linux-ext4 - Re: [RFC PATCH 00/10] fs-verity: filesystem-level integrity protection

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Date:   Fri, 31 Aug 2018 22:05:23 +0200
From:   Jan Lübbe <jlu@...gutronix.de>
To:     Eric Biggers <ebiggers@...nel.org>, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net
Cc:     linux-integrity@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Victor Hsieh <victorhsieh@...gle.com>
Subject: Re: [RFC PATCH 00/10] fs-verity: filesystem-level integrity
 protection

On Fri, 2018-08-24 at 09:16 -0700, Eric Biggers wrote:
[...]
> Since fs-verity provides the Merkle tree root hash in constant time and
> verifies data blocks on-demand, it is useful for efficiently verifying
> the authenticity of, or "appraising", large files of which only a small
> portion may be accessed -- such as Android application (APK) files.  It
> can also be useful in "audit" use cases where file hashes are logged.
> fs-verity also provides better protection against malicious disk
> firmware than an ahead-of-time hash, since fs-verity re-verifies data
> each time it's paged in.
[...]
> Feedback on the design and implementation is greatly appreciated.

Hi,

I've looked at the series and the slides linked form the recent lwn.net
article, but I'm not sure how fs-verity intends to protect against
malicious firmware (or offline modification). Similar to IMA/EVM, fs-
verity doesn't seem to include the name/location of the file into it's
verification. So the firmware/an attacker could replace one fs-verity-
protected file with another (maybe a trusted system APK with another
one for which a vulnerability was discovered, or /sbin/init with
/bin/sh).

Is the expected root hash of the file provided from somewhere else, so
this is not a problem on Android? Or is this problem out-of-scope for
fs-verity?

For IMA/EVM, there were patches by Dmitry to address this class of
attacks (they were not merged, though):
https://lwn.net/Articles/574221/

Thanks,
Jan

[1] https://events.linuxfoundation.org/wp-content/uploads/2017/11/fs-ve
rify_Mike-Halcrow_Eric-Biggers.pdf
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