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Message-ID: <YalGQHTq51SHqC1a@sol.localdomain>
Date: Thu, 2 Dec 2021 14:18:40 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Mimi Zohar <zohar@...ux.ibm.com>
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, 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
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