[Nexus 9 Android Development] gladiac:
Hello everybody,

I created a tool for the nexus 9 that gets rid of the ForceEncrypt flag in a generic way (meaning it should work no matter what rom you are on). It does that by patching the currently installed boot.img.

Background

The Android CDD (Compatibility Definition Document) suggests that all devices SHOULD enable full disk-encryption (FDE) by default. Even though I support every step towards more security I have to criticize this approach. FDE comes at a price. Encryption takes time because some component has to de- and encrypt the stuff on the disk at some point and in the case of the nexus 9 (aka flounder) it's the CPU's task. Even though the nexus 9's CPU has 2 pretty fast cores you can still easily feel the difference between FDE in the on- or off-state. The I/O is faster and boot-times take only half as long. (I did not do any measurements)

There is an ongoing discussion about this topic in cyanogenmod's gerrit. Although it's a fun read it is pretty clear that this exchange of views is not going anywhere near a useful outcome.

Because performance is important to me and my tablet does not need the extra security I created the FED-Patcher (ForceEncrypt Disable Patcher)

How does it work?

FED-Patcher is a simple flashable ZIP that is supposed to be run in a recovery that has busybox integrated (like TWRP or CWM). This is what it does:

Checks if your device is compatible

Dumps the currently installed boot.img.

Unpacks the dump of your currently installed boot.img. The unpacking process is done via a self-compiled, statically linked version of unmkbootimg.

It patches the filesystem tables which include the force-encrypt flags. This process will change "forceencrypt" to "encryptable".

Then it patches the filesystem tables to not use dm-verity. This is done by removing the "verify" mount-parameter.

Creates a new boot.img. The unpacking process is done via a self-compiled, statically linked version of mkbootimg.

Flashes the modified boot.img

Supported devices

HTC Nexus 9 WiFi (flounder)

HTC Nexus 9 LTE (flounder_lte)

Motorola Nexus 6 (shamu)

Version History

v1 - Initial version with HTC Nexus 9 WiFi (flounder) support

v2 - Added Motorola Nexus 6 (shamu) support

v3 - Added support for HTC Nexus 9 LTE (flounder_lte)

v4 - Added support for signed boot-images

v5 - Changed error handling to compensate for missing fstab files. Some roms seem not to ship with the complete set of boot-files from AOSP.

v6 - FED-Patcher will enforce the same structure for the patched boot.img that the original boot.img had. Additionally, the kernel commandline will also be taken over. This should fix pretty much every case where devices would not boot after patching.

v7 - FED-Patcher will now disable dm-verity in fstab to get rid of the red error sign on marshmallow roms.

What do I need to make this work?

A supported device (Your nexus 9)

An unlocked bootloader

An already installed ROM with forceencrypt flag. (like cyanogenmod CM12.1)

A recovery that includes busybox (TWRP, CWM)

How do I use it?

Make a thorough, conservative backup of your data if there is any on your device

Go into your recovery (TWRP, CWM)

Flash fed_patcher-signed.zip

If your device is already encrypted (You booted your ROM at least once) you need to do a full wipe to get rid of the encryption. This full wipe will clear all your data on your data-partition (where your apps as well as their settings are stored) as well as on your internal storage so please, do a backup before. If you don't do a backup and want to restore your data... well... Call obama.

How do I know if it worked?

Go into your "Settings"-App. In "Security", if it offers you to encrypt your device it is unencrypted. If it says something like "Device is encrypted" it indeed is encrypted.

IMPORTANT: If you update your ROM you have to run FED-Patcher again because ROM-updates also update the boot-partition which effectively removes my patch. So, if you are on CM12.1 for example and you used my patch and do an update to a newer nightly you have to run FED-Patcher again. If you don't do so Android will encrypt your device at the first boot.

Is it dangerous?

Well, I implemented tons of checks that prevent pretty much anything bad from happening. But, of course, we're dealing with the boot-partition here. Even though I tested FED-Patcher quite a lot there is still room for crap hitting the fan.

Screenshot

Scroll down to the attached thumbnails.

Credits

* pbatard for making (un)mkbootimg (dunno if he is on xda)

* [MENTION=4419114]rovo89[/MENTION] for his xposed framework - I used some of his ideas by reading the source of his xposed installer flashable ZIP for FED-Patcher.