Re: [RFC PATCH 2/5] char: rpmb: provide a user space interface

[Alex Bennée]

Winkler, Tomas <tomas.winkler@xxxxxxxxx> writes:

>> "Winkler, Tomas" <tomas.winkler@xxxxxxxxx> writes:
>> 
>> >> The user space API is achieved via a number of synchronous IOCTLs.
>> >>
>> >>   * RPMB_IOC_VER_CMD - simple versioning API
>> >>   * RPMB_IOC_CAP_CMD - query of underlying capabilities
>> >>   * RPMB_IOC_PKEY_CMD - one time programming of access key
>> >>   * RPMB_IOC_COUNTER_CMD - query the write counter
>> >>   * RPMB_IOC_WBLOCKS_CMD - write blocks to device
>> >>   * RPMB_IOC_RBLOCKS_CMD - read blocks from device
>> >>
>> >> The keys used for programming and writing blocks to the device are
>> >> key_serial_t handles as provided by the keyctl() interface.
>> >>
>> >> [AJB: here there are two key differences between this and the
>> >> original proposal. The first is the dropping of the sequence of
>> >> preformated frames in favour of explicit actions. The second is the
>> >> introduction of key_serial_t and the keyring API for referencing the
>> >> key to use]
>> >
>> > Putting it gently I'm not sure this is good idea, from the security point of
>> view.
>> > The key has to be possession of the one that signs the frames as they are,
>> it doesn't mean it is linux kernel keyring, it can be other party on different
>> system.
>> > With this approach you will make the other usecases not applicable. It
>> > is less then trivial to move key securely from one system to another.
>> 
>> OK I can understand the desire for such a use-case but it does constrain the
>> interface on the kernel with access to the hardware to purely providing a
>> pipe to the raw hardware while also having to expose the details of the HW
>> to userspace. 
> This is the use case in Android. The key is in the "trusty" which
> different os running in a virtual environment. The file storage
> abstraction is implemented there. I'm not sure the point of
> constraining the kernel, can you please elaborate on that.

Well the kernel is all about abstracting differences not baking in
assumptions. However can I ask a bit more about this security model?

Is the secure enclave just a separate userspace process or is it in a
separate virtual machine? Is it accessible at all by the kernel running
the driver?

The fact that key id is passed down into the kernel doesn't have to
imply the kernel does the final cryptographic operation. In the ARM
world you could make a call to the secure world to do the operation for
you. I note the keyctl() interface already has support for going to
userspace to make queries of the keyring. Maybe what is really needed is
an abstraction for the kernel to delegate the MAC calculation to some other
trusted process that also understands the keyid.

>
> Also doesn't this break down after a PROGRAM_KEY event as
>> the key will have had to traverse into the "untrusted" kernel?
>
> This is one in a life event of the card happening on the manufacturing
> floor, maybe even not performed on Linux.

In an off list conversation it was suggested that maybe the PROGRAM_KEY
ioctl should be disabled for locked down kernels to dissuade production
use of the facility (it is handy for testing though!).

>> I wonder if virtio-rpmb may be of help here? You could wrap up up the front-
>> end in the security domain that has the keys although I don't know how easy
>> it would be for a backend to work with real hardware?
>
> I'm open to see any proposal, not sure I can wrap may head about it right now. 
>
> Anyway I was about to send the new round of my code,  but let's come to common ground first. 
>

OK - I'll see what the others say.

-- 
Alex Bennée