Locked C139 bootloader reverse-engineered

Hello fellow hackers, Following my successful reverse eng of Tracfone "unlocking" utility mot931c.exe, I wrote a native Unix/Linux program (tfc139, based on FreeCalypso rvinterf tools) that breaks into locked-down TF C139 phones in the same fashion, with an IRAM payload that enables and jumps to the Calypso boot ROM. Doing so allowed me to run fc-loadtool against one of these Tracfones that still has its original locked-down bootloader intact (*not* overwritten with mot931c), and make a dump of the latter for examination. So here is how the bootloader lock works: the bootloader which sits in the first 0x2000 bytes of flash sector 0 is clearly built together with the main fw image as a single whole (TI's reference firmware is built the same way, so no surprise), and thus the bootloader version changes in sync with the main fw version. Most of the versions seen in the wild are almost byte-identical, the only diffs being some unused signature (?) words at 0x20 and the word at 0x964 giving the initial value to be loaded into the stack pointer - variations in the latter are a linker artifact resulting from how this bootloader is built together with the main fw. Despite the way in which they must have been compiled, the older versions of the bootloader have one good quality: if you are going to break into the bootloader serially (in our familiar way), then only the first 0x2000 bytes of the flash (in fact, even less than that) need to be good; the flash from 0x2000 onward can be blank or filled with garbage or malware or whatever - as long as the boot process is interrupted with a serial download, the jump to 0x20f8 doesn't happen and nothing from 0x2000 onward affects anything. So far, so good. But the newer versions of the bootloader that are part of the newer firmwares for both C11x/123 and C139/140 have an added malicious feature. Before sending what we call PROMPT1 out the serial port and waiting for a possible serial download, the boot code now checks the word in flash at 0x2060. This word needs to equal 0xDDDDDDDD (was it some developer's fascination with bras? - scnr); if this word contains any other value, no serial download opportunity is offered, and the code proceeds directly to the silly routine that emits that "ftmtool error" nonsense, followed by the jump to 0x20f8. All TF-branded C139 phones I've seen have fw version 8.8.17, which features the new malware bootloader. And the word at 0x2060 is zeroed out, resulting in the observed behavior of no serial download opportunity being given on boot. I speculate that perhaps the newer fw versions containing boot code with this malicious feature start out with 0xDDDDDDDD in the word at 0x2060, so that their own developers could do their job, but when shipping phones to end users, the bastards issued some ETM or whatever command to zero that word out, disabling the serial download access - remember that any NOR flash bit can be changed from 1 to 0 at any time, but going the other way around requires erasing and rewriting the whole sector. So what do we do about it? Well, at least the TF-branded C139s still have that ETM memory write command that allows us to break in by writing a little payload into IRAM and smashing the stack while the main fw is running, and we now have a free, source-enabled, Unix/Linux- based tool for performing this break-in. Not too long ago there was someone else on this list who had a newer C139 with Cingular branding (not TF) that also featured the maliciously locked bootloader. Not surprisingly, mot931c wouldn't work on that phone, as this closed source Weendoze binary does a fw version query and refuses to work with any versions other than TF's 8.8.x. But now that we have our own free tool for the hack in question, it may be worth testing if one can break into non-TF phones by the same method. The addresses for the IRAM payload download and the stack smashing may need to be tweaked experimentally, but hey... When the time comes to flash our own FreeCalypso firmware into these phones, I'm thinking of adopting one of the old bootloader versions as our "standard". In fact, the only diff between C11x/123 and C139/140 versions of this boot code is that the latter adds the check for "1003" at 0x803ce0, whereas the former has no such check. Thus we can use the more basic C11x/123 version of the boot code on all hw versions, and make our chainloading more efficient by loading 32 bytes instead of 15332. :) All of the work described above has already been pushed into the freecalypso-sw and freecalypso-reveng Hg repositories on Bitbucket. VLR, SF