Re: [PATCH v1 4/5] ima: support fs-verity file digest based signatures

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Dec 31, 2021 at 10:35:00AM -0500, Mimi Zohar wrote:
> Hi Eric,
> 
> On Thu, 2021-12-02 at 14:07 -0800, Eric Biggers wrote:
> > On Thu, Dec 02, 2021 at 04:55:06PM -0500, Mimi Zohar wrote:
> > >  	case IMA_VERITY_DIGSIG:
> > > -		fallthrough;
> > > +		set_bit(IMA_DIGSIG, &iint->atomic_flags);
> > > +
> > > +		/*
> > > +		 * The IMA signature is based on a hash of IMA_VERITY_DIGSIG
> > > +		 * and the fs-verity file digest, not directly on the
> > > +		 * fs-verity file digest.  Both digests should probably be
> > > +		 * included in the IMA measurement list, but for now this
> > > +		 * digest is only used for verifying the IMA signature.
> > > +		 */
> > > +		verity_digest[0] = IMA_VERITY_DIGSIG;
> > > +		memcpy(verity_digest + 1, iint->ima_hash->digest,
> > > +		       iint->ima_hash->length);
> > > +
> > > +		hash.hdr.algo = iint->ima_hash->algo;
> > > +		hash.hdr.length = iint->ima_hash->length;
> > 
> > This is still wrong because the bytes being signed don't include the hash
> > algorithm.  Unless you mean for it to be implicitly always SHA-256?  fs-verity
> > supports SHA-512 too, and it may support other hash algorithms in the future.
> 
> IMA assumes that the file hash algorithm and the signature algorithm
> are the same.   If they're not the same, for whatever reason, the
> signature verification would simply fail.
> 
> Based on the v2 signature header 'type' field, IMA can differentiate
> between regular IMA file hash based signatures and fs-verity file
> digest based signatures.  The digest field (d-ng) in the IMA
> meausrement list prefixes the digest with the hash algorithm. I'm
> missing the reason for needing to hash fs-verity's file digest with
> other metadata, and sign that hash rather than fs-verity's file digest
> directly.

Because if someone signs a raw hash, then they also implicitly sign the same
hash value for all supported hash algorithms that produce the same length hash.
Signing a raw hash is only appropriate when there is only 1 supported algorithm.

All the other stuff you mentioned is irrelevant.

- Eric