Jan 082016
 

This post will serve to help step you through the process of rooting your Google Nexus device with the systemless root method. As any technique’s change due to software updates, I intend to keep this post updated with that latest information. First a bit of background to clarify the traditional root with what is now known as systemless root.
As google releases newer versions of Android, they are also attempting to increase the security of their handsets. They have enabled SELinux by default, added safetynet checks, and some boot up warning messages. The traditional root methods would install a superuser binary, “su”, onto the system partition of your phone, allowing apps to ask for root rights and escalate their privileges. Within the android community, we want the benefit’s that root access provides but we don’t necessarily want it at the cost of discarding all the security protections google is trying to include. In other words, we can attempt to be better citizens of the android rooting community by trying to have our root methods work within these security confines as much as possible. The first bit that happened along these lines was that instead of rooting the new marshmallow OS and disabling SELinux with custom kernels, the new root method patched the kernels and updated SELinux policies to include what is needed for root applications to run and request the escalation’s they typically need.  The second bit was the ability to root without touching the system partition at all, keeping it completely stock which has the unintended side effect of also allowing Android Pay to work on a systemless rooted device.

What is root and why care about systemless?

I’d like to define what root really means.  It’s quite simple really, root is obtaining elevated privileges on the device.  It does not guarantee anything else but those escalated privileges.  With other security methods being employed on devices, this does NOT mean you will be able to do everything with those escalated privileges.  Depending on your device, selinux policies may still hamper what you want to do, the system partition may still not be writable or may have no free space from the stock image.  Root doesn’t mean you can modify anything it just means you have the elevated root privileges on that device.

It’s important to have this understanding as we discuss systemless root.  Some of the benefits of a systemless root is being able to have those escalated privileges while not tripping many of the other security flags set in place.  You are able to keep a stock system image for the first time, in fact never even mounting it for rw access at all.  This currently allows for safetynet checks to pass and as such Android Pay even still works on devices rooted in this manner.  As the development of this method matures, receiving OTA updates may be easier as you’d just need to revert the modified boot.img (kernel) back to stock to apply the OTA update, then patch the new one to get root back.

Obtaining a pure systemless root

Getting systemless root and even manually installing any of the monthly security updates is pretty easy once you’ve done it.  It takes me about 5 minutes to install a monthly update and re-root on my Nexus devices.  It will only get easier if SuperSu adds reverting the modified boot.img.  First let’s get some pre-requisites out of the way.

Pre-requisites:

  1. Install the Android SDK.  From the link, grab the SDK Tools only for your platform.  Extract it to some folder in your OS, doesn’t really matter where, let’s just say you extracted it to the SDK folder.  In windows you want to go into android-sdk-windows that was extracted and run the “SDK Manager.exe”.  For mac/linux go into your extracted folder and look for a tools/ folder.  In there you’ll see and android executable file.  Run that.
  2. Once you have the Android SDK Manager open, click Deselect All on the bottom and just select to install the “Android SDK Platform-tools”, which is in the Tools section, and the “Android Support Library” down in the Extras section.  If you are on Windows also check the Google USB Driver in that section. Click to install.  Once completed the only real tools we now care about live in a platform-tools folder that is in your extracted sdk directory.  Everytime you go to update your device you should run the Android SDK Manager again and see if any of these 3 check boxes have updates available.  If they do, update them as it’ll help avoid potential future issues.
  3. Download the latest TWRP recovery for your device.  Using a web browser navigate to http://twrp.me, click on the Devices link in the upper right and search for your device, ie “Nexus 6p” or “Nexus 5X”, etc.  Once found, click on it, go down to the Download Links: section and click on the “Primary (Recommended)”.  Download the latest version of twrp-x.x.x.x.img file you see there.
  4. Download the Google Factory Image for your device.  Just find your device in that list and download the latest version which will be the last link in the column for your device.  You’ll want to extract this .tgz file but not the zip inside it.
  5. Next we need SuperSu itself.  Currently the version of SuperSu we want is available from Post #3 in this thread.  Since it is a Work in Progress (WIP).  You’ll want the latest version you see there which currently is “BETA-SuperSU-v2.66-20160103015024.zip”.  Don’t extract this zip just download it.

