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- A new device state, “Unknown Update Detected” (Abbreviated as UUD) will be introduced in EVC, to indicate to in EVC that software running on a given device is not one of the intended/certified images
- An optional feature, "Location-Lock" may be introduced in EVC, to additionally check the Geo-Location reported by device, and flag if the location has changed since its last-seen location. This feature may be critical in some deployments, where a given EVE node may be mounted permanently, and any change in its physical location should be flagged, and optionally considered along with software measurements to conclude if the EVE node is trusted. In this regard, another new device flag, “Unknown Movement Detected” (Abbreviated as UMD) will be introduced in EVC, to indicate that a change in the location of the device has been detected.
- EVC maintains a central database of all the supported EVE software images, and their hash values, indexed with the EVE image version tag
- EVC also maintains a central database of mostly used BIOS firmware images, their signatures, and the certificate provided by the BIOS vendor for validating the signatures, indexed by a combined tag of BIOS version string + Manufacturer
- On every reboot, EVE sends PCR quote to EVC, along with optional Geo-Location information, along with its EVE image version and BIOS firmware version.
- After normal reboot operation of EVE, EVC checks PCR Quote against the baseline value. If there is a change, the edge-node is marked as “UUD”. If "Location-Lock" feature is enabled and the location of the EVE node has changed, EVE node is marked "UMD"
- After EVE software upgrade, it is expected that the baseline value will change for the PCR values. However, after comparing the reported values with the expected values(since EVC knows about the new image version and its hash values), EVC makes a decision: If the reported values match, the baseline for the EVE node is updated. If they don’t, it is a UUD detection. Optionally, if administrator enables “Location-Lock” feature, Geo-Location of the device will also be included in the checks, before sending the latest configuration.
- If edge-node is in “UUD” or "UMD" state, only the latest baseOS configuration is sent as config request response, all other configuration items will be omitted in config response. This any config request from EVE will be responded with response code 403 - Forbidden. This is to protect any new sensitive images/credentials from getting exposed to the compromised device. The response from EVC in such cases will carry an error code to indicate that there was an attestation failure, and hence partial configuration is being sent. EVE, upon receiving such error codes , will apply the latest baseOS configuration, and also schedule will schedule re-attestation immediately (instead of waiting for the next attestation timer to expire)
- If EVE node is not in “UUD" or "UMD" state, all the latest configuration items are included.
- Frequency of attestation: EVE node will be required to periodically attest itself via attestation requests. The interval is fixed to be every 4 hours, or whenever the device connects back to the cloud again, or after the next reboot, whichever is earlier. This 4 hours is a tentative interval, and can be changed based on deployment requirements.
- If EVE reboots with a different software image from the configured version, EVC should be able to detect as quickly as possible and force attestation. To this effect a new token is introduced, called "Integrity Token". This token is a random nonce that will be stored in EVE under an encrypted folder. The master-key to decrypt this folder will be sealed against the TPM PCRs. Therefore, if EVE reboots with a different software, unsealing of this key will fail, and hence the new software will not be able to recover the Integrity Token inside it.
- This token is sent during attestation requests, and if the attestation is successful, the provided token value is locked against the EVE node.
- Every configuration request from EVE will include this Integrity Token. If there is a token mismatch, HTTP code 403 - Forbidden will be sent to the device as the response, indicating that device should do re-attestation.
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- Device-steps starts client.go (the provisioning client) which will check and do the following:
- If certificates from EVC are not yet fetched, fetches them
- Retrieves UUID from EVC
- Device-steps starts Vault Mgr
- Vault Mgr tries to retrieve the master decryption key from TPM with Unseal operation, and Unseal operation fails since the PCR values have changed
- Publishes vault status (as "locked")
- Waits for Integrity-Token and/or master-key from EVC - it will block here forever
- Device-steps starts TPM mgr
- TPM manger retrieves the attestation certificate and publishes to Zedagent
- Waits for Quote requests on pubsub channel from Zedagent
- Device-steps starts Zedagent (and Zedagent starts 3 concurrent tasks: attest, info and config)
- Attest task picks up the attestation certificate and publishes to EVC
- Attest task requests for a nonce from EVC (to prepare PCR quote)
- Attest task sends the nonce back to TPM Mgr and waits for PCR quote
- Attest task, once notified about the quote readiness, creates a random nonce value for Integrity-Token
- Attest task sends { Quote, Location, Event Log, Integrity-Token, Image Version } to EVC
- EVC, since the quote is different, tries to compare EventLog entries with its known hashes against the version reported. Since this is some arbitrary software, there will not be a match.
- Even if Location-Lock is enabled, location can not be trusted here, since the software reporting the location is not trusted.
- EVC sends an error back to EVE, to retry attestation.
- In the mean time, configuration task keeps requesting config from EVC (expected to fail till attestation goes through).
- EVC replies to configuration request , sends partial configuration (only BaseOS upgrade config is sent), and indicates with HTTP Error Code 403 - Forbidden. Indicates attestation failure (due to no or invalid Integrity Token)
- Config task communicates to Attest Task to re-trigger attestation
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