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

[Peter Gonda]

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?

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

key-exchange

> + * 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?

> + * 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?

> + *
> + * 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?

> + * 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?

> +
> +/*
> + * 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

> + *                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.


> + *     format=    string modifier for the format, if the platform TSM
> + *                supports multiple formats. To date only SEV accepts an
> + *                "extended" argument. Default "".
> + *
> + * On success returns 0, otherwise -EINVAL.
> + */