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

[Eric Biggers]

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.

- Eric