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

[Eric Biggers]

On Thu, Dec 02, 2021 at 05:13:15PM -0500, Mimi Zohar wrote:
> 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.
> 
> The signature stored in security.ima is prefixed with a header
> (signature_v2_hdr).

Yes, but the byte that identifies the hash algorithm is not included in the
bytes that are actually signed, as far as I can tell.

- Eric