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

[James Bottomley]

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@xxxxxxxxxxxxx>
> > > 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.

James