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
