Verified boot integration

[phhusson]

Coming with Android M, verified boot will become mandatory on many devices. ( https://static.googleusercontent.com/media/source.android.com/fr//compatibility/android-cdd.pdf section 9.10) It is unclear whether this refers to http://source.android.com/devices/tech/security/verifiedboot/verified-boot.html, but we might expect so.

This enables users to keep a trusted (from a user point-of-view) system, by having a verifiable (by viewing fingerprint of the keystore at boot). We could integrate this verified boot in our SU. Multiple possibilities:

• "Superuser's organisation" generation: The "Superuser" organisation has one keystore, and generates root for many ROMs/devices. This way, assuming all bootloaders uses the same algorithm, every devices rooted by our root will have the same fingerprint, easier to remember, and users can compare their fingerprints to ensure noone got MITMed
• auto-generation without destroying: We could generate a keystore on first boot, and ask the user to protect it with a password on first boot, and then all updates would resign boot.img/system partition with this key
• auto-generation with destroying: Generate a new keystore everytime we flash a new boot.img/system partition (makes it almost impossible for the user to ensure the fingerprint is the expected one)
• on-device user generation: As auto-generation without destroying, but only with a keystore provided by the user if he wants to put one
• off-device user generation: Mostly the same as Superuser's organisation generation, as script will be public anyway, but the user takes care of the generation itself.

[phhusson]

I did some tests on Nexus 9 about this:

• keystore_signer/boot_signer works perfectly
• Verified => Locked doesn't need the unlock setting to be ticked
• Verified => Locked works with a custom keystore, and still boots
• Verified => Locked factory resets
• With a custom keystore, on the Google bootlogo, the open lock at the bottom is no more displayed, but at the top there is a "This device is only for testing, not for production" but there is no display of the fingerprint
• Unlocked => Verified factory resets
• In verified mode, fastboot boot fails, after 5s, saying something like "Buffer too big". I feel like there is a bug there...

Moment of terror: once in locked, you can't unlock back(obviously since it factory reseted, and thus reseted the "unlockable setting"), and you can't flash anything either. But it did boot. I think that when I did unlocked -> verified and verified -> locked, the bootloader booted the boot.img in a special mode with fast reboot to bootloader to ensure the system did boot, but I'm not too sure.

I think that's all I've tested yet.

[phhusson]

Ok, we see pictures of new Nexus-es about it: http://www.androidauthority.com/verified-boot-warnings-in-android-6-0-marshmallow-650368 It seems like they properly implemented their whitepaper on latest Nexus.

[phhusson]

ERM. It seems like they didn't implement the "show the fingerprint of the keystore" part...

[lbdroid]

The last bit of section 9.10 specifies an exemption for devices that shipped a previous android version withOUT verified boot. The interesting thing about the wording, is that rather than making it an optional exemption, it actually FORBIDS verified boot from being enabled/forced. This is probably in order to continue to support the expectations of the device owner, rather than forcing an error message on them that they previously didn't have.

This is the reason why you need to manually enable verified boot on a Nexus 9 (and presumably a 6), and why the indicated message is consistent with being a test device rather than appropriate for end users. Likely they just added the feature in order to allow people like US to work with the features without forcing us into buying this year's 5x/6p.

I wouldn't be concerned over the missing fingerprint, it would probably be present on a 5x or 6p.