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

[Eric Biggers]

On Sat, Jan 15, 2022 at 08:31:01AM +0300, Vitaly Chikunov wrote:
> 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.
> 

I was going to reply to this thread again, but I got a bit distracted by
everything else being broken.  Yes, the kernel needs to be restricting which
hash algorithms can be used with each public key algorithm, along the lines of
what you said.  I asked the BoringSSL maintainers for advice, and they confirmed
that ECDSA just signs/verifies a raw hash, and in fact it *must* be a raw hash
for it to be secure.  This is a design flaw in ECDSA, which was fixed in newer
algorithms such as EdDSA and SM2 as those have a hash built-in to the signature
scheme.  To mitigate it, the allowed hash algorithms must be restricted; in the
case of ECDSA, that means to the SHA family (preferably excluding SHA-1).

akcipher_alg::verify doesn't actually know which hash algorithm is used, except
in the case of rsa-pkcs1pad where it is built into the name of the algorithm.
So it can't check the hash algorithm.  I believe it needs to happen in
public_key_verify_signature() (and I'm working on a patch for that).

Now, SM2 is different from ECDSA and ECRDSA in that it uses the modern design
that includes the hash into the signature algorithm.  This means that it must be
used to sign/verify *data*, not a hash.  (Well, you can sign/verify a hash, but
SM2 will hash it again internally.)  Currently, public_key_verify_signature()
allows SM2 to be used to sign/verify a hash, skipping the SM2 internal hash, and
IMA uses this.  This is broken and must be removed, since it isn't actually the
SM2 algorithm as specified anymore, but rather some homebrew thing with unknown
security properties. (Well, I'm not confident about SM2, but homebrew is worse.)

Adding fs-verity support to IMA also complicates things, as doing it naively
would introduce an ambiguity about what is signed.  Naively, the *data* that is
signed (considering the hash as part of the signature algorithm) would be either
the whole file, in the case of traditional IMA, or the fsverity_descriptor
struct, in the case of IMA with fs-verity.  However, a file could have contents
which match an fsverity_descriptor struct; that would create an ambiguity.

Assuming that it needs to be allowed that the same key can sign files for both
traditional and fs-verity hashing, solving this problem will require a second
hash.  The easiest way to do this would be sign/verify the following struct:

	struct ima_file_id {
		u8 is_fsverity;
		u8 hash_algorithm;
		u8 hash[];
	};

This would be the *data* that is signed/verified -- meaning that it would be
hashed again as part of the signature algorithm (whether that hash is built-in
to the signature algorithm, as is the case for modern algorithms, or handled by
the caller as is the case for legacy algorithms).  Note that both traditional
and fs-verity hashes would need to use this same method for it to be secure; the
kernel must not accept signatures using the old method at the same time.

- Eric