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

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Date:   Thu, 02 Dec 2021 17:13:15 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-integrity@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/5] ima: support fs-verity file digest based
 signatures

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).

Mimi