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Dynamic System Updates

Dynamic System Updates (DSU) allows you to make an Android system image that users can download from the internet and try out without the risk of corrupting the current system image. This document describes how to support DSU.

Kernel requirements

See Implementing Dynamic Partitions for kernel requirements.

In addition, DSU relies on the device-mapper-verity (dm-verity) kernel feature to verify the Android system image. So you must enable the following kernel configs:

  • CONFIG_DM_VERITY=y
  • CONFIG_DM_VERITY_FEC=y

Partition requirements

Starting in Android 11, DSU requires the /data partition to use the F2FS or ext4 file system. F2FS gives better performance and is recommended, but the difference should be insignificant.

Here are some examples of how long a dynamic system update takes with a Pixel device:

  • Using F2FS:
    • 109s, 8G user, 867M system, file system type: F2FS: encryption=aes-256-xts:aes-256-cts
    • 104s, 8G user, 867M system, file system type: F2FS: encryption=ice
  • Using ext4:
    • 135s, 8G user, 867M system, file system type: ext4: encryption=aes-256-xts:aes-256-cts

If it takes much longer on your platform, you may want to check if the mount flag contains any flag that makes “sync” write, or you can specify an “async” flag explicitly to get better performance.

The metadata partition (16 MB or larger) is required for storing data related to the installed images. It must be mounted during first stage mount.

The userdata partition must use F2FS or ext4 file system. When using F2FS, include all F2FS related patches available in the Android common kernel.

DSU was developed and tested with kernel/common 4.9. It's recommended to use kernel 4.9 and higher for this feature.

Vendor HAL behavior

Weaver HAL

The weaver HAL provides a fixed number of slots for storing user keys. The DSU consumes two extra key slots. If an OEM has a weaver HAL, it needs to have enough slots for a generic system image (GSI) and a host image.

Gatekeeper HAL

The Gatekeeper HAL needs to support large USER_ID values, because the GSI offsets UIDs to the HAL by +1000000.

Verify boot

If you want to support booting Developer GSI Images in LOCKED state without disabling verified boot, include Developer GSI keys by adding the following line to the file device/<device_name>/device.mk:

$(call inherit-product, $(SRC_TARGET_DIR)/product/developer_gsi_keys.mk)

Rollback protection

When using DSU, the downloaded Android system image must be newer than the current system image on the device. This is done by comparing the security patch levels in the Android Verified Boot (AVB) AVB property descriptor of both system images: Prop: com.android.build.system.security_patch -> '2019-04-05'.

For devices not using AVB, put the security patch level of the current system image into the bootloader kernel cmdline: androidboot.system.security_patch=2019-04-05.

Hardware requirements

When you launch a DSU instance, two temporary files are allocated:

  • A logical partition to store GSI.img (1~1.5 G)
  • An 8 GB empty /data partition as the sandbox for running the GSI

We recommend reserving at least 10 GB of free space before launching a DSU instance. DSU also supports allocation from an SD card. When an SD card is present, it has the highest priority for the allocation. SD card support is critical for lower-powered devices that might not have enough internal storage. When an SD card is present, make sure it's not adopted. DSU doesn't support adopted SD cards.

Available frontends

You can launch DSU using adb, an OEM app, or the one-click DSU loader (in Android 11 or higher).

Launching DSU using adb

To launch DSU using adb, enter these commands:

$ simg2img out/target/product/.../system.img system.raw
$ gzip -c system.raw > system.raw.gz
$ adb push system.raw.gz /storage/emulated/0/Download
$ adb shell am start-activity \
-n com.android.dynsystem/com.android.dynsystem.VerificationActivity  \
-a android.os.image.action.START_INSTALL    \
-d file:///storage/emulated/0/Download/system.raw.gz  \
--el KEY_SYSTEM_SIZE $(du -b system.raw|cut -f1)  \
--el KEY_USERDATA_SIZE 8589934592

Launching DSU using an app

The main entry point to DSU is the android.os.image.DynamicSystemClient.java API:

public class DynamicSystemClient {


...
...

