Category: lang:en

• Over the weekend, I’ve talked at the ChaosBBQ in Dortmund, Germany. A nice and small conferency gathering.
• This week I had only a few bugs (that I reported):
  • Rhythmbox crashed again (hacking without music is dreadful)
  • Weird SELinux problems after resuming from suspend
• The filter stuff is actually more complex than I initially thought: I do indeed have to handle async USB URBs which makes the code look more ugly and taste more like spaghetti. Anyway, the filter stuff should fully work now *yay*.

  I hope the video works. If it doesn’t, please download first. So we can replace packets on the fly, in and out. And of course, you could design a more complex scenario: Let the first read pass by unmodified but after that modify the packets.

• Read a bit in “Essential Linux Device Drivers” which is an interesting to read book. I like the relaxed writing style. I haven’t gotten to the nitty gritty USB details yet though.
• Wireshark can sniff USB communication, too. And it can save as a pcap file. And it dissects bits of the protocols. At least it shows the SCSI request for an USB Massstorage. I have to test whether it knows, say, webcams. First investigations show that it only supports USB Massstorage though. And it’d be interesting whether it can sniff the communication I’m filtering with QEmu.
• Apparently, USB network magic for pcap is 0xdc:

  typedef struct pcap_hdr_s { guint32 magic_number; /* magic number */ guint16 version_major; /* major version number */ guint16 version_minor; /* minor version number */ gint32 thiszone; /* GMT to local correction */ guint32 sigfigs; /* accuracy of timestamps */ guint32 snaplen; /* max length of captured packets, in octets */ guint32 network; /* data link type */ } pcap_hdr_t;

  typedef struct pcap_hdr_s { guint32 magic_number; /* magic number */ guint16 version_major; /* major version number */ guint16 version_minor; /* minor version number */ gint32 thiszone; /* GMT to local correction */ guint32 sigfigs; /* accuracy of timestamps */ guint32 snaplen; /* max length of captured packets, in octets */ guint32 network; /* data link type */ } pcap_hdr_t;

  Wirehsark dumps the following:

  0000000: d4 c3 b2 a1 02 00 04 00 00 00 00 00 00 00 00 00  ................
  0000010: ff ff 00 00 dc 00 00 00 43 8e 44 4c d7 96 05 00  ........C.DL....

  So I might be able to implement saving to PCap at some stage.

• Next week will be GUADEC! *yay* But that will also mean that I can’t work that much.
• So my emulation seems to work now, too. I now need to speak the right protocol. So if you know a good resource that describes how a, say, USB Webcam behaves on the bus, drop me a line. Or anything that can dissect USB packets would be fine.

• Read about Radare. Apparently, they have “USB support” but I could only see a USB communication sniffer. So Radare doesn’t dissect USB pakets 🙁
• Installed GDB from git, because the GDB in Fedora 13 crashes way too often. I didn’t file as many new bugs this week though 😉 I seem to have worked around all my crashers…
• Fought a lot with git 🙁 It’s incredibly hostile. I tried to rebase stuff and it keeps bugging me with old commits still being visible although I’ve changed them 🙁 I probably haven’t understood what it does yet. Tried to fix as much as possible using git reflog. Of course, the man page references options (–verbose in my case) that are not existant. Brilliant. I don’t know why I actually expected git to help me.
  This is hilarious, too:

  muelli@bigbox ~/git/qemu $ git rebase setup_fds 
  First, rewinding head to replay your work on top of it...
  Applying: Temporary migration to usb_packet_filter_setup_fds
  Using index info to reconstruct a base tree...
  Falling back to patching base and 3-way merge...
  Auto-merging usb-linux.c
  CONFLICT (content): Merge conflict in usb-linux.c
  Failed to merge in the changes.
  Patch failed at 0001 Temporary migration to usb_packet_filter_setup_fds
  
  When you have resolved this problem run "git rebase --continue".
  If you would prefer to skip this patch, instead run "git rebase --skip".
  To restore the original branch and stop rebasing run "git rebase --abort".
  
  
  muelli@bigbox ~/git/qemu $ nano usb-linux.c # hack hack hack
  muelli@bigbox ~/git/qemu $ git add usb-linux.c
  muelli@bigbox ~/git/qemu $ git rebase --continue
  Applying: Temporary migration to usb_packet_filter_setup_fds
  No changes - did you forget to use 'git add'?
  
