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

[Mimi Zohar]

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.