On Mon, 2020-08-10 at 10:13 -0700, James Bottomley wrote: > On Mon, 2020-08-10 at 12:35 -0400, Mimi Zohar wrote: > > On Mon, 2020-08-10 at 08:35 -0700, James Bottomley wrote: > [...] > > > > Up to now, verifying remote filesystem file integrity has been > > > > out of scope for IMA. With fs-verity file signatures I can at > > > > least grasp how remote file integrity could possibly work. I > > > > don't understand how remote file integrity with existing IMA > > > > formats could be supported. You might want to consider writing a > > > > whitepaper, which could later be used as the basis for a patch > > > > set cover letter. > > > > > > I think, before this, we can help with the basics (and perhaps we > > > should sort them out before we start documenting what we'll do). > > > > I'm not opposed to doing that, but you're taking this discussion in a > > totally different direction. The current discussion is about NFSv4 > > supporting the existing IMA signatures, not only fs-verity > > signatures. I'd like to understand how that is possible and for the > > community to weigh in on whether it makes sense. > > Well, I see the NFS problem as being chunk at a time, right, which is > merkle tree, or is there a different chunk at a time mechanism we want > to use? IMA currently verifies signature on open/exec and then > controls updates. Since for NFS we only control the client, we can't > do that on an NFS server, so we really do need verification at read > time ... unless we're threading IMA back to the NFS server? Yes. I still don't see how we can support the existing IMA signatures, which is based on the file data hash, unless the "chunk at a time mechanism" is not a tree, but linear. Mimi > > > > The first basic is that a merkle tree allows unit at a time > > > verification. First of all we should agree on the unit. Since we > > > always fault a page at a time, I think our merkle tree unit should > > > be a page not a block. Next, we should agree where the check gates > > > for the per page accesses should be ... definitely somewhere in > > > readpage, I suspect and finally we should agree how the merkle tree > > > is presented at the gate. I think there are three ways: > > > > > > 1. Ahead of time transfer: The merkle tree is transferred and > > > verified > > > at some time before the accesses begin, so we already have a > > > verified copy and can compare against the lower leaf. > > > 2. Async transfer: We provide an async mechanism to transfer > > > the > > > necessary components, so when presented with a unit, we check > > > the > > > log n components required to get to the root > > > 3. The protocol actually provides the capability of 2 (like the > > > SCSI > > > DIF/DIX), so to IMA all the pieces get presented instead of > > > IMA > > > having to manage the tree > > > > > > There are also a load of minor things like how we get the head > > > hash, which must be presented and verified ahead of time for each > > > of the above 3. > > > > > > I was under the impression that IMA support for fs-verity signatures > > would be limited to including the fs-verity signature in the > > measurement list and verifying the fs-verity signature. As fs- > > verity is limited to immutable files, this could be done on file > > open. fs-verity would be responsible for enforcing the block/page > > data integrity. From a local filesystem perspective, I think that > > is all that is necessary. > > The fs-verity use case is a bit of a crippled one because it's > immutable. I think NFS represents more the general case where you > can't rely on immutability and have to verify at chunk read time. If > we get chunk at a time working for NFS, it should work also for fs- > verity and we wouldn't need to have two special paths. > > I think, even for NFS we would only really need to log the open, so > same as you imagine for fs-verity. As long as the chunk read hashes > match, we can be silent because everything is going OK, so we only need > to determine what to do and log on mismatch (which isn't expected to > happen for fs-verity). > > > In terms of remote file systems, the main issue is transporting and > > storing the Merkle tree. As fs-verity is limited to immutable files, > > this could still be done on file open. > > Right, I mentioned that in my options ... we need some "supply > integrity" hook ... or possibly multiple hooks for a variety of > possible methods.
