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

[Dan Williams]

Peter Gonda wrote:
> On Mon, Jul 31, 2023 at 11:48 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > 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.
> 
> Makes sense. Can we tweak the language of these comments then?

Will do.

> 
> >
> > >
> > > > + * (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".
> 
> 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.

[1]: http://lore.kernel.org/r/c6576d1682b576ba47556478a98f397ed518a177.camel@xxxxxxxxxxxxxxxxxxxxx

> You could imagine some userspace application that receives RPCs and
> does some attestation for each one, would adding then deleting a huge
> number of keys present any issues?

I can imagine a lot of scenarios, but reading the SEV-SNP whitepaper it
seems to imply that this is a launch-time one-off report that
establishes a channel to convey other secrets. So my expectation is that
this interface is used to bootstrap a guest and never again. Are you
aware of a high frequency use case for these reports?```

> > > > + * 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.
> 
> Thanks for the explanation.
> 
> >
> > >
> > > > + * 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.
> 
> But you haven't allowed userspace to add any data into the quote other
> than just the raw public key.

That is not allowed by the SNP firmware interface. The only input is the
64-byte user-buffer that the SNP whitepaper calls a public-key.

> The current sevguest ioctl allows users to pass arbitrary userdata.
> This would allow for some nonce to be included.

It's not arbitrary user-data, it is only meant to a pubkey per the "VM
Launch and Attestation" section of the SNP whitepaper.

> At a highlevel I'm not sure why this is better than each vendor having
> their own driver. It doesn't seem that difficult for userspace to deal
> with these systems given userspace will need to be written carefully
> for these PKI protocols anyways.

The common facilities can still be made common. Namely, the interface to
take in a pubkey / user-data and the location to pull the report need
not have any vendor specificity.