Now we are prepped, outside of checking for newer versions of these pre-req’s once in a while you don’t have to do these steps for every update.  Now lets root our Nexus device without modifying (or even mounting the /system partition).

Apply Systemless Root:

  1. First you’ll want to reboot your device into the bootloader.  So that I don’t have to baby step you here feel free to use google like ‘how to reboot <device name> into bootloader’ but basically power it off then hold vol down + power until you see the logo then release power.  Done successfully, you should see a screen with the android logo on its back and a bunch of smaller sized text on the bottom.
  2. Once in the bootloader, usb your device to your computer and using a command line go into your platform-tools folder from above.  Run the command fastboot devices
    Side Note: If you are in windows you’d probably have to type fastboot as fastboot.exe and if you are in mac/linux you may have to use ./fastboot to reference the one in the folder you are in. I’ve added the platform-tools directory to my path so if you do that in any OS, you can just type it as “fastboot” and not even be in the platform-tools folder. Regardless, once you see your device show up in the output of fastboot devices you can then proceed to unlock your bootloader. For the Nexus 6p and Nexus 5x you would use fastboot flashing unlock and for older Nexus phones you can use fastboot oem unlock. Look at your device and select Yes with the volume and power keys and now your device should show that it is bootloader unlocked.
  3. Now we can flash our previously downloaded factory image to either update to this version or do a full factory return to stock.  If you look in the extracted factory image you’ll see a .zip, a radio.img, bootloader.img and scripts called “flash-all”.  If you want to wipe your data and return to stock while flashing all these to their latest versions you can just run the flash-all script for your OS.  If you look on your device in the bootloader screen it will display the version of your bootloader and radio (could be labeled Baseband). You can then see the version of the two .img files and determine if they have been updated.  Since google is releasing monthly security updates, many times these are not updated.  If they are newer than what is on your device you would flash them with:
    fastboot flash bootloader bootloader-blah.img
    fastboot reboot-bootloader
    fastboot flash radio radio-blah.img
    fastboot reboot-bootloader
    

    To flash the rest of the device partitions you can type
    fastboot update image-blah.zip
    This will NOT wipe your data, you can run this to update every partition with that factory image. If you want to factory reset your device with this version then you’d add a “-w” to that command to tell it to wipe as in: fastboot -w update image-blah.zip
    After flashing the factory image, even if you intend to systemless root, let your device boot the first time and go through updating the apps. Once booted then power back off and go to the bootloader again.

    [UPDATE 05/2016] – Google has released the OTA images for Nexus devices available here.  So optionally, instead of running the fastboot update image-blah.zip command from above that is a few hundred megs and reflashes everything.  You can instead just apply your OTA via adb sideload as detailed on their site.  This should be a tad quicker and then you can continue on below just as before.

  4. Now we have the latest factory image on our device and we need to systemless root it. In order to flash the SuperSu zip we need to use a special recovery. That is the TWRP software we downloaded. You can either flash this recovery with fastboot flash recovery twrp.img or you can just boot into it temporarily in order to get root applied. I prefer the latter since it is one less thing to revert if I want to take an OTA update in the future. To just boot into twrp recovery we issue: fastboot boot twrp.img.  When twrp boots it will ask if you’d like to keep system read-only. You’ll want to say yes to that since we don’t want to even mount system for writing at all.
    Next we use the adb tool that is also in the platform-tools folder. We want to push our SuperSu zip file we downloaded from pre-requisite step 5. If you have that zip in your platform tools folder the command would be adb push BETA-SuperSU-vX.x.x.zip /sdcard/Download/. Don’t forget about that period at the end. Now before we install SuperSu we want to ensure it doesn’t bind mount a /system/xbin as that will trip SafetyNet checks. So we use adb again but this time the command is adb shell "echo BINDSYSTEMXBIN=false>>/data/.supersu"
    Now on your device you can click the Install button, navigate to the /sdcard/Download folder and flash the SuperSu zip file you see there.
  1. That’s it! Reboot your device, if TWRP asks to install SuperSu be sure to say NO.  This looks like a lot but once you go through it once, and have the pre-req’s all set, you can install the monthly updates in a matter of 5 minutes. The benefit is they post the updates to the google factory images before developer’s will even see the code in AOSP and before they ever push OTA updates. So by using this method you can get onto these updates right away without waiting for anything.

