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

[Vitaly Chikunov <vt@altlinux.org>]

Eric,

On Sun, Jan 09, 2022 at 01:07:18PM -0800, Eric Biggers wrote:
> On Sun, Jan 09, 2022 at 11:45:37PM +0300, Vitaly Chikunov wrote:
> > On Wed, Jan 05, 2022 at 03:37:39PM -0800, Eric Biggers wrote:
> > > On Fri, Dec 31, 2021 at 10:35:00AM -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.
> > > > 
> > > > 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.
> > 
> > Unless there is broken hash algorithm allowing for preimage attacks this
> > is irrelevant. If there is two broken algorithms allowing for collisions,
> > colliding hashes could be prepared even if algo id is hashed too.
> > 
> 
> Only one algorithm needs to be broken.  For example, SM3 has the same hash
> length as SHA-256.  If SM3 support were to be added to fs-verity, and if someone
> were to find a way to find an input that has a specific SM3 digest, then they
> could also make it match a specific SHA-256 digest.  Someone might intend to
> sign a SHA-256 digest, but if they are only signing the raw 32 bytes of the
> digest, then they would also be signing the corresponding SM3 digest.  That's
> why the digest that is signed *must* also include the algorithm used in the
> digest (not the algorithm(s) used in the signature, which is different).

I think it will be beneficial if we pass hash algo id to the
akcipher_alg::verify. In fact, ecrdsa should only be used with streebog.
And perhaps, sm2 with sm3, pkcs1 with md/sha/sm3, and ecdsa with sha family
hashes.

Thanks,


> 
> - Eric