Re: [PATCH v3 7/8] ima: support fs-verity file digest based version 3 signatures

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2022-01-31 at 17:06 -0800, Eric Biggers wrote:
> On Tue, Jan 25, 2022 at 07:06:57PM -0500, Mimi Zohar wrote:
> > Instead of calculating a file hash and verifying the signature stored
> > in the security.ima xattr against the calculated file hash, verify
> > fs-verity's signature (version 3).
> > 
> > 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.
> > 
> > Update the 'ima-sig' template field to display the new fs-verity signature
> > type as well.
> > 
> > For example:
> >   appraise func=BPRM_CHECK digest_type=hash|verity
> > 
> > Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
> > ---
> >  Documentation/ABI/testing/ima_policy      | 10 +++++
> >  Documentation/security/IMA-templates.rst  |  4 +-
> >  security/integrity/ima/ima_appraise.c     | 49 ++++++++++++++++++++++-
> >  security/integrity/ima/ima_template_lib.c |  3 +-
> >  security/integrity/integrity.h            |  5 ++-
> >  5 files changed, 65 insertions(+), 6 deletions(-)
> 
> All this IMA-specific stuff is confusing to me, so let me ask a question about
> what the end result actually is.  Let's say I want to use IMA to authenticate
> ("appraise") a file.  I've signed its fs-verity digest with a key.  I put only
> that one key in the IMA keyring, and that key was only ever used to sign that
> one fs-verity digest.  Can an attacker (who controls the file's contents and IMA
> xattr) replace the file with one with a different contents and still pass the
> IMA check?  For example, could they replace the file's contents with the
> ima_file_id of the authentic file, and then downgrade the signature version to
> v2?  If they can do that, then the goal of authentication wasn't met.  It might
> be necessary to enforce that only one signature version is used at a time, to
> avoid this kind of ambiguity.

Instead of only allowing a single signature version, the signature
verification could be based on policy rules.   "ima: include fsverity's
file digests in the IMA measurement list" defines the new policy rule
'digest_type=' option, which currently permits either IMA or fsverity
signatures to match.  Instead only allow IMA or fsverity signatures,
not both, on a per policy rule basis.

From an IMA perspective, this would be safe since the builtin policies
do not support fs-verity signatures.  After loading a custom policy,
additional rules can only extend the custom policy.

thanks,

Mimi