Apps with workarounds for Systemless root

Since we installed systemless root here and the idea is to keep the system partition completely pristine and stock I’d like to point out some of the more valuable root applications you can run while in this mode and a couple that typically need to modify system but you can have them function perfectly fine still.  First, I have root apps like Titanium Backup, Greenify, CF Lumen, Nova Launcher that all have root access and don’t touch my system.  One of the most important apps for me to use however is AdAway which needs to modify the /system/etc/hosts file in order to function properly.  Many people also use busybox which installs to system so both of these apps we need to do a couple workarounds to keep them from not touching our /system partition.

AdAway

For AdAway it has been made fairly simple.  You’ll want to go this XDA forum thread and download both the latest AdAway application and the zip mentioned for systemless hosts file.  Boot your device into that twrp recovery again.  Use adb to adb push these two files to /sdcard/Download. just as we did for the SuperSu zip.  In the recovery flash the AdAway_systemless_hosts.zip and reboot your phone. Back in your OS install AdAway from your /sdcard/Download folder using any file explorer like FX File Explorer or even from F-Droid.  Now run AdAway, leaving it at its default target hosts location of /system/etc/hosts.

How can AdAway write to /system/etc/hosts yet we still aren’t modifying our system partition?  Well that systemless_hosts zip you flashed setup a special mount on the filesystem of your phone such that /su/etc/hosts (our systemless root is /su) was bind mounted to /system/etc/hosts.  So even though AdAway thinks its writing to /system/etc/hosts it is really going to /su/etc/hosts which is mounted in our /data partition.  This might be hard for some to follow but just know if you flash that zip you can use AdAway as normal and it will not be touching your system.

Busybox

Getting busybox to not modify system is a bit more tricky.  Will update this post as I gather that information in detail.

Nov 302011
 
Mastering SSH Keys

Welcome to the wonderful world of SSH keys! If you don’t yet share my enthusiasm you soon will! SSH keys are a perfect way for you to control access to your machine, whether that be a very secure way for only you to have access, locking down other authorized users and preventing their passwords from getting distributed or stolen, or even allowing access to scripts for very specific purposes. SSH keys accomplish all of it and more. First, let me just lay out how simple SSH keys are in case you aren’t clear and maybe get discouraged by the length of this post. The basics of it are, you have a host server create a private and public key pair. You can optionally provide a password for the keys. You copy the public key to servers you wish to log on to, while keeping private key secured and on your system. Then when you login to the system the private key is used to decrypt the public (also with optional password) and you get in. It is that simple at its core, there are just a lot of various configuration options you can use to accomplish certain features or functionality that may be more desirable for you. The keys are very secure as using this method alone prevents against man-in-middle attacks and eliminates a few other possible break-ins from password only authentication.
The most common way I see SSH keys being used is as a password-less easy login for an admin to use for his remote linux systems. Although this is better than using a password only system you still are vulnerable to someone obtaining your private key and using it for instant access to any system with the corresponding public key. I prefer to use them as a type of two factor authentication for the ssh logins where I need to provide the password to the key plus (of course) have the actual key. Regardless, let’s get a basic configuration going and start using ssh-keys. First on the server side you can follow a config I’ve used in a previous post but general settings to ensure:

  • Port 2992 – (I always recommend changing the default port)
  • Make sure both “RSAAuthentication” and “PubkeyAuthentication” are both set to yes
  • PermitRootLogin no
  • AuthorizedKeysFile %h/.ssh/authorized_keys
  • I’d also only allow users in a certain group to use ssh for further protection so “AllowGroups sshlogin” for example and add users who will ssh into this system into the sshlogin group.

