| Internet-Draft | EAT DA | June 2025 |
| Poirier & Fossati | Expires 4 December 2025 | [Page] |
In confidential computing, device assignment (DA) is the method by which a device (e.g., network adapter, GPU), whether on-chip or behind a PCIe Root Port, is assigned to a Trusted Virtual Machine (TVM). For the TVM to trust the device, the device must provide the TVM with attestation Evidence confirming its identity and the state of its firmware and configuration.¶
This document defines an attestation Evidence format for DA as an EAT (Entity Attestation Token) profile.¶
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In confidential computing, device assignment (DA) is the method by which a device (e.g., network adapter, GPU), whether on-chip or behind a PCIe Root Port, is assigned to a Trusted Virtual Machine (TVM). Most confidential computing platforms (e.g., Arm CCA, AMD SEV-SNP, Intel TDX) provide DA capabilities. Such capabilities prevent agents which are untrusted by the TVM (including other TVMs and the host hypervisor) from accessing or controlling a device that has been assigned to the TVM. This includes, for example, protection of device MMIO interfaces and device caches. From a trust perspective, DA allows a device to be included in the TVM's Trusted Computing Base (TCB). For the TVM to trust the device, the device must provide the TVM with attestation Evidence confirming its identity and the state of its firmware and configuration.¶
This document defines an attestation Evidence format for DA as an EAT [RFC9711] profile. The format is designed to be generic, extensible and architecture agnostic. Ongoing work on DA concentrates on PCIe devices that support the SPDM protocol, but other bus architecture and protocols are expected to be supported as the technology gains wider adoption. As such we focus on the formalization of an Evidence format for SPDM compliant devices while leaving room for the definition of other Evidencee format such as CXL and CHI. This list is by no means exhaustive and is expected to expand.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The Device Attestation claim is the encompassing envelope for the individual device claims to be presented. It can be used as a standalone entity but typically enclosed in a wider platform specific attestation token. The Device attestation claim consists of an EAT profile number, a nonce and any number of individual device claims. Each device claim is a tuple made of the device name and a standard claims format based on the bus or protocol the device supports. As previously mentioned, a definition for SPDM compliant devices is presented as part of this draft but we leave room for future expansion.¶
da-token = {
&(eat_profile: 265) => "tag:linaro.org,2025:device#1.0.0"
&(eat_nonce: 10) => bytes .size 64 ; same as realm nonce
&(eat_submods: 266) => {
+ device-name => $device-claims
}
}
device-name = text .regexp "dev-[A-Za-z0-9]+"
$device-claims /= #6.1000000(spdm-claims)
$device-claims /= #6.1000001(cxl-claims)
$device-claims /= #6.1000002(chi-claims)
; Thomas, we need to talk about representing legacy devices
; that may be assigned to a TVM but that are too old to
; generate evidence claims.
$device-claims /= #6.1000003(legacy-claims)
¶
A SPDM claim instance is expected to be present for each SPDM compatible device to be attested. Each instance consists of measurements and a certificates section. There can be up to 239 measurements per device with the entire measurement log optionally signed by the certificate populated in one of the 8 certificate slots. It should be noted that measurements formalized herein follow the DMTF measurement specification.¶
spdm-claims = {
&(measurements: 1) => {
+ block-id => spdm-measurement
? "signature" => spdm-measurement-blocks-signature
}
&(certificates: 2) => spdm-certificates
}
block-id = 1..239
¶
SPDM measurements start with a component type that reflects one of the 10 category defined by the SPDM specification. Following is the measurement itself represented by either a raw bitstream or a digest. The size of the digest value is derived from the measurement hash algorithm conveyed by the SPDM ALGORITHMS message response.¶
spdm-measurement = {
&(component-type: 1) => component-type
measurement
}
measurement //= ( &(digest-measurement: 2) => digest-measurement )
measurement //= ( &(raw-measurement: 3) => raw-measurement )
component-type /= &(immutable-rom: 0)
component-type /= &(mutable-firmware: 1)
component-type /= &(hardware-config: 2)
component-type /= &(firmware-config: 3)
component-type /= &(freeform-measurement-manifest: 4)
component-type /= &(device-mode: 5)
component-type /= &(mutable-firmware-version: 6)
component-type /= &(mutable-firmware-svn: 7)
component-type /= &(hash-extend-measurement: 8)
component-type /= &(informational: 9)
component-type /= &(structured-measurement-manifest: 10)
raw-measurement = bytes
digest-measurement = digest
digest = [
alg: uint / text
val: bytes
]
¶
SPDM compliant devices can optionally support the capability to sign measurements. Included in the measurement claim signature are all the elements needed by a third party entity to reconstruct the original measurement log signed by the device. Those elements include L1 (see CDDL below), the combined SPDM prefix, the hash algorithm used to generate a digest of the measurement log and nonces provided by the requester and responder. The slot number of the leaf certificate used to sign the measurement log is also provided.¶
;
; What follows is based on SPDM v1.3.2 (DSP0274_1.3.2.pdf)
;
;
; Algorithms currently supported by SPDM.
; See "MeasurementHashAlgo", table 21, page 79.
;
hash-algorithm-type /= &(tpm_alg_sha_256: 0)
hash-algorithm-type /= &(tpm_alg_sha_384: 2)
hash-algorithm-type /= &(tpm_alg_sha_512: 4)
hash-algorithm-type /= &(tpm_alg_sha3_256: 8)
hash-algorithm-type /= &(tpm_alg_sha3_384: 16)
hash-algorithm-type /= &(tpm_alg_sha3_512: 32)
hash-algorithm-type /= &(tpm_alg_sm3_256: 64)
;
; See signature generation and verification algorithms for
; MEASUREMENTS messages on page 126.
