Re: [PATCH v7 0/5] Allow guest access to EFI confidential computing secret area

Nayna
2022-02-09T00:26:25Z

On 2/2/22 03:25, Greg KH wrote:
> On Wed, Feb 02, 2022 at 08:04:01AM +0000, Matthew Garrett wrote:
>> On Wed, Feb 02, 2022 at 08:22:03AM +0100, Ard Biesheuvel wrote:
>>> On Wed, 2 Feb 2022 at 08:10, Matthew Garrett <mjg59@srcf.ucam.org> wrote:
>>>> Which other examples are you thinking of? I think this conversation may
>>>> have accidentally become conflated with a different prior one and now
>>>> we're talking at cross purposes.
>>> This came up a while ago during review of one of the earlier revisions
>>> of this patch set.
>>>
>>> https://lore.kernel.org/linux-efi/YRZuIIVIzMfgjtEl@google.com/
>>>
>>> which describes another two variations on the theme, for pKVM guests
>>> as well as Android bare metal.
>> Oh, I see! That makes much more sense - sorry, I wasn't Cc:ed on that,
>> so thought this was related to the efivars/Power secure boot. My
>> apologies, sorry for the noise. In that case, given the apparent
>> agreement between the patch owners that a consistent interface would
>> work for them, I think I agree with Greg that we should strive for that.
>> Given the described behaviour of the Google implementation, it feels
>> like the semantics in this implementation would be sufficient for them
>> as well, but having confirmation of that would be helpful.
>>
>> On the other hand, I also agree that a new filesystem for this is
>> overkill. I did that for efivarfs and I think the primary lesson from
>> that is that people who aren't familiar with the vfs shouldn't be
>> writing filesystems. Securityfs seems entirely reasonable, and it's
>> consistent with other cases where we expose firmware-provided data
>> that's security relevant.
>>
>> The only thing I personally struggle with here is whether "coco" is the
>> best name for it, and whether there are reasonable use cases that
>> wouldn't be directly related to confidential computing (eg, if the
>> firmware on a bare-metal platform had a mechanism for exposing secrets
>> to the OS based on some specific platform security state, it would seem
>> reasonable to expose it via this mechanism but it may not be what we'd
>> normally think of as Confidential Computing).
>>
>> But I'd also say that while we only have one implementation currently
>> sending patches, it's fine for the code to live in that implementation
>> and then be abstracted out once we have another.
> Well right now the Android code looks the cleanest and should be about
> ready to be merged into my tree.
>
> But I can almost guarantee that that interface is not what anyone else
> wants to use, so if you think somehow that everyone else is going to
> want to deal with a char device node and a simple mmap, with a DT
> description of the thing, hey, I'm all for it :)
>
> Seriously, people need to come up with something sane or this is going
> to be a total mess.
>

Thanks for adding us to this discussion. I think somehow my last post
got html content and didn't make to mailing list, so am posting it
again. Sorry to those who are receiving it twice.

If I have understood the discussion right, the key idea discussed here
is to unify multiple different interfaces(this one, and [1]) exposing
secrets for confidential computing usecase via securityfs.

And the suggestion is to see if the proposed pseries interface [2] can
unify with the coco interface.

At high level, pseries interface is reading/writing/adding/deleting
variables using the sysfs interface, but the underlying semantics and
actual usecases are quite different.

The variables exposed via pseries proposed interface are:

* Variables owned by firmware as read-only.
* Variables owned by bootloader as read-only.
* Variables owned by OS and get updated as signed updates. These support
both read/write.
* Variables owned by OS and get directly updated(unsigned) eg config
information or some boot variables. These support both read/write.

It can be extended to support variables which contain secrets like
symmetric keys, are owned by OS and stored in platform keystore.

Naming convention wise also, there are differences like pseries
variables do not use GUIDs.

The initial patchset discusses secure boot usecase, but it would be
extended for other usecases as well.

First, I feel the purpose itself is different here. If we still
continue, I fear if we will get into similar situation as Matthew
mentioned in context of efivars -

"the patches to add support for
manipulating the Power secure boot keys originally attempted to make it
look like efivars, but the underlying firmware semantics are
sufficiently different that even exposing the same kernel interface
wouldn't be a sufficient abstraction and userland would still need to
behave differently. Exposing an interface that looks consistent but
isn't is arguably worse for userland than exposing explicitly distinct
interfaces."

With that, I believe the scope of pseries interface is different and
much broader than being discussed here. So, I wonder if it would be
better to still keep pseries interface separate from this and have its
own platform specific interface.

I would be happy to hear the ideas.

[1] https://lore.kernel.org/linux-efi/YRZuIIVIzMfgjtEl@google.com/

[2] https://lore.kernel.org/all/20220122005637.28199-1-nayna@linux.ibm.com/

Thanks & Regards,

- Nayna