Re: [PATCH 2/4] ima: define a new signature type named IMA_VERITY_DIGSIG

[Mimi Zohar]

Hi Eric,

On Tue, 2021-11-30 at 13:14 -0500, Mimi Zohar wrote:
> 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@xxxxxxxxxxxxx>
> > 
> > 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.

By signing the file hash, the sig field in the IMA measurement list can
be directly verified against the digest field.  For appended
signatures, we defined a new template named ima-modsig which contains
two file hashes, with and without the appended signature.

Similarly, by signing a digest containing other metadata and fs-
verity's file digest, the measurement list should include both digests.
Otherwise the consumer of the measurement list would first need to
calculate the signed digest before verifying the signature.

Options:
- Include just fs-verity's file digest and the signature in the
measurement list.  Leave it to the consumer of the measurement list to
deal with.
- Define a new template format to include both digests, add a new field
in the iint for the signed digest.  (Much more work.)
- As originally posted, directly sign fs-verity's file digest.

thanks,

Mimi