     /**
     * Start installing DynamicSystem from URL with default userdata size.
     *
     * @param systemUrl A network URL or a file URL to system image.
     * @param systemSize size of system image.
     */
    public void start(String systemUrl, long systemSize) {
        start(systemUrl, systemSize, DEFAULT_USERDATA_SIZE);
    }

You must bundle/preinstall this app on the device. Because DynamicSystemClient is a system API, you can't build the app with the regular SDK API and you can't publish it on Google Play. The purpose of this app is:

  1. Fetch an image list and the corresponding URL with a vendor-defined scheme.
  2. Match the images in the list against the device and show compatible images for the user to select.
  3. Invoke DynamicSystemClient.start like this:

    DynamicSystemClient aot = new DynamicSystemClient(...)
       aot.start(
            ...URL of the selected image...,
            ...uncompressed size of the selected image...);
    
    

The URL points to a gzipped, non-sparse, system image file, which you can make with the following commands:

$ simg2img ${OUT}/system.img ${OUT}/system.raw
$ gzip ${OUT}/system.raw
$ ls ${OUT}/system.raw.gz

The filename should follow this format:

<android version>.<lunch name>.<user defined title>.raw.gz

Examples:

  • o.aosp_taimen-userdebug.2018dev.raw.gz
  • p.aosp_taimen-userdebug.2018dev.raw.gz

One-click DSU loader

Android 11 introduces the one-click DSU loader, which is a frontend in the developer settings.

Launching the DSU loader

Figure 1. Launching the DSU loader

When the developer clicks the DSU Loader button, it fetches a pre-configured DSU JSON descriptor from the web and displays all applicable images in the floating menu. Select an image to start the DSU installation, and the progress shows on the notification bar.

DSU image installation progress

Figure 2. DSU image installation progress

By default, the DSU loader loads a JSON descriptor that contains the GSI images. The following sections demonstrate how to make OEM-signed DSU packages and load them from the DSU loader.

Feature flag

The DSU feature is under the settings_dynamic_android feature flag. Before using DSU, make sure the corresponding feature flag is enabled.

Enabling the feature flag.

Figure 3. Enabling the feature flag

The feature flag UI might be unavailable on a device running a user build. In this case, use the adb command instead:

$ adb shell setprop persist.sys.fflag.override.settings_dynamic_system 1

Vendor host system images on GCE (optional)

One of the possible storage locations for the system images is the Google Compute Engine (GCE) bucket. The release administrator uses the GCP storage console to add/delete/change the released system image.

The images must be public access, as shown here:

Public access in GCE

Figure 4. Public access in GCE

The procedure to make an item public is available in the Google Cloud documentation.

Multiple-partition DSU in ZIP file

Starting in Android 11, DSU can have more than one partition. For example, it can contain a product.img in addition to the system.img. When the device boots, the first stage init detects the installed DSU partitions and replaces the on-device partition temporarily, when the installed DSU is enabled. The DSU package may contain a partition that doesn't have a corresponding partition on the device.

DSU process with multiple partitions

Figure 5. DSU process with multiple partitions

OEM-signed DSU

To make sure all images running on the device are authorized by the device manufacturer, all images within a DSU package must be signed. For example, assume there’s a DSU package that contains two partition images like below:

dsu.zip {
    - system.img
    - product.img
}

Both system.img and product.img must be signed by the OEM key before they are put into the ZIP file. The common practice is to use an asymmetric algorithm, for example, RSA, where the secret key is used to sign the package and the public key is used to verify it. The first stage ramdisk must include the paring public key, for example, /avb/*.avbpubkey. If the device already adopted AVB, the existing signing procedure will suffice. The following sections illustrate the signing process and highlight the placement of the AVB pubkey that is used to verify the images in the DSU package.

DSU JSON descriptor

The DSU JSON descriptor describes DSU packages. It supports two primitives. First, the include primitive includes additional JSON descriptors or redirects the DSU loader to a new location. For example:

{
    "include": ["https://.../gsi-release/gsi-src.json"]
}

Second, the image primitive is used to describe released DSU packages. Inside the image primitive there are several attributes:

  • The name and details attributes are strings that are shown on the dialog for the user to select.

  • The cpu_api, vndk, and os_version attributes are used for compatibility checks, which are described in the next section.

  • The optional pubkey attribute describes the public key that pairs with the secret key that's used to sign the DSU package. When it’s specified, the DSU service can check if the device has the key used to verify the DSU package. This avoids installing an unrecognized DSU package, for example installing a DSU signed by OEM-A to a device made by OEM-B.

  • The optional tos attribute points to a text file that describes the terms of service for the corresponding DSU package. When a developer selects a DSU package with the terms of service attribute specified, the dialog box shown in Figure 6 opens, asking the developer to accept the terms of service before installing the DSU package.

