linux-kernel - Re: [PATCH 2/4] ima: define a new signature type named IMA_VERITY

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Date:   Tue, 30 Nov 2021 13:14:11 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-integrity@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] ima: define a new signature type named
 IMA_VERITY_DIGSIG

On Mon, 2021-11-29 at 18:33 -0800, Eric Biggers wrote:
> On Mon, Nov 29, 2021 at 12:00:55PM -0500, Mimi Zohar wrote:
> > To differentiate between a regular file hash and an fs-verity file digest
> > based signature stored as security.ima xattr, define a new signature type
> > named IMA_VERITY_DIGSIG.
> > 
> > Signed-off-by: Mimi Zohar <zohar@...ux.ibm.com>
> 
> For this new signature type, what bytes are actually signed?  It looks like it's
> just the raw digest, which isn't sufficient since it is ambiguous.  It needs to
> include information that makes it clear what the signer is actually signing,
> such as "this is an fs-verity SHA-256 file digest".  See
> 'struct fsverity_formatted_digest' for an example of this (but it isn't
> necessary to use that exact structure).
> 
> I think the existing IMA signatures have the same problem (but it is hard for me
> to understand the code).  However, a new signature type doesn't have
> backwards-compatibility concerns, so it could be done right.

As this change should probably be applicable to all signature types,
the signature version in the  signature_v2_hdr should be bumped.  The
existing signature version could co-exist with the new signature
version.

thanks,

Mimi