FreeCalypso > hg > freecalypso-tools

annotate doc/TFC139-breakin @ 995:74024eb17e04

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fc-loadtool help: improve language regarding 16 MiB flash chips In FC project history, 16 MiB flash originally meant Pirelli DP-L10. Then we got FCDEV3B with the same flash (our own design), but now we are discovering more Calypso devices that used such large flash, both late Calypso era (Sony Ericsson K2x0) as well as much earlier ones (FIC FLUID devices.txt file with 2004 dates, Leonardo+ rev 5). Hence we need to migrate to more generic or neutral language in associated documentation, without giving elevated status to specific examples that drove our early project history.
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 03 Dec 2023 21:11:12 +0000
parents e7502631a0f9
children
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1 Maliciously locked bootloader
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2 =============================
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3
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4 When Compal (Motorola's ODM who designed and built their C1xx phones for them)
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Mychaela Falconia <falcon@freecalypso.org>
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5 designed the firmware architecture and flash memory layout for their phones,
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6 they made a bad design decision by putting the boundary between their bootloader
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7 and the main fw image at 0x2000, even though the flash erase block boundary
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8 doesn't come until 0x10000 - thus every time the main fw needs to be reflashed
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9 to a different version, the dangerous boot sector has to be reflashed too.
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10
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11 But then they made things even worse in the newer versions of their fw by
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12 introducing a bootloader lock malfeature whereby the ability to interrupt boot
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13 and load code serially may be artificially disabled. This malfeature is
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14 implemented as follows:
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15
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16 * In the original firmware layout (before the addition of the malfeature in
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17 question) the boot code occupies the flash range from 0 through 0x1FFF, then
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18 there are some ID strings at 0x2000, 0x2020 and 0x2040, and then the part of
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19 the firmware that used to be at 0x10000 in TI's reference fw starts at 0x20A0,
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20 with the entry point at 0x20F8 (corresponding to TI's 0x10058).
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21
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22 With the addition of the bootloader lock malfeature the 32-bit word at 0x2060
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23 (previously unused and filled with 0xFFFFFFFF) became a control word telling
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24 the bootloader whether diversion of the boot path to serial code download
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25 should be allowed or not.
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26
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27 * When firmware images with this malfeature present are first built, the word
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28 at 0x2060 contains 0xDDDDDDDD. (Does D stand for debug or development, or
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29 was the developer who implemented this malfeature fascinated by large bra
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30 cups? We may never know.) This word MUST read as 0xDDDDDDDD in order for
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31 the boot code to allow serial download: if it reads as any other value (e.g.,
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32 if it contains 0xFFFFFFFF because only the 8192 byte boot code has been
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33 programmed into flash sector 0, with blank flash from 0x2000 onward), no
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34 serial download opportunity will ever be offered and the phone is effectively
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35 bricked!
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36
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37 * For as long as the word at 0x2060 still contained 0xDDDDDDDD, Compal's
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38 developers could continue gaining access through the bootloader and reflashing
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39 their firmware. But when phones were to be shipped to customers with the
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40 malicious bootloader lock activated, they probably sent some Test Mode command
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41 (see RVTMUX write-up) to their running fw that caused it to write 0x00000000
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42 into the flash word at 0x2060. (Remember that any bit in a NOR flash memory
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43 can be programmed from 1 to 0 at any time in any combination, but changing
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44 bits from 0 back to 1 is only possible with full sector erasure.)
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45
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46 * Once the word at 0x2060 has been programmed (in the flash memory sense) from
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47 0xDDDDDDDD down to 0x00000000, the phone is irreversibly locked and has lost
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48 its ability to ever run a different firmware version, like a kamikaze pilot's
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49 plane that has discarded its landing gear and can only crash now.
