linux-ext4 - Re: [PATCH 1/6] fscrypt: add v2 encryption context and policy

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]

Date:   Fri, 14 Jul 2017 13:08:10 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     Michael Halcrow <mhalcrow@...gle.com>,
        linux-fscrypt@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, linux-crypto@...r.kernel.org,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Alex Cope <alexcope@...gle.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 1/6] fscrypt: add v2 encryption context and policy

On Jul 13, 2017, at 3:58 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> 
> Hi Michael,
> 
> On Thu, Jul 13, 2017 at 03:29:44PM -0700, Michael Halcrow wrote:
>> On Wed, Jul 12, 2017 at 02:00:30PM -0700, Eric Biggers wrote:
>>> From: Eric Biggers <ebiggers@...gle.com>
>>> 
>>> Currently, the fscrypt_context (i.e. the encryption xattr) does not
>>> contain a cryptographically secure identifier for the master key's
>>> payload.  Therefore it's not possible to verify that the correct key was
>>> supplied, which is problematic in multi-user scenarios.  To make this
>>> possible, define a new fscrypt_context version (v2) which includes a
>>> key_hash field, and allow userspace to opt-in to it when setting an
>>> encryption policy by setting fscrypt_policy.version to 2.  For now just
>>> zero the new field; a later patch will start setting it for real.
>> 
>> The main concern that comes to mind is potentially blowing past the
>> inline xattr size limit and allocating a new inode block.  The
>> security benefit probably outweighs that concern in this case.
>> 
> 
> The way it adds up now for ext4 is:
> 
> 128 bytes for base inode
> + 32 bytes for i_extra fields
> + 4 bytes for in-inode xattrs header
> + 20 bytes for encryption xattr header + name
> + 28 bytes for encryption xattr value
> ----------------------------------
> = 212 bytes total.
> 
> By adding the 16-byte 'key_hash' field it grows to 228 bytes total.  So it still
> fits in a 256-byte inode, though it's getting closer to the limit.  We could
> save 8 bytes by instead using the design where master_key_descriptor is extended
> to 16 bytes and redefined as a cryptographically secure hash.  But as noted,
> that has some significant disadvantages.
> 
> Also note that we don't really have to worry about leaving space for a SELinux
> xattr anymore because with 256-byte inodes + encryption the SELinux xattr is
> already being written to an external block, given that it requires about 52-62
> bytes (at least when using Android's SELinux policy; different SELinux policies
> may use different values), and 212 + 52 > 256.  So if someone wants both xattrs
> in-inode they need to use 512-byte inodes already.

It is probably time to consider changing to a default of 512-byte inodes for
larger filesystems anyway.  In our testing, this affected performance only by
a couple of percent under normal usage, and avoided a significant performance
drop if the xattrs ever fall out of the inode.

Cheers, Andreas






Download attachment "signature.asc" of type "application/pgp-signature" (196 bytes)