Bootloader Unlock for the Atrix HD, Razr HD, and Razr M
Dan Rosenberg (a.k.a. XDA Recognized Developer djrbliss) gets the credit for finding exploits on a lot of devices, and now you can add to it the line of Motorola units that use the Qualcomm MSM8960 chipset. There are currently three models included in this category, the Atrix HD, Razr HD, and Razr M. They’re based on the processor marketed as the Qualcomm Snapdragon, and they’re hiding some interesting tricks that may eventually keep users from loading their own ROMs. Dan’s investigation did lead to an exploit, but I find some of the pseudocode he authored based on the disassembly an interesting look at what the future might bring from Motorola.
Dan’s article is posted over at the Azimuth Security blog. In it, he starts by covering an online bootloader unlocking tool provided by Motorola that allows you to upload a blob generated using fastboot with your handset. It will return an unlock key, which is used in the same fashion. Great, if you have a developer version of the hardware. If not, you’re out of luck because this chipset includes internal fuses, which when blown can never be reset again. One of the QFuses is blown on every user model to differentiate it from an otherwise identical developer version.
The most interesting part about this is what Dan uncovered when disassembling the parts of a leaked Atrix HD update, which pertains to these fuses. Luckily he rebuilt the program as psuedo-code that is readable by the non-assembly-guru crowd. In addition to a fuse for the developer edition and one to signify that the bootloader is unlocked, he discovered a rather sinister flag. He calls it is_unlocking_allowed() as his psuedo-function label. He thinks that once burnt, that fuse will never allow the ‘bootloader unlocked’ QFuse to be burned, something that would have prevented all exploits including the one he eventually found. Dan is fairly certain that the units Motorola is shipping now do not have that QFuse blown.
At this point Dan started looking at how the official unlock method manages to write to the ‘bootloader unlocked’ fuse. It ends up that the ARM kernel is doing the work,which brings us to a familiar package, the TrustZone kernel running inside the ARM core. XDA Elite Recognized Developer Adam Outler wrote a terrific post covering TrustZone earlier in the year that will help fill you in on the role that TZ plays in the chain of trust. Having already worked with the kernel many times, Dan was able to quickly find an arbitrary memory write which he used to clear the global flag preventing writing to the bootloader unlock QFuse location. He posted the details for those who don’t mind getting their hands dirty.