Re: [PATCH 1/4] keys: Introduce tsm keys

[Dan Williams]

Peter Gonda wrote:
> On Fri, Jul 28, 2023 at 1:31 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > One of the common operations of a TSM (Trusted Security Module) is to
> > provide a way for a TVM (confidential computing guest execution
> > environment) to take a measurement of its run state and use that with a
> > key-exchange protocol to establish a shared secret with a third-party /
> > remote attestation agent. The concept is common across TSMs, but the
> > implementations are unfortunately vendor specific. While the industry
> > grapples with a common definition of this attestation format [1], Linux
> > need not make this problem worse by defining a new ABI per TSM that
> > wants to perform a similar operation. The current momentum has been to
> > invent new ioctl-ABI per TSM per function which at best is an abdication
> > of the kernel's responsibility to make common infrastructure concepts
> > share common ABI.
> >
> > The proposal, targeted to conceptually work with TDX, SEV, COVE if not
> > more, is to define a new key type that produces a TSM common blob format
> > and moves the vendor specificity inside that envelope. The common Linux
> > definition is:
> >
> >     "<hex encoded pubkey> <blob descriptor> <hex encoded attestation blob>"
> >
> > This approach later allows for the standardization of the attestation
> > blob format without needing to change the Linux ABI. TSM specific
> > options are encoded in the frontend request format where the options
> > like SEV:vmpl (privilege level) can be specified and TSMs that do not
> > support them can decide to ignore them or fail if they are specified.
> > For now, "privlevel=" and "format=" are the only implemented options.
> >
> > Example of establishing a tsm key and dumping the provider-specific
> > report:
> >
> >     dd if=/dev/urandom of=pubkey bs=1 count=64
> >     keyctl add tsm tsm_test "auth $(xxd -p -c 0 < pubkey) privlevel=2" @u
> >     keyctl print 280877394 | awk '{ print $3 }' | xxd -p -c 0 -r | hexdump -C
> 
> What is the purpose of this report? What does it prove to whom? I'm a
> bit confused because it doesn't seem like there is an ability for a
> remote party to participate in a challenge and response to introduce
> any freshness into this protocol.
> 
> Also shouldn't the report have a little more context into the key we
> are signing? For instance what type of public key is this? And what is
> its purpose? In your example this isn't even a valid public key.
> 
> >
> > Now, this patch ends up being a fairly simple custom-key format because
> > most of the follow-on work that happens after publishing a TSM-wrapped
> > public-key is performed by userspace. The TSM key is just one step in
> > establishing a shared secret that can be used to unlock other keys. For
> > example a user-key could be populated with the resulting shared secret
> > and that could be used as a master-key for an encrypted-key
> > (security/keys/encrypted-keys/encrypted.c).
> >
> > While the discussion that led to this proposal hinted at a new
> > trusted-key (security/keys/trusted-keys/trusted_core.c) type rooted in
> > the TSM [2], more work is needed to fetch a secret from the TSM
> > directly. The trusted-key core expects a pre-established secure channel
> > to seal and unseal secrets locally. For that reason a "tsm" flavor
> > trusted-key is saved for follow on work. That will likely starting as a
> > wrapper around SNP_GET_DERIVED_KEY.
> >
> > Link: http://lore.kernel.org/r/64961c3baf8ce_142af829436@xxxxxxxxxxxxxxxxxxxxxxxxx.notmuch [1]
> > Link: http://lore.kernel.org/r/CAAH4kHYLETfPk-sMD-QSJd0fJ7Qnt04FBwFuEkpnehB5U7D_yw@xxxxxxxxxxxxxx [2]
> > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > Tested-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > Cc: David Howells <dhowells@xxxxxxxxxx>
> > Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > Cc: Dionna Amalie Glaze <dionnaglaze@xxxxxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Samuel Ortiz <sameo@xxxxxxxxxxxx>
> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > ---
> >  include/keys/tsm.h     |   71 ++++++++++++
> >  security/keys/Kconfig  |   12 ++
> >  security/keys/Makefile |    1
> >  security/keys/tsm.c    |  282 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  4 files changed, 366 insertions(+)
> >  create mode 100644 include/keys/tsm.h
> >  create mode 100644 security/keys/tsm.c
> >
> > diff --git a/include/keys/tsm.h b/include/keys/tsm.h
> > new file mode 100644
> > index 000000000000..61a81017d8f5
> > --- /dev/null
> > +++ b/include/keys/tsm.h
> > @@ -0,0 +1,71 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef __TSM_H
> > +#define __TSM_H
> > +
> > +#include <linux/types.h>
> > +#include <linux/module.h>
> > +
> > +/*
> > + * @TSM_DATA_MAX: a reasonable max with enough space for known attestation
> > + * report formats. This mirrors the trusted/encrypted key blob max size.
> > + */
> > +#define TSM_DATA_MAX 32767
> > +#define TSM_PUBKEY_MAX 64
> > +#define TSM_FORMAT_MAX 16
> > +
> > +/**
> > + * DOC: TSM Keys
> > + *
> > + * Trusted Security Module Keys are a common provider of blobs that
> > + * facilitate key-exchange between a TVM (confidential computing guest)
> > + * and an attestation service. A TSM key combines a user-defined blob
> 
> Are we limited to only doing key-exchanges between guests and
> attestation services? What if some user would like to handle the
> attestation verification without a service?

