IBM's remote firmware configuration protocol
I spent last week looking into the firmware configuration protocol used on current IBM system X servers. IBM provide a tool called ASU for configuring firmware settings, either in-band (ie, running on the machine you want to reconfigure) or out of band (ie, running on a remote computer and communicating with the baseboard management controller - IMM in IBM-speak). I'm not a fan of using vendor binaries for this kind of thing. They tend to be large (ASU is a 20MB executable) and difficult to script, so I'm gradually reimplementing them in Python. Doing that was fairly straightforward for Dell and Cisco, both of whom document their configuration protocol. IBM, on the other hand, don't. My first plan was to just look at the wire protocol, but it turns out that it's using IPMI 2.0 to communicate and that means that the traffic is encrypted. So, obviously, time to spend a while in gdb.
The most important lesson I learned last time I did this was "Check whether the vendor tool has a debug option". ASU didn't mention one in its help text and there was no sign of a getopt string, so this took a little longer - but nm helpfully showed a function called DebugLog and setting a breakpoint on that in gdb showed it being called a bunch of the time, so woo. Stepping through the function revealed that it was checking the value of a variable, and looking for other references to that variable in objdump revealed a -l argument. Setting that to 255 gave me rather a lot of output. Nearby was a reference to --showsptraffic, and passing that as well gave me output like:
BMC Sent: 2e 90 4d 4f 00 06 63 6f 6e 66 69 67 2e 65 66 69
BMC Recv: 00 4d 4f 00 21 9e 00 00
which was extremely promising. 2e is IPMI-speak for an OEM specific command, which seemed pretty plausible. 4d 4f 00 is 20301 in decimal, which is the IANA enterprise number for IBM's system X division (this is required by the IPMI spec, it wasn't an inspired piece of deduction on my behalf). 63 6f 6e 66 69 67 2e 65 66 69 is ASCII for config.xml. That left 90 and 06 to figure out. 90 appeared in all of the traces, so it appears to be the command to indicate that the remaining data is destined for the IMM. The prior debug output indicated that we were in the QuerySize function, so 06 presumably means… query the size. And this is supported by the response - 00 (a status code), 4d 4f 00 (the IANA enterprise number again, to indicate that the responding device is speaking the same protocol as you) and 21 9e 00 00 - or, in rational endianness, 40481, an entirely plausible size.
Once I'd got that far the rest started falling into place fairly quickly. 01 rather than 06 indicated a file open request, returning a four byte file handle of some description. 02 was read, 03 was write and 05 was close. Hacking this together with pyghmi meant I could open a file and read it. Success!
Well, kind of. I was getting back a large blob of binary. The debug trace showed calls to an EfiDecompress function, so on a whim I tried just decompressing it using the standard UEFI compression format. Shockingly, it worked and now I had a 345K XML blob and presumably many more problems than I'd previously had. Parsing the XML was fairly straightforward, and now I could retrieve the full set of config options, along with the default, current and possible values for them.
Making changes was pretty much just the reverse of this. A small XML blob containing the new values is compressed and written to asu_update.efi. One of the elements is a random identifier, which will then appear in another file called config_log along with a status. Just keep re-reading this until the value changes to CM_DONE and we're good.
The in-band configuration appears to be identical, but rather than sending commands over the wire they're send through the system's IPMI controller directly. Yes, this does mean that it's sending compressed XML through a single io port. Yes, I'd rather be drinking.
I've documented the protocol here and hope to be able to release this code before too long - right now I'm trying to nail down the interface a little, but it's broadly working.
Syndicated 2014-02-10 18:57:28 from Matthew Garrett