Now on your machine, the computer you will be ssh’ing from, you need to create the key pair and copy the public key to the host(s). Generate the key pair as follows:
Create the .ssh directory in your linux home directory if it doesn’t exist already. Ensure correct permissions with chmod 0700 .ssh. Then create the actual keys and when prompted specify a pass-phrase to use (This pass-phrase is the optional part if you just want to authenticate with the keys alone but as I mentioned above I prefer supplying one so you have a two factor authentication going on). Keep in mind this can be a passphrase not just a password.
ssh-keygen -b 2048 -t rsa -f ~/.ssh/MainKey
The -b specifies the length of the key and the -t the type (which is rsa or dsa).
I’d use RSA 2048 or if you are the more paranoid type RSA 4096. I won’t delve into the RSA/DSA debate except to say that I’ve done a LOT of reading on the topic and I’m choosing RSA keys and with sensitive servers I’d go with RSA at 4096.
[Update, Dec 2015] :: DSA is being deprecated in OpenSSH so I now strongly recommend RSA over it. If you are running version 6.5 or newer of openssh I’d actually recommend using ed25519 keys over RSA and you can read this blog post for more details. Typically you can check your version with ssh -V command. Your command to generate this key would simply be
ssh-keygen -t ed25519 -f ~/.ssh/MainKey
Note for Mac OSX users, the version of ssh is too old for ed25519 unless you are on OSX 10.11, El Capitan. If you are on an older Mac OSX version simply google for homebrew install of openssh to upgrade your version and use that.
[End Update]
Once the keys are created scp the public key to the server but copy the public key into the .ssh directory named as “authorized_keys” (or wherever you specified in sshd_config the AuthorizedKeysFile). You can append multiple public keys to the authorized keys file just cat mykey.pub >> authorized_keys for each public key.

Now you may have noticed above when generating the key pair we passed the -f option. This way we can name our private key something other than the default. If you do not specify the -f the default will be id_rsa with the corresponding id_rsa.pub. So now to ssh into our system we will want to specify the identity file to use as:
ssh -i /home/user/.ssh/MainKey user@myserver.com -p2992
The reason I want to show the more complicated use case of specifying a particular identity file is that you may want to have different key pairs for servers that are yours versus your employers’ or any other separation that would be important to you. You can simplify the management of which keys are used for which hosts by specifying in the ~/.ssh/config file.
vi ~/.ssh/config

Add both host names and their identity file as follows:
Host server1.myserver.com
IdentityFile ~/home/user/.ssh/MainKey
Host server1.workserver.com
IdentityFile ~/home/employee/.ssh/id_rsa

That’s all there is to your basic key based login! Now lets briefly go over many of the popular use cases for these ssh-keys so you can see how powerful and helpful they can be for you.

First, as I mentioned earlier, I frequently see administrators using these without a password so they can quickly login to the servers they manage. The problem is you are now solely relying on the security of that private key for complete access to those systems. I will strongly recommend (yet again) putting a pass-phrase on your keys and then if you want a quick password-less login then simply configure ssh-agent to cache your pass-phrase on your system for that session. For details just search for “ssh-agent cache password” and you’ll find a few examples. In this way if your key is somehow stolen that person still has no access to the servers as they don’t know the password, yet you have the efficiency of not typing a pass-phrase once you set up and configure ssh-agent to cache it on your system. Win-win here.

Next, we can limit by IP or IP subnet where users with ssh-keys are able to login FROM. So say you are the server admin and you have other users you manage on the server. Maybe you (especially after reading this) have a requirement that they only login with ssh-keys. Being security conscious you might have a few concerns, not the least are:

  • Did they create a passphrase or leave blank?
  • Will they give their key and pass to someone else to use?
  • What if their key is stolen or compromised?