>From the kernel perspective it does not matter, it is just marshalling
the quote data. I assume local attestation could be built around this,
but that's all user-space policy.

> 
> > + * (likely a public-key for a key-exchance protocol) with a signed
> 
> key-exchange

got it.

> 
> > + * attestation report. That combined blob is then used to obtain
> > + * secrets provided by an agent that can validate the attestation
> > + * report.
> > + *
> > + * A full implementation uses a tsm key to, for example, establish a
> 
> Should 'TSM' be capitalized everywhere? Or does it not matter?

Probably should be.

> > + * 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".

> 
> > + *
> > + * The attestation report format is TSM provider specific, when / if a
> 
> I'm confused about the TSM terminology and what a TSM provider is. Is
> TSM the confidential compute framework of the vendor? So for Intel
> this is TDX, and the TSM provider is the SEAM module?

Yes, I borrowed this term from the TDISP specification where the "Trusted
Security Module" is the overarching term for the confidential compute
infrastructure for the vendor. Yes, TDX + SEAM is a TSM and SEV-SNP +
PSP is a TSM.

> 
> > + * standard materializes it is only a change to the auth_blob_desc
> > + * member of 'struct tsm_key_payload', to convey that common format.
> > + */
> > +
> > +/**
> > + * struct tsm_key_payload - generic payload for vendor TSM blobs
> > + * @privlevel: optional privilege level to associate with @pubkey
> > + * @pubkey_len: how much of @pubkey is valid
> > + * @pubkey: the public key-exchange blob to include in the attestation report
> > + * @auth_blob_desc: base ascii descriptor of @auth_blob
> > + * @auth_blob_format: for TSMs with multiple formats, extend @auth_blob_desc
> > + * @auth_blob_len: TSM provider length of the array it publishes in @auth_blob
> > + * @auth_blob: TSM specific attestation report blob
> > + */
> > +struct tsm_key_payload {
> > +       int privlevel;
> > +       size_t pubkey_len;
> > +       u8 pubkey[TSM_PUBKEY_MAX];
> > +       const char *auth_blob_desc;
> > +       char auth_blob_format[TSM_FORMAT_MAX];
> > +       size_t auth_blob_len;
> > +       u8 *auth_blob;
> > +};
> 
> How is freshness incorporated into the key exchange protocol? Wouldn't
> we need to do a challenge response between each remote party that we
> need to attest the provenance of @pubkey too?

That's left to userspace.

> 
> > +
> > +/*
> > + * tsm_parse - parse the tsm request data
> > + *
> > + * input format: "auth <hex pubkey data> [options]"
> > + *
> > + * Checks for options and parses a hex blob of data to be wrapped by the
> > + * TSM attestation format.
> > + *
> > + * options:
> > + *     privlevel= integer for selecting the privelege level of the
> 
> privilege

got it.

> 
> > + *                request, if the platform TSM supports that concept. To
> > + *                date only SEV accepts this option. Default 0.
> 
> SEV-SNP or just SNP? Plain SEV or SEV-ES doesn't actually support this
> interface at all.

Yeah, I was using "SEV" as a shorthand for the AMD "TSM". So maybe just
"SNP" is appropriate?