  When you have resolved this problem run "git rebase --continue".
  If you would prefer to skip this patch, instead run "git rebase --skip".
  To restore the original branch and stop rebasing run "git rebase --abort".
  
  muelli@bigbox ~/git/qemu $ 
  

  WTF?!

  That one is brilliant, too:

  muelli@bigbox ~/git/qemu $ git rebase -i setup_fds
  # Stupid me: I selected "f" for the very first entry in that edit window
  Cannot 'fixup' without a previous commit
  # Fair enough, let me restart then:
  muelli@bigbox ~/git/qemu $ git rebase setup_fds 
  Interactive rebase already started
  # O_o WTF? What else, besides aborting, could I possibly do anyway?!
  muelli@bigbox ~/git/qemu $ git rebase --abort
  muelli@bigbox ~/git/qemu $ git rebase setup_fds 
  # Now it works...
  

• Reimplemented host side USB filters to obtain valid USB communication. I have various simple filters: PassThrough, Logging and Replacing. The first one does nothing but return the data w/o any modification. The second one writes the bytes it reads and writes to files. The third one replaces 512 “A”s with 512 “B”s. Still need separate packets from the device in question to the host from packets from the host to the device to obtain valid device behaviour without reading all of the documentation. That will give me a good starting point to actually do the fuzzing.

  That replace filter produced interesting results. I replaced every “A” transmitted by a “B”. On the host, I created a file on a mass storage with 4KB “A”s. When “cat”ting the file from the guest, I saw “A”s. But copying the file in the guest resulted in the new file having all “B”s. I expected the “cat” showing all “B”s, too. And as far as I can see, the “A”s are actually replaced for the “cat”.

  Of course, Istanbul crashed while trying to make that screencast.
  Note that the filter code actually changed by now, not only because I enhanced the protocol (in the version you’re seeing, only USB payload is exchanged. In the new version, also the PID, device address and device endpoint are filtered) but also because I refactored the communication bits into a USBPacket class.
  I missed to show the pen drive from the host point of view after having copied the file in the guest, but the “bbbb” file is full of “B”s.

• I’m on my way to emulating a USB device, i.e. make the guest think it has a USB device attached but the device is a program running on the guest. I basically copied the USB serial driver and the HID driver and modified them to get packets from a pipe and send them to a pipe. I had serious problems with QEmu: QEmu didn’t register my new “device”. Now I called the right function to initialize the USB device and voila, it attaches it like it should.
  Now I need to obtain valid USB communication using the filter so that I can respond to incoming packets properly.
• Dear lazyweb, I’m wondering whether I could make my OS load an application but then break on main() so that I can attach a debugger. I cannot run the application *with* GDB. Instead, I want to attach a GDB after the program is fully loaded. Maybe LD_PRELOADing on main() will work?

• Implemented adding and removal of a filter. It works via the monitor or command line. And it does indeed seem to work:
  

  
  Sorry for that video being so poorly embedded in this wordpress instance. You might want to try to download the video directly.

  So yeah, we can potentially filter USB packets by now, which allows us in-place fuzzing. But that’s cumbersome because we need to have a device attached to the host. So the goal must be to be able to do USB communication without a device being attached to the host but with a program that emulates the USB device in question.

• Of course I had problems to build Istanbul, the software I created the screencast with. Other stuff, including my IDE 🙁 just crashes, too…
• I used qemu to pass a usb device through to the guest. Hence the Linux on the host detached the device. I desperately tried make Linux reattach the device. I tried to use ioctl() with USBDEVFS_CLAIMINTERFACE but it didn’t really work. After spending many hours, I just unplugged and replugged the pendrive…
  My code is pretty much

      int interface = atoi(argv[1]);
      fd = open("/dev/bus/usb/002/006", O_RDWR);
      result = ioctl(fd, USBDEVFS_RELEASEINTERFACE, &interface);
      //result = ioctl(fd, USBDEVFS_CLAIMINTERFACE, &interface);
      printf("Result: %d, errno: %d %s\n", result, errno, strerror(errno));
  

  and fails with Invalid Argument for RELEASE or for CLAIM with -EINVAL (Invalid Argument) or -ENOENT (No such file or directory). I have no idea what I am doing wrong. So if you do, please tell me 😛
  Oh, and these ioctls are not exported to Python I think. At least I couldn’t find the correct ioctl number for USBDEVFS_CLAIMINTERFACE without progamming a tiny C program to print it out for me.

