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

> On Aug 11, 2020, at 1:43 AM, James Bottomley <James.Bottomley@Hanse > nPartnership.com> wrote: > > On Mon, 2020-08-10 at 19:36 -0400, Chuck Lever wrote: > > > On Aug 10, 2020, at 11:35 AM, James Bottomley > > > <James.Bottomley(a)HansenPartnership.com&gt; wrote:

> > > 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. > > > > Remote filesystems will need to agree that the size of that unit > > is the same everywhere, or the unit size could be stored in the > > per-filemetadata. > > > > > > > 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 > > > > A Merkle tree is potentially large enough that it cannot be > > stored in an extended attribute. In addition, an extended > > attribute is not a byte stream that you can seek into or read > > small parts of, it is retrieved in a single shot. > > Well you wouldn't store the tree would you, just the head > hash. The rest of the tree can be derived from the data. You need > to distinguish between what you *must* have to verify integrity > (the head hash, possibly signed) We're dealing with an untrusted storage device, and for a remote filesystem, an untrusted network. Mimi's earlier point is that any IMA metadata format that involves unsigned digests is exposed to an alteration attack at rest or in transit, thus will not provide a robust end-to-end integrity guarantee. Therefore, tree root digests must be cryptographically signed to be properly protected in these environments. Verifying that signature should be done infrequently relative to reading a file's content.

> and what is nice to have to speed up the verification > process. The choice for the latter is cache or reconstruct > depending on the resources available. If the tree gets cached on > the server, that would be a server implementation detail invisible > to the client. We assume that storage targets (for block or file) are not trusted. Therefore storage clients cannot rely on intermediate results (eg, middle nodes in a Merkle tree) unless those results are generated within the client's trust envelope.

So: if the storage target is considered inside the client's trust envelope, it can cache or store durably any intermediate parts of the verification process. If not, the network and file storage is considered untrusted, and the client has to rely on nothing but the signed digest of the tree root. We could build a scheme around, say, fscache, that might save the intermediate results durably and locally.

> > For this reason, the idea was to save only the signature of the > > tree's root on durable storage. The client would retrieve that > > signature possibly at open time, and reconstruct the tree at that > > time. > > Right that's the integrity data you must have. > > > Or the tree could be partially constructed on-demand at the time > > each unit is to be checked (say, as part of 2. above). > > Whether it's reconstructed or cached can be an implementation > detail. You clearly have to reconstruct once, but whether you have > to do it again depends on the memory available for caching and all > the other resource calls in the system. > > > The client would have to reconstruct that tree again if memory > > pressure caused some or all of the tree to be evicted, so perhaps > > an on-demand mechanism is preferable. > > Right, but I think that's implementation detail. Probably what we > need is a way to get the log(N) verification hashes from the server > and it's up to the client whether it caches them or not. Agreed, these are implementation details. But see above about the trustworthiness of the intermediate hashes. If they are conveyed on an untrusted network, then they can't be trusted either.

> > > 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. > > > > Also, changes to a file's content and its tree signature are not > > atomic. If a file is mutable, then there is the period between > > when the file content has changed and when the signature is > > updated. Some discussion of how a client is to behave in those > > situations will be necessary. > > For IMA, if you write to a checked file, it gets rechecked the next > time the gate (open/exec/mmap) is triggered. This means you must > complete the update and have the new integrity data in-place before > triggering the check. I think this could apply equally to a merkel > tree based system. It's a sort of Doctor, Doctor it hurts when I > do this situation. I imagine it's a common situation where a "yum update" process is modifying executables while clients are running them. To prevent a read from pulling refreshed content before the new tree root is available, it would have to block temporarily until the verification process succeeds with the updated tree root.