One thing you can do if you just control the server with the public key, is limit by IP or subnet where they can be coming from. In this way if the users key is stolen it can’t be used anyway if that person isn’t trying to login from the same IP (or subnet) as where the original user is from. You can either wait till they login the first time and see what IP they are coming from or ask them to send you the subnet or possible IP’s they might come from. Once you have this IP or list of IP’s just open the authorized_keys file on your server and add a “from=” line to the beginning of their public key as shown below:

[root@mainserver .ssh]# cat authorized_keys
from="10.20.30.*,172.16.31.*,192.168.1.*" ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEArAkcHTOXZiDxcJEHNmrJRoM2HJE9Rq1uoiVHuTSjgl0THp0UFDNepnmCvk5bX22KzjAUa8PWnq1ZDw5Uf9/i6N/SiXittnxT0dFnZGj6RRep5Ae3AmOJbg0i69XL9o2zBJnYo2JVPXJkCDhSvVWokZUn5QjaJiGNigP9plA1He94Slkhn2jTxx1iehx9Vy/ojnxsJDqpTa8hX1GQK/b1jzvWdN3Qg+EhlxBDfKSA8u4uGsPP+6hXGgFLRluG/6yizj8LDF1LRWIKYaBvaLNJ+720sAI9O4miHyxY4n3ghBDbULPoLGz5a6bYRJ9pY9i0ySQEmXNSD+0u31+fAaGGpQ== user@sauronseye

So if you have this public key and it is supposed to be used by a user at Company A and that user gives it to someone else at Company B or it is compromised by evil haxor, then that key whether it had a password or not will not allow them access. Then with your normal audits you do (right?) you see failed logins from a foreign IP and realize the key was compromised. Server is still secure your world is safe from harm :).

The last and yet another very common way of using ssh-keys is for specialized scripts use. You have a script on one server and at a certain point in the script you’d like it to kick off a command or script on another server. How do you do this? Well you can create an ssh-key specific for this purpose. Create your keys just as above but name its identity for the job at hand so you don’t get confused.
ssh-keygen -t rsa -f ~/.ssh/login_audit
For an example lets say you have primary internal server and you want it to go out and gather a report of all the last logins from your remote servers. You could put a script like this on the remote server:

# Script: last_logins.sh
last -a > /home/user/last_logins.barney
scp /home/user/last_logins.barney fred@mainserver:.
# End Script

Then in the authorized_keys file on the remote server edit the public key just as we added the “from=” line but instead use “command=”:

command="/home/user/scripts/last_logins.sh",no-port-forwarding,no-X11-forwarding,no-agent-forwarding [rest of pub key...]

Now whenever you ssh in using that identity file the only thing it will do is run that command and exit. There are countless uses for this alone and being aware that you can use ssh-keys like this will help you realize WHEN you should use them like this.
There is a bunch more you can do with ssh and ssh-keys so use this as a starting point and take advantage of the tools at your disposal!

~~~~~BONUS TWO-FACTOR AUTHENTICATION SECTION~~~~~
We discussed above using ssh-keys as two-factor authentication to login to a server via ssh. Some of you may be aware that Google offers a two factor authentication to login to your Google services (gmail, apps, etc.). They accomplish this with an application you can install called Google Authenticator.
If you don’t use google authenticator and 2-factor authenication on your google account you should strongly consider it. Gmail accounts are prime targets for being compromised and a lot of people use that account for a wide array of things that would make the compromise that much more damaging. Read some of the links below for starters:
http://www.multitasked.net/2011/jun/27/hacked-gmail-google-account/
http://threatpost.com/en_us/blogs/new-password-not-enough-secure-hacked-e-mail-account-100410

Once you have google authenticator installed cause now you fear for the safety of your gmail… what else can you do with this thing? Well we can use it for two-factor authentication to login to our machines, much in the same way we used the ssh-keys.
I’ve found a great post that details how to do this and don’t forget to read the comments there as they include a lot of valuable information as well:
http://www.mnxsolutions.com/security/two-factor-ssh-with-google-authenticator.html

Linked also in that post is how to use a YubiKey to login with two factor authentication. These are all great and really secure methods to login and authenticate to your systems.

Hopefully you can realize the importance of two factor authentication to secure the things that really matter to you. There are countless hacks and exploits and more found every day. Trust me it is much easier to take the extra 10 minutes to set this up than the hours or days of recovery from a security breach. One needs to be proactive about security because if you are reactive then you are too late.