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

[Mimi Zohar]

On Mon, 2020-08-10 at 08:35 -0700, James Bottomley wrote:
> 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).

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.

> 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.

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.

Mimi