linux-kernel - Re: [PATCH 1/4] keys: Introduce tsm keys

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Date:   Fri, 4 Aug 2023 15:24:24 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Peter Gonda <pgonda@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     <dhowells@...hat.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dionna Amalie Glaze <dionnaglaze@...gle.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Samuel Ortiz <sameo@...osinc.com>, <peterz@...radead.org>,
        <linux-coco@...ts.linux.dev>, <keyrings@...r.kernel.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] keys: Introduce tsm keys

Peter Gonda wrote:
> > > >
> > > > > > + * shared secret and then use that communication channel to instantiate
> > > > > > + * other keys. The expectation is that the requester of the tsm key
> > > > > > + * knows a priori the key-exchange protocol associated with the
> > > > > > + * 'pubkey'.
> > > > >
> > > > > Can we instead be very specific about what protocols and cryptography
> > > > > are being used?
> > > >
> > > > Again this is a contract to which the kernel is not a party. The
> > > > requester knows the significance of the user-data, and it knows where to
> > > > send the combined user-data plus quote to provision further secrets.
> > > >
> > > > Not that I like that arrangement, but the kernel is not enabled by these
> > > > TSM implementations to know much more than "user-data in", "report out".
> > >
> > > Can you explain why using this key API is better than the ioctl
> > > version? Is there an overhead to adding keys?
> >
> > Setting aside that folks that have been involved in the Keyring
> > subsystem a lot longer than I are not keen on this usage [1], I expect
> > the overhead is negligible. Keys are already used in RPC scenarios and
> > can be destroyed immediately after being instantiated and read.
> 
> OK the overhead is negligible. But why is this any better?
> 
> To me this seems strictly worse to me as a user since I have much less
> input into the hardware attestation which is one of the primary
> benefits of confidential compute. I don't want the kernel limiting
> what cryptographic algorithm I use, or limiting attestation reports to
> signing pubkeys.

The current proposal on the table is not to have the kernel enforce
anything with respect to the format of the "pubkey" payload. The only
feedback so far I have seen about improving the semantics here is
enforce a nonce which the ioctl() interface just has to trust userspace
is handling and the Keyring approach can enforce a callout for that
input.

> I understand having a proliferation of similar drivers may not be
> ideal but given the hardware lift required to make confidential
> compute happen will we really see too many?

>From my perspective this discussion has already been worth it as some
people were unaware that security relevant development had started under
drivers/virt/coco/. The details of the kernel/user contract are coming
into focus.

In general, the point at which to have these types of discussions is at
the 1 -> 2 implementation transition, to my knowledge we are already up
to 5 of these things (AMD, Intel, RISC-V, ARM, S390).