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 # Updatable VM
 
-(To be filled)
+From Android V+, AVF (with Microdroid) supports Updatable VMs. This allows the VM instances to
+remain stable even when the VM core components and payload are upgraded. This includes (but is not
+limited to) update of payload apk and Microdroid OS.
+
+## Background
+
+The following constructs have been used (and are critical) to support Updatable VM:
+
+1. [Secretkeeper][sk_project] is the critical piece of solution. It provides secure storage for VM's
+   secrets. It is specified as [a HAL][secretkeeperhal] and needs to be implemented in an
+   environment with privilege higher than protected VM.
+1. [DICE Policies][dice_policy]: DICE policy is the mechanism for setting constraints on a DICE
+   chain(i.e., identities of a VM). VM seals its secrets using DICE policies, and Secretkeeper
+   serves as a policy verifier.
+1. [AuthGraph key exchange][authgraphke]: The requests/responses between pVM and Secretkeeper are
+   ferried via Android (which is untrusted). A cryptographically secure channel is setup using
+   AuthGraph key exchange.
+
+## VmSecrets::V2
+
+Updatable VMs are achieved by changing Microdroid's secret management. It now supports
+`VmSecrets::V2` which is derived from 2 independently secured secrets:
+
+1. Secretkeeper protected secret: This is random 64 bytes generated by VM on first boot & stored in
+   Secretkeeper.
+1. DICE Sealing CDIs (similar to legacy secrets V1): These are defined by
+   [Open Profile for DICE][open_dice_spec_cdi] and must remain the same across software updates.
+
+Secretkeeper protected secret is protected against rollback of boot images i.e. VM instance rebooted
+with downgraded images will not have access to these secrets. This is done using
+[Policy Gated Storage feature](policy_gated_storage) of Secretkeeper. On the first boot of the VM
+instance, Microdroid Manager (on behalf of the VM payload) generates a secret, stores it in
+Secretkeeper and on further reboots, this is retrieved from it. Along with this secret, a
+[sealing policy](#sealing-policy) is also stored (in Secretkeeper) that ensures that secrets are not
+released to the VM instance booted with downgraded images.
+
+Each Secretkeeper client needs a 64 bytes' Id to store an entry in Secretkeeper. For Microdroid,
+this is Instance Id. It is allocated by host (when the VM instance is created) and relayed to VM via a property (`instance-id`)
+in device tree node (`/avf/untrusted`)
+
+## Sealing Policy
+
+Sealing Policy is a DICE policy on the DICE chain of the payload running in Microdroid. This is
+constructed by Microdroid Manager on behalf of the payload and is stored along with the secret.
+
+A highly simplified view - Sealing policy built by Microdroid has the following constraints:
+
+- ExactMatch on DiceCertChainInitialPayload (root public key)
+- ExactMatch of Instance salt, this is present in DiceChainEntry corresponding to OS (and is derived
+  deterministically from Instance Id). This is needed to prevent the secrets of one instance from
+  being accessible to another instance running with the same VM images.
+- For each DiceChainEntry:
+  1. ExactMatch on AUTHORITY_HASH.
+  1. ExactMatch on MODE - Secret should be inaccessible if any of the runtime
+     configuration changes. For example, the secrets stored with a boot stage being in Normal mode
+     should be inaccessible when the same stage is booted in Debug mode.
+  1. GreaterOrEqual on SECURITY_VERSION: The secrets will be accessible if version of any
+     image is greater or equal to the set version.
+- For each Subcomponent on the last DiceChainEntry (which corresponds to VM payload, See
+  [vm_config.cddl][vm_config_cddl]): - GreaterOrEqual on SECURITY_VERSION - ExactMatch on
+  AUTHORITY_HASH.
+
+The sealing policy is updated each time the secret is retrieved. This ensures the secrets are only
+released if the security version of the images are non-decreasing.
+
+## Deferring rollback protection
+
+Traditionally in Android, each boot stage is responsible for rollback protection of the next boot
+image. ABL has access to tamper evident storage to ensure that. VM (Android U and lower) use
+instance.img where the boot stages (pvmfw/Microdroid) would store information about packages they
+boot (exact code_hash) and on subsequent boot of the instance ensure that the same images are
+allowed to run. This prevented running of older images, but also prevented running newer images and
+hence VMs were not updatable.
+
+Secretkeeper HAL then introduced the capability of storing secrets in a TA such that the owner of
+the secret ( for ex. VM) while storing it, includes a corresponding sealing policy such that only
+entities with DICE chain that adheres to those policies can access the secrets.
+
+This allows the bootloaders to defer rollback protection to the payload. Host relays this intention
+to pVM (both pVM firmware and OS) using the property (`defer-rollback-protection`) in device tree
+node (`/avf/untrusted`). If this is set and the guest OS is capable of `SecretkeeperProtection` then
+VMs use Secretkeeper based rollback protection.
+
+### Note on legacy support
+
+If the device does not support Secretkeeper, Microdroid will fallback to legacy secrets
+(`VmSecrets::V1`). These are not protected against the rollback of boot images and hence pVM
+firmware cannot defer rollback protection. Instance image is used to record information about the
+images on the first boot of the instance, and any further boot prevents any different image from
+running i.e, Updatable VMs are not supported.
+
+[authgraphke]: https://cs.android.com/android/platform/superproject/main/+/main:hardware/interfaces/security/authgraph/aidl/android/hardware/security/authgraph/IAuthGraphKeyExchange.aidl
+[dice_policy]: https://android.googlesource.com/platform/system/secretkeeper/+/refs/heads/main/dice_policy/
+[open_dice_spec_cdi]: https://pigweed.googlesource.com/open-dice/+/HEAD/docs/specification.md#cdi-values
+[secretkeeperhal]: https://cs.android.com/android/platform/superproject/main/+/main:hardware/interfaces/security/secretkeeper/aidl/android/hardware/security/secretkeeper/ISecretkeeper.aidl
+[sk_project]: https://android.googlesource.com/platform/system/secretkeeper/
+[vm_config_cddl]: https://cs.android.com/android/platform/superproject/main/+/main:packages/modules/Virtualization/microdroid_manager/src/vm_config.cddl