    Terms of service dialog box

    Figure 6. Terms of service dialog box

For reference, here's a DSU JSON descriptor for the GSI:

{
   "images":[
      {
         "name":"GSI+GMS x86",
         "os_version":"10",
         "cpu_abi": "x86",
         "details":"exp-QP1A.190711.020.C4-5928301",
         "vndk":[
            27,
            28,
            29
         ],
         "pubkey":"",
         "tos": "https://dl.google.com/developers/android/gsi/gsi-tos.txt",
         "uri":"https://.../gsi/gsi_gms_x86-exp-QP1A.190711.020.C4-5928301.zip"
      },
      {
         "name":"GSI+GMS ARM64",
         "os_version":"10",
         "cpu_abi": "arm64-v8a",
         "details":"exp-QP1A.190711.020.C4-5928301",
         "vndk":[
            27,
            28,
            29
         ],
         "pubkey":"",
         "tos": "https://dl.google.com/developers/android/gsi/gsi-tos.txt",
         "uri":"https://.../gsi/gsi_gms_arm64-exp-QP1A.190711.020.C4-5928301.zip"
      },
      {
         "name":"GSI ARM64",
         "os_version":"10",
         "cpu_abi": "arm64-v8a",
         "details":"exp-QP1A.190711.020.C4-5928301",
         "vndk":[
            27,
            28,
            29
         ],
         "pubkey":"",
         "uri":"https://.../gsi/aosp_arm64-exp-QP1A.190711.020.C4-5928301.zip"
      },
      {
         "name":"GSI x86_64",
         "os_version":"10",
         "cpu_abi": "x86_64",
         "details":"exp-QP1A.190711.020.C4-5928301",
         "vndk":[
            27,
            28,
            29
         ],
         "pubkey":"",
         "uri":"https://.../gsi/aosp_x86_64-exp-QP1A.190711.020.C4-5928301.zip"
      }
   ]
}

Compatibility management

Several attributes are used to specify the compatibility between a DSU package and the local device:

  • cpu_api is a string that describes the device architecture. This attribute is mandatory and is compared with the ro.product.cpu.abi system property. Their values must match exactly.

  • os_version is an optional integer that specifies an Android release. For example, for Android 10, os_version is 10 and for Android 11, os_version is 11. When this attribute is specified, it must be equal to or greater than the ro.system.build.version.release system property. This check is used to prevent booting an Android 10 GSI image on an Android 11 vendor device, which is currently not supported. Booting an Android 11 GSI image on an Android 10 device is allowed.

  • vndk is an optional array that specifies all VNDKs that are included in the DSU package. When it's specified, the DSU loader checks if the number extracted from the ro.vndk.version system property is included.

Revoking DSU keys for security

In the extremely rare case when the RSA key pair used to sign the DSU images is compromised, the ramdisk should be updated as soon as possible to remove the compromised key. In addition to updating the boot partition, you can block compromised keys using a DSU key revocation list (key blacklist) from an HTTPS URL.

The DSU key revocation list contains a list of revoked AVB public keys. During DSU installation, the public keys inside the DSU images are validated with the revocation list. If the images are found to contain a revoked public key, the DSU installation process stops.

The key revocation list URL should be a HTTPS URL to ensure the security strength, and is specified in a resource string:

frameworks/base/packages/DynamicSystemInstallationService/res/values/strings.xml@key_revocation_list_url

The string's value is https://dl.google.com/developers/android/gsi/gsi-keyblacklist.json, which is a revocation list for Google-released GSI keys. This resource string can be overlayed and customized, so that OEMs who adopt the DSU feature can provide and maintain their own key blacklist. This provides a way for the OEM to block certain public keys without updating the device's ramdisk image.

The format of the revocation list is:

{
   "entries":[
      {
         "public_key":"bf14e439d1acf231095c4109f94f00fc473148e6",
         "status":"REVOKED",
         "reason":"Key revocation test key"
      },
      {
         "public_key":"d199b2f29f3dc224cca778a7544ea89470cbef46",
         "status":"REVOKED",
         "reason":"Key revocation test key"
      }
   ]
}
  • public_key is the SHA-1 digest of the revoked key, in the format described in the generating the AVB pubkey section.
  • status indicates the revocation status of the key. Currently, the only supported value is REVOKED.
  • reason is an optional string describing the reason for revocation.

DSU procedures

This section describes how to perform several DSU configuration procedures.

Generating a new key pair

Use the openssl command to generate an RSA private/public key pair in .pem format (for example, with size 2048 bit):

$ openssl genrsa -out oem_cert_pri.pem 2048
$ openssl rsa -in oem_cert_pri.pem -pubout -out oem_cert_pub.pem

The private key might not be accessible and is only kept in a hardware security module (HSM). In this case, there might be a x509 public key certificate available after key generation. See the Adding the pairing pubkey to the ramdisk section for instructions to generate the AVB public key from a x509 certificate.

To convert an x509 certificate to PEM format:

$ openssl x509 -pubkey -noout -in oem_cert_pub.x509.pem > oem_cert_pub.pem

Skip this step if the certificate is already a PEM file.

