Re: [dm-devel] [RFC PATCH v5 00/11] Integrity Policy Enforcement LSM (IPE)

On Sun, 2020-08-09 at 13:16 -0400, Mimi Zohar wrote: > On Sat, 2020-08-08 at 13:47 -0400, Chuck Lever wrote: > > > On Aug 5, 2020, at 2:15 PM, Mimi Zohar <zohar(a)linux.ibm.com&gt; > > > wrote: > > <snip> > > > > If block layer integrity was enough, there wouldn't have been a > > > need for fs-verity. Even fs-verity is limited to read only > > > filesystems, which makes validating file integrity so much > > > easier. From the beginning, we've said that fs-verity signatures > > > should be included in the measurement list. (I thought someone > > > signed on to add that support to IMA, but have not yet seen > > > anything.) > > > > Mimi, when you and I discussed this during LSS NA 2019, I didn't > > fully understand that you expected me to implement signed Merkle > > trees for all filesystems. At the time, it sounded to me like you > > wanted signed Merkle trees only for NFS files. Is that still the > > case? > > I definitely do not expect you to support signed Merkle trees for all > filesystems. My interested is from an IMA perspective of measuring > and verifying the fs-verity Merkle tree root (and header info) > signature. This is independent of which filesystems support it. > > > The first priority (for me, anyway) therefore is getting the > > ability to move IMA metadata between NFS clients and servers > > shoveled into the NFS protocol, but that's been blocked for various > > legal reasons. > > 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).

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.