Re: Proposal: Yet another possible fs-verity interface

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Feb 12, 2019 at 12:24:33PM -0500, Theodore Y. Ts'o wrote:
> 
> > > > The existing file hashes included in the measurement list and the
> > > > audit log, are currently being used for remote attestation, forensics
> > > > and security analytics.
> > 
> > Again, the context for this comment was Linus' suggestion "each level
> > of the merkle tree needs to have a hash seeding thing or whatever."
> > Up to this point, I had assumed the Merkle tree file root hash could
> > be used as an identifier, similar to the file hash.  With his
> > suggestion, it sounds like the Merkle tree file root hash would be
> > system dependent, making it useless for the above usages.
> 
> Yeah, I have no idea what Linus was talking about there.  The only
> thing that really makes sense is that if you don't have any
> file-system place to store a seed, you don't use a seed for the Merkle
> tree, and for a given set of bytes, the Merkle root hash is the same.
> So it's basically an expensive to calculate crypto checksum, as I said.
> 

I think there's confusion due to the use of the phrase "Merkle tree root hash".
Linus's point was:

> Now, since I assume that only the merkle tree root hash would be
> returned by the "enable merkle tree" operation (so that the code
> enabling it can verify that the hash matches the expected value), you
> do have to worry about the preimage attack, and make sure that you
> can't fool the hashing by making the (bad) file contents themselves be
> just the hashes of the (good) blocks. So each level of the merkle tree
> needs to have a hash seeding thing or whatever.

This is already taken into account in the original design.  The file hash
reported by fs-verity is *not* the Merkle tree root hash directly, but rather a
hash of the Merkle tree root hash and additional metadata in the
fsverity_descriptor including the file size in bytes.  This resulting hash is
referred to in the code, documentation, etc. as the "fs-verity file measurement".

Thus you can't fool the hashing in the way that Linus mentioned, because the
file size is included in the hash too.  And I don't expect this part of the
design should change, even if we change the API.

It's been difficult to get people to start saying "fs-verity file measurement"
rather than "Merkle tree root hash", though, so if anyone has a suggestion for a
better name it would be appreciated.  An earlier name was "fs-verity root hash",
but I thought that would too easily be confused with the Merkle tree root hash.

fs-verity does support a salt as well, but it's optional and isn't needed to
prevent preimage attacks, assuming the user chose a strong cryptographic hash
function such as SHA-256 or SHA-512.

- Eric