Resending ... > On Aug 28, 2019, at 1:36 PM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > Last week I presented at the Linux Security Summit on a proposal > for handling IMA metadata on NFS files. My proposal enables storage > of per-file IMA metadata via the NFSv4 protocol. I have a prototype > and an IETF nfsv4 Working Group document that specifies a small > protocol extension. > > After the presentation, Mimi Zohar pointed out that although the > proposal extends protection from an NFS file server to time-of- > measurement on an NFS client, there is still a protection gap between > time-of-measurement and time-of-use on that client. > > I would like to find a way to extend IMA protection all the way > to time-of-use on NFS clients. The consensus is that a per-file > Merkle tree would be the most desirable approach, as that is the > same mechanism used for fs-verity protection. > > For a few important reasons, it will be challenging to plumb > support for durable Merkle trees into NFS, although that is an > eventual goal. > > My thought was to use an ephemeral Merkle tree for NFS (and > possibly other remote filesystems, like FUSE, until these > filesystems support durable per-file Merkle trees). A tree would > be constructed when the client measures a file, but it would not > saved to the filesystem. Instead of a hash of the file's contents, > the tree's root signature is stored as the IMA metadata. > > Once a Merkle tree is available, it can be used in exactly the > same way that a durable Merkle tree would, to verify the integrity > of individual pages as they are used, evicted, and then read back > from the server. > > If the client needs to evict part or all of an ephemeral tree, it > can subsequently be reconstructed by measuring the file again and > verifying its root signature against the stored IMA metadata. > > So the only difference here is that the latency-to-first-byte > benefit of a durable Merkle tree would be absent. > > I'm interested in any thoughts or opinions about this approach. -- Chuck Lever
