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A hacker's journey: freeing a phone from the ground up, first part

Written by Paul Kocialkowski - - no comments

Every once in a while, an unexpected combination of circumstances ends up enabling us to do something pretty awesome. This is the story of one of those times. About a year ago, a member of the Replicant community started evaluating a few targets from CyanogenMod and noticed some interesting ones. After some early research, he picked a device: the LG Optimus Black (P970), bought one and started porting Replicant to it. After a few encouraging results, he was left facing issues he couldn't overcome and decided to give up with the port. As the device could still be an interesting target for Replicant, we decided to buy the phone from him so that I could pick up the work where he stalled.

Documentation

Upon receiving the item, I decided to dig a little more into what that device was made of and found that very precise technical documentation about it leaked online (EN_LG-P970_SVC_ENG_110415.pdf). This mostly contains all the electrical schematics of the phone, a precise list of the components used and a few other interesting things. Such in-depth documentation is rather rare for such mainstream devices: we usually get service manuals with some of that information, but seldom something to reach this level of completeness. That really got me interested in the device.

Bootloaders

In the meantime, I found a few interesting threads at XDA mentioning the bootloader of a couple of OMAP4 devices from LG. Apparently, LG leaked the private keys for signing the first bootloader on these devices in a release of the bootloader's (copylefted) source code. As often on OMAP devices, the bootloader is X-Loader, a minimalistic copy of U-Boot source code that can run with a small memory footprint. While all of this is great fun, it's nothing we can seriously use in Replicant. At best, it enables developers to play around with the device's bootloader. And we know all too well how much of an issue signed bootloaders represent to software freedom on those devices, so this is kinda nice. Ironically, those private keys aren't the only things that we leaked aside free software releases for these LG OMAP4 devices: that's exactly where the recent PowerVR Series 5 source code leak originates, rising its own share of issues for free software.

Seeing that people were playing around with bootloaders on other LG OMAP devices got me interested in my recently-acquired LG Optimus Black. A quick look at the first bootloader's binary data revealed that it is X-Loader as well, but without the usual headers for signature checking. In addition, the second stage bootloader turned out to be U-Boot on early versions of the devices, which was replaced by LK on the ICS update. Nevertheless, X-Loader and U-Boot are copylefted free software, so I went ahead and tried to grab the released source code from LG. After downloading a few different archives, I finally spotted the one with the bootloaders' source code. All of it looked nice and nearly complete (only a small piece of software specific to LG, called FOTA, was missing from the release and broke the build, but I found equivalent code in one of the OMAP4 device's source code and it turned out to be nothing of interest).

Signature checks

At this point, I just couldn't stand in place as one question was left unanswered: can I actually build and run that code on the device? That is, are signature checks enforced on the LG Optimus Black (P970)? Even if the lack of dedicated header on the binary was a good sign, I am usually not too optimistic about these things, so I wanted to double check. As it turns out, OMAP platforms are also incredibly well documented by TI, so I just grabbed a copy of the Technical Reference Manual (TRM as we call it) and looked for a way to identify clearly whether the device enforces signature checks. Of course, that part is not publicly documented in-depth, but it seemed very clear from the documentation that OMAP SoCs are produced in (at least) two different fashions: a General Purpose (GP) version that doesn't check the first bootloader's signature and a High Security (HS) version that does check those signatures. Usually, GP versions of OMAP SoCs are shipped on reference or development boards, such as the BeagleBoard. They also enable projects like the GTA04 to create a device that runs a free bootloader. However, it is rare to see GP OMAP chips being used on mainstream devices. While very happy from that finding, I still needed a reliable way to figure out whether the Optimus Black is using a GP or HS version of the OMAP3630. And it turns out that a couple of registers hold that information. One of them, CONTROL_STATUS is 4 byte-long and holds the device type with a 1 byte offset, at 0x480022f0. It also holds the boot order information (SYS_BOOT) on the lower byte. On android, it can easily be accessed with devmem:

devmem 0x480022f0 16
The higher byte is 0x03, which clearly indicates a GP device: HS devices usually return 0x02.



This post is part of a series of articles about freeing the LG Optimus Black (P970):

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