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
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