;
; L1 = Concatenate(VCA, GET_MEASUREMENTS_REQUEST1,
; MEASUREMENTS_RESPONSE1, ...,
; GET_MEASUREMENTS_REQUESTn-1,
; MEASUREMENTS_RESPONSEn-1,
; GET_MEASUREMENTS_REQUESTn, MEASUREMENTS_RESPONSEn)
;
spdm-measurement-blocks-signature = {
&(slot: 1) => 0..7, ; Slot of the certificate chain used to
; authenticate the measurement. Default
; should be 0.
&(requester-nonce: 2) => bytes .size 32,
&(responder-nonce: 3) => bytes .size 32,
&(combined-spdm-prefix: 4) => bytes .size 100,
&(IL1: 5) => bytes, ; L1 (see comment above)
&(base-hash-algo: 6) => hash-algorithm-type,
&(signature: 7) => bytes
}
¶
According to the specification, SPDM compliant devices should support at most 8 slots, with slot 0 populated by default. Slot 0 shall contain a certificate chain that follow the Device certificate model or the Alias certificate model. Regardless of the certificate model used, a certificate chain is encoded in a ASN.1 DER-encoded X.509 v3 format.¶
spdm-certificates = {
default-cert-slot => cert-chain
? aux-cert-slots => cert-chain
}
; ASN.1 DER-encoded certificates concatenated with no intermediate
; padding.
cert-chain = bytes
default-cert-slot = 0
aux-cert-slots = 1..7
¶
=============== NOTE: '\' line wrapping per RFC 8792 ================
da-token = {
&(eat_profile: 265) => "tag:linaro.org,2025:device#1.0.0",
&(eat_nonce: 10) => bytes .size 64,
&(eat_submods: 266) => {+ device-name => $device-claims},
}
device-name = text .regexp "dev-[A-Za-z0-9]+"
$device-claims /= #6.1000000(spdm-claims) / #6.1000001(cxl-claims) \
/ #6.1000002(chi-claims) / #6.1000003(legacy-claims)
spdm-claims = {
&(measurements: 1) => {
+ block-id => spdm-measurement,
? "signature" => spdm-measurement-blocks-signature,
},
&(certificates: 2) => spdm-certificates,
}
block-id = 1 .. 239
spdm-measurement = {
&(component-type: 1) => component-type,
measurement,
}
measurement //= (&(digest-measurement: 2) => digest-measurement // &\
(raw-measurement: 3) => raw-measurement)
component-type /= &(immutable-rom: 0) / &(mutable-firmware: 1) / &(\
hardware-config: 2) / &(firmware-config: 3) / &(freeform-measurement\
-manifest: 4) / &(device-mode: 5) / &(mutable-firmware-version: 6) \
/ &(mutable-firmware-svn: 7) / &(hash-extend-measurement: 8) / &(\
informational: 9) / &(structured-measurement-manifest: 10)
raw-measurement = bytes
digest-measurement = digest
digest = [
alg: uint / text,
val: bytes,
]
spdm-certificates = {
default-cert-slot => cert-chain,
? aux-cert-slots => cert-chain,
}
cert-chain = bytes
default-cert-slot = 0
aux-cert-slots = 1 .. 7
hash-algorithm-type /= &(tpm_alg_sha_256: 0) / &(tpm_alg_sha_384: 2\
) / &(tpm_alg_sha_512: 4) / &(tpm_alg_sha3_256: 8) / &(\
tpm_alg_sha3_384: 16) / &(tpm_alg_sha3_512: 32) / &(tpm_alg_sm3_256\
: 64)
spdm-measurement-blocks-signature = {
&(slot: 1) => 0 .. 7,
&(requester-nonce: 2) => bytes .size 32,
&(responder-nonce: 3) => bytes .size 32,
&(combined-spdm-prefix: 4) => bytes .size 100,
&(IL1: 5) => bytes,
&(base-hash-algo: 6) => hash-algorithm-type,
&(signature: 7) => bytes,
}
cxl-claims = {}
chi-claims = {}
legacy-claims = {}
¶
TODO Security¶
TODO IANA CWT allocations¶
=============== NOTE: '\' line wrapping per RFC 8792 ================
{
/ profile / 265: "tag:linaro.org,2025:device#1.0.0",
/ nonce / 10: h'\
f9efc3341597f75f8d94432ad39566a8c5704b2004ba001c094f475bfc057f9f25d7\
aa40cd86cd30ebaae746fb19f008c1e6a1f23ad6a178e18dceda918f7f6e',
/ submods / 266: {
"dev-a": 1000000({
/ measurements / 1: {
1: {
/ component-type / 1: 2, / hardware config /
/ raw-measurement / 3: h'4f6d616861'
}
},
/ certificates / 2: {
/ device certs / 0: h'\
676f616e6e61747261646974696f6e6d6f6e676572'
/ no aux certs /
}
}),
"dev-b": 1000000({
/ measurements / 1: {
1: {
/ component-type / 1: 1, / mutable firmware /
/ digest-measurement / 2: [
/ alg / 1,
/ val / h'6b656e6e656c6c79'
]
},
6: {
/ component-type / 1: 2, / hardware config /
/ digest measurement / 2: [
/ alg / 0,
/ val / h'756e646572637279'
]
}
},
/ certificates / 2: {
/ device certs / 0: h'61746865697A656178696C6C6172',
/ aux certs (slot=2) / 2: h'23451576923AE99106783948598A'
}
})
}
}
¶
TODO acknowledge.¶