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50
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51 Recovery procedure
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52 ==================
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53
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54 While it probably was Compal's, Motorola's and various carriers' intent that the
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55 bootloader lock on their phones be truly irreversible, the unlocking community
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56 has now developed a method for recovering these phones (restoring their ability
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57 to run any firmware of the user's choice) which (we hope) will work with all of
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58 the existing locked-down firmware versions. It works as follows:
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59
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60 * Even though the bootloader is locked down, if one boots the full fw regularly,
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61 one can still access the RVTMUX interface which the TI-based fw implements
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62 for debug trace and factory programming functions. One needs to key in the
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63 magic sequence **16379# into the running fw, and a hidden menu will appear,
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64 giving the operator the option to enable trace. Selecting this option will
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65 cause the fw to switch the headset jack to the UART carrying RVTMUX.
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66
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67 * Mot/Compal's firmware is based on a quite old version of TI's chipset
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68 reference fw (relative to late TCS211 from the Openmoko/Pirelli era), and it
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69 does not feature the Enhanced Test Mode (ETM) component with which we are
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70 most familiar. However, it does implement the older set of non-enhanced
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71 Test Mode commands, and these TM commands just happen to include raw memory
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72 read and write operations at an arbitrary address. (For a while we were
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73 under a mistaken belief that these commands were Compal's inventions, until
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74 we discovered TI's original TM predating ETM.)
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75
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76 * The ability to write arbitrary bytes into arbitrary RAM locations while the
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77 phone firmware is running means that we can inject a piece of shellcode into
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78 an unused RAM location and then cause this shellcode to gain execution by
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79 overwriting a function return address on the stack.
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80
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81 * Once you can execute your own code on the Calypso, everything becomes possible
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82 once again. At that point one can trivially reverse the bootloader lock by
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83 erasing flash sector 0 and rewriting it with 0xDDDDDDDD in the 0x2060 word,
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84 or even better, rewriting this boot sector with an older version of the boot
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85 code that lacks the locking malfeature altogether.
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86
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87 Procedure variations: old mot931c.exe vs. new tfc139
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88 ====================================================
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89
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90 We first became aware of the possibility of recovering locked-down phones as
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91 described above in the spring of 2014 when FreeCalypso developer Space Falcon
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92 became aware of the existence of Windows utility mot931c.exe (binary w/o source)
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93 that performs a variant of this unlocking procedure specific to one particular
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Mychaela Falconia <falcon@freecalypso.org>
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94 locked-down firmware version: C139 phones with TracFone branding, fw version
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95 8.8.17. At first we had replicated the operation of this Windows tool verbatim
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96 in our own Unix/Linux-based tfc139 libre tool; this variant of the shellcode-
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Mychaela Falconia <falcon@freecalypso.org>
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97 based unlocking procedure worked well on TFC139 units, but could not crack other
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98 locked-down fw versions, e.g., Cingular-branded C139 phones with fw version
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99 1.9.24.
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100
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101 Subsequent investigation revealed that whoever wrote that mot931c.exe Windows
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102 tool had not studied the operation of Motorola/Compal's TI-based firmware deeply
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103 enough, and implemented their shellcode injection quite suboptimally: the stack
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104 smashing process is hitting the wrong stack (not the stack of the L1A task in
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105 whose context the Test Mode commands sent over the UART are executing), and it
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106 is only through dumb luck that this version of the break-in procedure worked
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
107 at all. The limitation of working only with one specific fw version results
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
108 from this poor method of shellcode injection (mindless choice of the wrong stack
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
109 for smashing), and instead of adapting it in a version-specific manner to other
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
110 particular locked-down fw versions at hand, I (Space Falcon) reimplemented our
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
111 tfc139 utility to smash the right stack (that of the L1A task), and thereby
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
112 made it generic to all Mot C1xx firmware versions.
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
113
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
114 Our Compal firmware break-in utility is still called tfc139, but it is no longer
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
115 specific to TFC139 phones; instead it should work with all Mot C1xx firmwares.
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
116 The shellcode injected by tfc139 re-enables the Calypso chip's own boot ROM and
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
117 jumps to it; this boot ROM will endlessly wait for a serial download because
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
118 the word at 0x2000 contains neither 0 nor 1 (it is part of an identifying ASCII
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
119 string in Mot/Compal's fw), and the operator can then run fc-loadtool to
e7502631a0f9 initial import from freecalypso-sw rev 1033:5ab737ac3ad7
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
120 perform arbitrary flash operations.