Adding the pairing pubkey to the ramdisk

The oem_cert.avbpubkey must be put under /avb/*.avbpubkey to verify the signed DSU package. First, convert the public key in PEM format to AVB public key format:

$ avbtool extract_public_key --key oem_cert_pub.pem --output oem_cert.avbpubkey

Then include the public key in the first stage ramdisk with the following steps.

  1. Add a prebuilt module to copy the avbpubkey. For example, add device/<company>/<board>/oem_cert.avbpubkey and device/<company>/<board>/avb/Android.mk with content like this:

    include $(CLEAR_VARS)
    
    LOCAL_MODULE := oem_cert.avbpubkey
    LOCAL_MODULE_CLASS := ETC
    LOCAL_SRC_FILES := $(LOCAL_MODULE)
    ifeq ($(BOARD_USES_RECOVERY_AS_BOOT),true)
    LOCAL_MODULE_PATH := $(TARGET_RECOVERY_ROOT_OUT)/first_stage_ramdisk/avb
    else
    LOCAL_MODULE_PATH := $(TARGET_RAMDISK_OUT)/avb
    endif
    
    include $(BUILD_PREBUILT)
    
  2. Make the droidcore target depend on the added oem_cert.avbpubkey:

    droidcore: oem_cert.avbpubkey
    

Generating the AVB pubkey attribute in the JSON descriptor

The oem_cert.avbpubkey is in the AVB public key binary format. Use SHA-1 to make it readable before putting it into the JSON descriptor:

$ sha1sum oem_cert.avbpubkey | cut -f1 -d ' '
3e62f2be9d9d813ef5........866ac72a51fd20

This will be the content of the pubkey attribute of the JSON descriptor.

   "images":[
      {
         ...
         "pubkey":"3e62f2be9d9d813ef5........866ac72a51fd20",
         ...
      },

Signing a DSU package

Use one of these methods to sign a DSU package:

  • Method 1: Reuse the artifact made by the original AVB signing process to make a DSU package. An alternative approach is to extract the already signed images from the release package and use the extracted images to make the ZIP file directly.

  • Method 2: Use the following commands to sign DSU partitions if the private key is available. Each img within a DSU package (the ZIP file) is signed separately:

    $ key_len=$(openssl rsa -in oem_cert_pri.pem -text | grep Private-Key | sed -e 's/.*(\(.*\) bit.*/\1/')
    $ for partition in system product; do
        avbtool add_hashtree_footer \
            --image ${OUT}/${partition}.img \
            --partition_name ${partition} \
            --algorithm SHA256_RSA${key_len} \
            --key oem_cert_pri.pem
    done
    

For more information about adding add_hashtree_footer using avbtool, see Using avbtool.

Verifying the DSU package locally

It’s recommended to verify all local images against the pairing public key with these commands:


for partition in system product; do
    avbtool verify_image --image ${OUT}/${partition}.img  --key oem_cert_pub.pem
done

The expected output looks like this:

Verifying image dsu/system.img using key at oem_cert_pub.pem
vbmeta: Successfully verified footer and SHA256_RSA2048 vbmeta struct in dsu/system.img
: Successfully verified sha1 hashtree of dsu/system.img for image of 898494464 bytes

Verifying image dsu/product.img using key at oem_cert_pub.pem
vbmeta: Successfully verified footer and SHA256_RSA2048 vbmeta struct in dsu/product.img
: Successfully verified sha1 hashtree of dsu/product.img for image of 905830400 bytes

Making a DSU package

The following example makes a DSU package that contains a system.img and a product.img:

dsu.zip {
    - system.img
    - product.img
}

After both images are signed, use the following command to make the ZIP file:

$ mkdir -p dsu
$ cp ${OUT}/system.img dsu
$ cp ${OUT}/product.img dsu
$ cd dsu && zip ../dsu.zip *.img && cd -

Customizing the one-click-DSU

By default, the DSU loader points to a metadata of GSI images which is https://...google.com/.../gsi-src.json.

OEMs can overwrite the list by defining the persist.sys.fflag.override.settings_dynamic_system.list property that points to their own JSON descriptor. For example, an OEM may provide JSON metadata that includes GSI as well as OEM proprietary images like this:

{
    "include": ["https://dl.google.com/.../gsi-src.JSON"]
    "images":[
      {
         "name":"OEM image",
         "os_version":"10",
         "cpu_abi": "arm64-v8a",
         "details":"...",
         "vndk":[
            27,
            28,
            29
         ],
         "spl":"...",
         "pubkey":"",
         "uri":"https://.../....zip"
      },

}

It’s possible for an OEM to chain published DSU metadata as shown in Figure 7.

Chaining published DSU metadata

Figure 7. Chaining published DSU metadata