Re: [PATCH RFC 17/20] attestation: Add CTLV operation handling

[Yeoreum Yun <yeoreum.yun@arm.com>]

> Fri, Nov 06, 2026 at 09:57:25AM +0100, yeoreum.yun@arm.com wrote:
> >On Fri, Nov 06, 2026 at 09:27:42AM +0100, Jiri Pirko wrote:
> >> Thu, Nov 05, 2026 at 02:52:19PM +0100, yeoreum.yun@arm.com wrote:
> >> >On Thu, Nov 05, 2026 at 01:32:49PM +0100, Jiri Pirko wrote:
> >> >> Wed, Nov 04, 2026 at 08:46:22PM +0100, jgg@ziepe.ca wrote:
> >> >> >On Wed, Nov 04, 2026 at 07:11:44PM +0000, Yeoreum Yun wrote:
> >> >> >> [..snip..]
> >> >> >> 
> >> >> >> > +static int attestation_evidence_request_decode(struct attestation *att, struct ctlv_op_ctx *ctx,
> >> >> >> > +					       const struct ctlv_cursor *request,
> >> >> >> > +					       struct attestation_evidence_request *decoded,
> >> >> >> > +					       struct attestation_evidence_object_request **objects)
> >> >> >> > +{
> >> >> >> > +	const struct attestation_evidence_caps *caps = att->provider->evidence_caps;
> >> >> >> > +	u32 challenge_length;
> >> >> >> > +
> >> >> >> > +	decoded->challenge =
> >> >> >> > +		ctlv_attestation_evidence_get_evidence_challenge_get(request, &challenge_length);
> >> >> >> > +	decoded->challenge_length = challenge_length;
> >> >> >> > +	if (challenge_length < caps->min_challenge_length ||
> >> >> >> > +	    challenge_length > caps->max_challenge_length)
> >> >> >> 
> >> >> >> Hmm, I'm not sure whether this check is required for
> >> >> >> ATTESTATION_EVIDENCE_BLOB_EVENT_LOG blob type.
> >> >> >> 
> >> >> >> for example, if the provider provides two evidence:
> >> >> >>   - ATTESTATION_EVIDENCE_BLOB_PRIMARY (might attestation token)
> >> >> >>   - ATTESTATION_EVIDENCE_BLOB_EVENT_LOG
> >> >> >> 
> >> >> >> might challenge is reuiqred for generating the token
> >> >> >> and this would be marked at min/max_challenge_size in evidence_cap.
> >> >> >> 
> >> >> >> However, in case of eventlog, it wouldn't require challenge.
> >> >> >> for example, TPM event log or CCEL event log.
> >> >> >
> >> >> >Right
> >> >> >
> >> >> >> Do we need some field on attestation_evidence_supported_object whether
> >> >> >> it requires challenge or not and applies this sanity check per the object?
> >> >> >> 
> >> >> >> Otherwise, to get eventlog, it would need to send *dummy* challenge
> >> >> >> always.
> >> >> >
> >> >> >I wonder if we should have two interfaces here for signed and unsigned
> >> >> >evidence? It would be clearer for userspace then when it needs to
> >> >> >provide a challenge or not.
> >> >> 
> >> >> Isn't the challenge field provided/not-provided enough?
> >> >> 
> >> >> Currently on the introspection side, device-get op would provide
> >> >> challenge-lengths array:
> >> >> 
> >> >>         evidence?:
> >> >>           challenge-lengths[8]:
> >> >>             length?: uint(..4096)
> >> >>             min-length?: uint(..4096)
> >> >>             max-length?: uint(..4096)
> >> >>  	  objects[16]:
> >> >>             blob-class: evidence-blob-class
> >> >>             max-length: uint
> >> >>             blob-instance?: uint(..u32-max)
> >> >> 
> >> >> I think it would make sense to have challenge-lengths array per object:
> >> >> 
> >> >>  	  objects[16]:
> >> >>             blob-class: evidence-blob-class
> >> >>             max-length: uint
> >> >>             blob-instance?: uint(..u32-max)
> >> >>             challenge-lengths[8]:
> >> >>               length?: uint(..4096)
> >> >>               min-length?: uint(..4096)
> >> >>               max-length?: uint(..4096)
> >> >> 
> >> >> Then the unsigned object would just simply have challenge-lengths empty?
> >> >
> >> >This looks good to me. :)
> >> 
> >> The challenge is one of several open points that all depend on what an
> >> evidence object is: Rick's TDREPORT, legacy versus DICE Quotes,
> >> "primary", "raw" and the format strings. Here is my proposal to settle
> >> these together:
> >> 
> >> The model:
> >> 
> >> 1. device-get lists each object with an id, a class (signed, event-log,
> >>    cert-chain or auxiliary), the formats it may come in, its max-length
> >>    and, for signed objects only, challenge-lengths.
> >> 2. evidence-get names objects by id, as MR ops name MRs, each with its
> >>    output blob. It carries a challenge exactly when it names a signed
> >>    object, and the challenge has to fit each of them.
> >> 3. The reply gives each object's format, length and, where the TCB
> >>    vouches for it, the TCB's digest. The output is always {label, data}.
> >> 4. A signed object is made by the call, over the challenge, and never
> >>    reused. The rest are read as they stand.
> >> 
> >> This settles following issues discussed during this patchset review:
> >> 
> >> 1. One op, not two. Per-object challenge-lengths tell userspace when to
> >>    pass a challenge, and one call returns the signed object with the log
> >>    and certificates needed to check it.
> >> 2. TDX's TDREPORT goes: it is MAC'd for the platform, not signed.
> >>    /dev/tdx_guest keeps TDX_CMD_GET_REPORT0.
> >> 3. TDX's Quote object lists the legacy and the DICE format. The provider
> >>    labels what the host returned and fails on a Quote it cannot label.
> >> 4. "primary" becomes "signed", manifest joins auxiliary, spdm-vca,
> >>    spdm-measurements and tdisp-report become formats, and signature
> >>    goes, as a signed object carries its own.
> >> 5. "raw" goes with the representation attr. The TCB digest is a reply
> >>    field, so a blob always holds the object's own bytes.
> >> 
> >> Formats become an enum each provider family defines, instead of free
> >> strings. I'm still unsure if format should be entirely provider-specific
> >> and defined as "extension", or it should be somehow mixture of common
> >> and provider-specific. Looks there is not really much common
> >> anyway, so I lean towards provider-specific only. So far I have:
> >
> >Overall, This is plausible for me with some small question.
> >
> >> 
> >> ======  ============  ==========  =========  ===========================
> >> Family  Format        Class       Challenge  Bytes
> >> ======  ============  ==========  =========  ===========================
> >[...]
> >> tpm     ek-cert       cert-chain  none       EK certificate from NV
> >
> >Just a thought, but how are we planning to retrieve
> >the NV index for EK certificate?
> >
> >As far as I know, TPM NV storage doesn't support labeling.
> >
> >Should I assume that the NV index would be specified as part of the
> >challenge to retrieve the certificate from TPM NV storage?
> 
> evidence-get allows to accept mr-selection as part of the request
> alongside the challenge.

Sorry. Could you explain a little bit detail for this?.
AFAIU, this is to get the EK certificate from TPM non-volatile storage.
But, I don't know why *EK* certificate get would be relevant for
*mr selection*.  AFAIK, the TPM_Quote will be used AK not a EK.

Also, AFAIK, mr_selection is meaningful when the evidence_accept is
supported.

What is the relation-ship between mr-selecation and getting the TPM's
EK certificate getting from NV storage?

-- 
Sincerely,
Yeoreum Yun