Reversing the V740, part 3: The V740 abstraction layer module

Last time, I described (at a high level) the interface used by the Verizon connection manager to talk to the V740, and how it related to my goal of coercing the V740 to re-upgrade to EVDO after failing over to 1xRTT, without idling the connection long enough for dormant mode to take over.

As we saw previously, it turned out that the V740’s abstraction layer module (WmcV740.dll) was static linked to a debug version of a Novatel SDK that encapsulated the mechanism to talk to the V740’s firmware and instruct the device to perform various tasks.

Now, the WmcV740.dll module contains code that is specific to the V740 (or at least specific to Novatel devices), which knows how to talk to a V740 that is connected to a computer. Internally, the way this appears to work is that the Novatel driver creates a virtual serial port (conveniently named Novatel Wireless Merlin CDMA EV-DO Status Port (COMxx)), which is then used by the DLL to send and receive data from the firmware interface. In other words, the virtual serial port is essentially a “backdoor” control channel to talk to the firmware, separate from the dial-up modem aspect of the device that is also presented by the driver in the form of a modem device that can be controlled via standard “AT” commands. (The advantage to taking this approach is that a serial port is typically only accessible by one program at a time, and if one is using the standard RAS/dial-up modem support in Windows to dial the connection, then this precludes being able to use the modem serial port to perform control functions on the device, as it’s already being used for the data session.)

By simply looking at the debug print strings in the binary, it’s possible to learn a fair amount at what sort of capabilities the SDK functions baked into the binaries might hold. Most functions contained at the very least a debug print at the start and end, like so:

Diag_Get_Time   proc near

push    ebp
mov     ebp, esp

[...]

 ; "Diag_Get_Time: Begin\\n"
push    offset aDiag_get_timeB
push    2               ; int
call    DebugPrint
add     esp, 8
cmp     IsNewFirmware, 0
jnz     short FirmwareOkay1

cmp     FirmwareRevision, 70h
jnb     short FirmwareOkay1

 ; "Diag_Get_Time: End: Old Firmware\\n"
push    offset aDiag_get_timeE
push    2               ; int
call    DebugPrint
add     esp, 8
mov     ax, 14h
jmp     retpoint

[...]

Although most of the routines named in debug prints didn’t appear all that relevant to what I was planning on doing, at least one stood out as worth further investigation:

strings WmcV740.dll | find /i "Begin"
[..]
Diag_ERI_Clear: Begin
Diag_Call_Origination: Begin
Diag_Call_End: Begin
Diag_Read_DMU_Entry: Begin
[...]

(Strings is a small SysInternals utility to locate printable strings within a file.)

In particular, the Diag_Call_End routine looked rather interesting (recall that at least for my handset, pressing the “end call” button on the device while a data connection is active places it into dormant mode). If this routine performed the same function as my handset’s “end call” button, and if “ending” the data call also put the V740 into dormant mode (such that it would re-select which network to use), then it just might be what I was looking for. There were several other functions that looked promising as well, though experimentation with them proved unhelpful in furthering my objective in this particular instance.

At this point, there were essentially two choices: Either I could try and re-implement the code necessary to speak the custom binary protocol used by WmcV740.dll to communicate with the device, or I could try and reuse as much of the already-existing code in the DLL as possible to achieve the task. There are trade-offs to both approaches; reimplementing the protocol would allow me to bypass any potential shortcomings in the DLL (it actually turns out that the DLL is slightly buggy, and the commands to reset power on the device will result in heap corruption on the process heap – ick! – fortunately, those functions were not required for what I wanted to do). Additionally, with a “from-scratch” implementation, assuming that I was correct about my theory that the the driver’s virtual serial port is just a pass-through to the firmware, such a re-implementation would possibly be more portable to other platforms for which Novatel doesn’t supply drivers (and might be extendable to do things that the parts of the SDK linked to WmcV740.dll don’t have support for).

However, taking the approach of reimplementing a client for the firmware control interface also would require a much greater investment of time (while not extremely complicated, extra work would need to be done to reverse enough of the protocol to get to the point of sending the desired commands as performed by the “Diag_Call_End” function that we’re interested in) compared to simply reusing the already existing protocol communications code in the DLL. Furthermore, reimplementing the protocol client from scratch carries a bit more risk here than if you were just reverse engineering a network protocol, because instead of talking to another program running on a conventional system, in this case, I would be talking to the firmware of a (non-serviceable-by-me) device. In other words, if I managed to botch my protocol client in such a way as to cause the firmware on the device to do something sufficiently bad as to break the device entirely, I’d have a nice expensive paperweight on my hands (without knowing a whole lot more about the firmware and the device itself, it’s hard to predict what it’ll do when given bad input, or the like – although it might well be quite robust against such things, that’s still a relatively risky decision to make, because it could just as well fall over and die in some horribly bad way on malformed input). Not fun, by any means, that. To add to that, at this point, I didn’t even know if the Diag_Call_End function would pan out at all, so if I spent all the time to go the full nine yards just to try it out, I might be blowing a lot of effort on another dead end.

Given that, I decided to go the more conservative route of trying to use the existing code in WmcV740.dll, at least initially. (Although I did research the actual protocol a fair bit after the fact, I didn’t end up writing my own client for it, merely just reusing what I could from the WmcV740.dll module). However, there’s a minor sticking point here; the DLL doesn’t provide any way to actually reach the code in Diag_Call_End that is externally visible to code outside the module. In other words, there are no exported functions that lead to Diag_Call_End. Actually, the situation was even more grim than that; after a bit of analysis by IDA, it became immediately clear that there weren’t any callers of Diag_Call_End present in the module, period! That meant that I wouldn’t have a “working model” to debug at runtime, as I did with the interface that WmcV740.dll exports for use by the Verizon connection manager GUI.

Nonetheless, the problem is not exactly an insurmountable one, at least not if one is willing to get their hands dirty and use some rather “under-handed tricks”, assuming that we can afford to tie ourselves to one particular version of WmcV740.dll.

Next time: Zeroing in on Diag_Call_End as a potential solution.

One Response to “Reversing the V740, part 3: The V740 abstraction layer module”

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