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comparison doc/TFC139-breakin @ 974:3f67d5bf96ef

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doc: TFC139-breakin written, Compal-unlock updated
author Mychaela Falconia <falcon@ivan.Harhan.ORG>
date Sun, 15 Nov 2015 03:47:19 +0000
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children 7a55a3eb985a
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973:285505f98013 974:3f67d5bf96ef
1 Maliciously locked bootloader
2 =============================
3
4 When Compal (Motorola's ODM who designed and built their C1xx phones for them)
5 designed the firmware architecture and flash memory layout for their phones,
6 they made a bad design decision by putting the boundary between their bootloader
7 and the main fw image at 0x2000, even though the flash erase block boundary
8 doesn't come until 0x10000 - thus every time the main fw needs to be reflashed
9 to a different version, the dangerous boot sector has to be reflashed too.
10
11 But then they made things even worse in the newer versions of their fw by
12 introducing a bootloader lock malfeature whereby the ability to interrupt boot
13 and load code serially may be artificially disabled. This malfeature is
14 implemented as follows:
15
16 * In the original firmware layout (before the addition of the malfeature in
17 question) the boot code occupies the flash range from 0 through 0x1FFF, then
18 there are some ID strings at 0x2000, 0x2020 and 0x2040, and then the part of
19 the firmware that used to be at 0x10000 in TI's reference fw starts at 0x20A0,
20 with the entry point at 0x20F8 (corresponding to TI's 0x10058).
21
22 With the addition of the bootloader lock malfeature the 32-bit word at 0x2060
23 (previously unused and filled with 0xFFFFFFFF) became a control word telling
24 the bootloader whether diversion of the boot path to serial code download
25 should be allowed or not.
26
27 * When firmware images with this malfeature present are first built, the word
28 at 0x2060 contains 0xDDDDDDDD. (Does D stand for debug or development, or
29 was the developer who implemented this malfeature fascinated by large bra
30 cups? We may never know.) This word MUST read as 0xDDDDDDDD in order for
31 the boot code to allow serial download: if it reads as any other value (e.g.,
32 if it contains 0xFFFFFFFF because only the 8192 byte boot code has been
33 programmed into flash sector 0, with blank flash from 0x2000 onward), no
34 serial download opportunity will ever be offered and the phone is effectively
35 bricked!
36
37 * For as long as the word at 0x2060 still contained 0xDDDDDDDD, Compal's
38 developers could continue gaining access through the bootloader and reflashing
39 their firmware. But when phones were to be shipped to customers with the
40 malicious bootloader lock activated, they probably sent some Test Mode command
41 (see RVTMUX write-up) to their running fw that caused it to write 0x00000000
42 into the flash word at 0x2060. (Remember that any bit in a NOR flash memory
43 can be programmed from 1 to 0 at any time in any combination, but changing
44 bits from 0 back to 1 is only possible with full sector erasure.)
45
46 * Once the word at 0x2060 has been programmed (in the flash memory sense) from
47 0xDDDDDDDD down to 0x00000000, the phone is irreversibly locked and has lost
48 its ability to ever run a different firmware version, like a kamikaze pilot's
49 plane that has discarded its landing gear and can only crash now.
50
51 TFC139 recovery
52 ===============
53
54 While it probably was Compal's, Motorola's and TracFone's intent that the
55 bootloader lock on their phones be truly irreversible, some genius out there
56 (we may never know who this person was/is) has found a way to recover the
57 reflashing capability on at least one very common flock of locked-down phones:
58 North American C139 units (1900+850 MHz hardware) sold with TracFone branding,
59 firmware version 8.8.17. Here is how it goes:
60
61 * Even though the bootloader is locked down, if one boots the full fw regularly,
62 one can still access the RVTMUX interface which the TI-based fw implements
63 for debug trace and factory programming functions. One needs to key in the
64 magic sequence **16379# into the running fw, and a hidden menu will appear,
65 giving the operator the option to enable trace. Selecting this option will
66 cause the fw to switch the headset jack to the UART carrying RVTMUX.
67
68 * Mot/Compal's firmware is based on a quite old version of TI's chipset
69 reference fw (relative to late TCS211 from the Openmoko/Pirelli era), and it
70 does not feature the Enhanced Test Mode (ETM) component with which we are
71 most familiar. However, it does implement the older set of non-enhanced
72 Test Mode commands, and these TM commands just happen to include raw memory
73 read and write operations at an arbitrary address. (For a while we were
74 under a mistaken belief that these commands were Compal's inventions, until
75 we discovered TI's original TM predating ETM.)
76
77 * The ingenious idea our hero came up with is that one can use the RVTMUX TM
78 memory write command to write a piece of "shellcode" into an unused RAM
79 location, and then use those very same memory write commands to cause a
80 transfer of control to this code by overwriting a function return address on
81 the stack!
82
83 * Once you can execute your own code on the Calypso, everything becomes possible
84 once again. At that point one can trivially reverse the bootloader lock by
85 erasing flash sector 0 and rewriting it with 0xDDDDDDDD in the 0x2060 word,
86 or even better, rewriting this boot sector with an older version of the boot
87 code that lacks the locking malfeature altogether.
88
89 In the FreeCalypso suite the tfc139 host utility performs the break-in using
90 the RVTMUX TM memory write and stack smashing method just described. The
91 "shellcode" injected by tfc139 re-enables the Calypso chip's own boot ROM and
92 jumps to it; this boot ROM will endlessly wait for a serial download because
93 the word at 0x2000 contains neither 0 nor 1 (it is part of an identifying ASCII
94 string in Mot/Compal's fw), and the operator can then run fc-loadtool to
95 perform arbitrary flash operations.