Re: Migration to trusted keys: sealing user-provided key?

[Sumit Garg]

Hi Jan,

On Sun, 31 Jan 2021 at 23:40, James Bottomley <jejb@xxxxxxxxxxxxx> wrote:
>
> On Sun, 2021-01-31 at 15:14 +0100, Jan Lübbe wrote:
> > On Sun, 2021-01-31 at 07:09 -0500, Mimi Zohar wrote:
> > > On Sat, 2021-01-30 at 19:53 +0200, Jarkko Sakkinen wrote:
> > > > On Thu, 2021-01-28 at 18:31 +0100, Ahmad Fatoum wrote:
> > > > > Hello,
> > > > >
> > > > > I've been looking into how a migration to using
> > > > > trusted/encrypted keys would look like (particularly with dm-
> > > > > crypt).
> > > > >
> > > > > Currently, it seems the the only way is to re-encrypt the
> > > > > partitions because trusted/encrypted keys always generate their
> > > > > payloads from RNG.
> > > > >
> > > > > If instead there was a key command to initialize a new
> > > > > trusted/encrypted key with a user provided value, users could
> > > > > use whatever mechanism they used beforehand to get a plaintext
> > > > > key and use that to initialize a new trusted/encrypted key.
> > > > > From there on, the key will be like any other trusted/encrypted
> > > > > key and not be disclosed again to userspace.
> > > > >
> > > > > What are your thoughts on this? Would an API like
> > > > >
> > > > >   keyctl add trusted dmcrypt-key 'set <content>' # user-
> > > > > supplied content
> > > > >
> > > > > be acceptable?
> > > >
> > > > Maybe it's the lack of knowledge with dm-crypt, but why this
> > > > would be useful? Just want to understand the bottleneck, that's
> > > > all.
> >
> > Our goal in this case is to move away from having the dm-crypt key
> > material accessible to user-space on embedded devices. For an
> > existing dm-crypt volume, this key is fixed. A key can be loaded into
> > user key type and used by dm-crypt (cryptsetup can already do it this
> > way). But at this point, you can still do 'keyctl read' on that key,
> > exposing the key material to user space.
> >
> > Currently, with both encrypted and trusted keys, you can only
> > generate new random keys, not import existing key material.
> >
> > James Bottomley mentioned in the other reply that the key format will
> > become compatible with the openssl_tpm2_engine, which would provide a
> > workaround. This wouldn't work with OP-TEE-based trusted keys (see
> > Sumit Garg's series), though.
>
> Assuming OP-TEE has the same use model as the TPM, someone will
> eventually realise the need for interoperable key formats between key
> consumers and then it will work in the same way once the kernel gets
> updated to speak whatever format they come up with.

IIUC, James re-work for TPM trusted keys is to allow loading of sealed
trusted keys directly via user-space (with proper authorization) into
the kernel keyring.

I think similar should be achievable with OP-TEE (via extending pseudo
TA [1]) as well to allow restricted user-space access (with proper
authorization) to generate sealed trusted key blob that should be
interoperable with the kernel. Currently OP-TEE exposes trusted key
interfaces for kernel users only.

[1] https://github.com/OP-TEE/optee_os/blob/master/ta/trusted_keys/entry.c

-Sumit

>
> > > We upstreamed "trusted" & "encrypted" keys together in order to
> > > address this sort of problem.   Instead of directly using a
> > > "trusted" key for persistent file signatures being stored as
> > > xattrs, the "encrypted" key provides one level of
> > > indirection.   The "encrypted" key may be encrypted/decrypted with
> > > either a TPM based "trusted" key or with a "user" type symmetric
> > > key[1].
> > >
> > > Instead of modifying "trusted" keys, use a "user" type "encrypted"
> > > key.
> >
> > I don't see how this would help. When using dm-crypt with an
> > encrypted key, I can't use my existing key material.
> >
> > Except for the migration aspect, trusted keys seem ideal. Only a
> > single exported blob needs to be stored and can only be loaded/used
> > again on the same (trusted) system. Userspace cannot extract the key
> > material.
>
> Yes, that's what I was thinking ... especially when you can add policy
> to the keys, which includes PCR locking.  Part of the problem is that
> changing policy, which you have to do if something happens to update
> the PCR values, is technically a migration, so your trusted keys for
> dm-crypt are really going to have to be migrateable.
>
> > To get to this point on systems in the field without re-encryption of
> > the whole storage, only the initial trusted/encrypted key creation
> > would need to allow passing in existing key material.
>
> What about a third option: why not make dm-crypt store the master key
> it uses as an encrypted key (if a parent trusted key is available)?
> That way you'd be able to extract the encrypted form of the key as
> root, but wouldn't be able to extract the actual master key.
>
> James
>
>