > 512-byte inodes in this case.  However, it's not the default.
> > 
> > Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> 
> Reviewed-by: Andreas Dilger <adilger@...ger.ca>
> 
> Note that we've been using 512-byte xattrs for Lustre metadata servers
> for ages.  We may want to consider enabling this by default when the
> filesystem features are set (e.g. crypto, inline data, etc).
> 
> Having a general mechanism to bias xattrs to in-inode or external xattr
> storage would be nice also, but I don't know any way to do this beyond
> just having a list of xattr names and then prioritizing the ones that
> go into the in-inode space.
> 

I think it's a good idea to have mke2fs default to 512-byte inodes if
'-O inline_data' is specified.  But with '-O encrypt' it may be more debatable
because there is no guarantee as to how many files userspace will actually
choose to encrypt.  It could be almost the whole filesystem, or just a few
files, or even nothing at all.

Regardless, I think adjusting the xattr inheritance order (as this patch does)
has advantages but no real disadvantages, so it might as well be done too.

Eric