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

[Yeoreum Yun]

On Fri, Nov 06, 2026 at 09:27:42AM +0100, Jiri Pirko wrote:
> Thu, Nov 05, 2026 at 02:52:19PM +0100, yeoreum.yun@xxxxxxx wrote:
> >On Thu, Nov 05, 2026 at 01:32:49PM +0100, Jiri Pirko wrote:
> >> Wed, Nov 04, 2026 at 08:46:22PM +0100, jgg@xxxxxxxx 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?

[...]
> armcca  event-log     event-log   none       Realm firmware log, if any

JFYI, this would be a CCEL log like tdx:
  - https://github.com/tianocore/edk2/issues/11384


> snp     report        signed      64         SNP attestation report
> snp     cert-table    cert-chain  none       VCEK or VLEK, ASK and ARK
> ======  ============  ==========  =========  ===========================
> 
> Did I miss something that would break us here?
> 
> CTLV layout draft:
> 
>     device-get reply, caps.evidence:
>       objects[16]:
>         id: uint(..15)
>         class: evidence-class
>         # The reply says which one was written.
>         formats[4]: evidence-format
>         max-length: uint
>         # Signed objects only, entries as today.
>         challenge-lengths[8]?: ...
>         # Descriptive, such as an SPDM slot.
>         instance?: uint(..u32-max)
>         # When the TCB vouches for the object with a digest.
>         tcb-digest-algo?: hash-algo
> 
>     evidence-get request:
>       # Required exactly when a signed object is named.
>       challenge?: binary(1..4K)
>       objects[16]:
>         id: uint(..15)
>         data: blob(out, va memfd dma-buf)
>       # selection, privlevel, service and options as now.
> 
>     evidence-get reply:
>       # In request order.
>       objects[16]:
>         format: evidence-format
>         length: uint
>         tcb-digest?:
>           hash-algo: hash-algo
>           data: binary(16..64)
>       # The generations as now.
> 
> Makes sense?

For me, It seems fine.

-- 
Sincerely,
Yeoreum Yun