• Trying to test anything with QEmu is a pain though: It takes ages to boot anything with QEmu without KVM 🙁 It takes literally a whole night to boot into an Ubuntu installation CD.
• trying to work with a minimal operating system created by the following command on my Ubuntu box:
  sudo ubuntu-vm-builder kvm lucid --addpkg openssh-server --addpkg screen --addpkg acpid --addpkg htop --addpkg cheese --flavour generic
• For the record: I build my QEmu with the following command ./configure --prefix=/opt/muelli/qemu/ --disable-strip --extra-cflags="-O0 -DDEBUG" --disable-docs --enable-io-thread --enable-attr --enable-kvm --disable-xen --target-list="i386-softmmu x86_64-softmmu" --enable-curses && make && make install

Again, a small summary of my last week.

• Filed a couple of bugs that annoyed me. My favourite: My main monitor dies randomly. Let’s hope it’s not a hardware issue. That’d seriously put me back. In fact, it’s quite cumbersome to reanimate my monitor in the middle of a working session… Oh. And qemu crashes 🙁 That’s really unfortunate for me atm.
• Subscribed and quickly unsubscribed qemu-devel mailinglist. Way too noisy. Those low-level people don’t seem to like using bug tracker or smth like ReviewPad to submit patches. Very stressful.
• Enjoyed a long weekend in Hamburg including watching some Worldcup games
• Read through Qemu code and tried to grasp how things play together.
• Started to implement simple USB packet filter. spent ages resolving a logical error: I checked for retval != -23 whereas I should have checked for retval == -23 🙁

  

  So I have exported a new command to the QEmu monitor. And we can even attach some logic to that new command:

  

  Everything returns -1 at this stage though. So the actual implementation still needs to be done.

• It literally takes a whole night for me to boot anything with qemu though 🙁 That’s a real pain and I cannot work that way. My CPU is one of the few modern Intel CPUs that does not support hardware virtualisation 🙁 I need to think of a solution.
• I still don’t really have a timeline 😐
• Our deadline is on 2010-08-20 and we are supposed to hand in 3 hard copies and one soft copy. I wondering whether I have to go back to Dublin to hand my hard copies in.

So I figured that we are supposed to write a blog during our practicum phase. Here I am.

• I missed the first official week, which was right after the exams anyway. I doubt anybody was able to do anything after the Biometrics exam.
• In the second week, I moved back to Germany. Slowly though: I attended LinuxTag and visited a friend…
• The third week began with some administrative stuff (i.e. taxes and care about a grant). I also almost finished running GNOME Foundation Board of Directors elections: Preliminary Result.
• More work related: I tried to updated from Fedora 12 to Fedora 13 (to get latest QEmu and tools). Didn’t work (as expected) out of the box. Encountered (and reported) a couple of annoying bugs. My favourite: The update tool tries to mount /boot and swap. But /boot is left unclean because the preupgrade tool apparently does a hard reboot (i.e. w/o unmounting the filesystems properly). And swap can’t be found by the upgrade tool (for whatever reason). In both cases the installer just stops working and reboots the machine (sic!), as opposed to just fsck /boot or continue w/o swap.
• Began to set up working environment: LaTeX Template, cloned qemu repository, looked a bit at QEmu code.
• Tried to install some Operating Systems to break. Microsoft didn’t let me.
• Read some stuff
• Filed two bugs against Zotero (my bibliography tool): One problem in fullscreen mode and one with proxied URLs.
• Went to a regulars’ table (for the first time after 9 month) and found out that one of them runs a company and they do USB security assessment atm. They are trying to make QEmu emulate a mass storage that returns a good file on the first read and a bad file (i.e. virus) on the second read. Sounded interesting, we’ll keep in touch and exchange details.
• Right now I’m missing kind of a plan for my work. I haven’t really structured my work or broken it up. So I’m trying to see how many weeks I actually have (I know that I’ll go at least to GUADEC, the annual GNOME conference, for one week. I might even be invited to GNOME.Asia in Taiwan…) and what I could possibly do in that time.
• I do have a high level idea of what needs to be done, i.e.
  • Patch QEmu to pipe USB communication in and out,
  • write some backend that uses these pipes to communicate with the guest,
  • find a smart algorithm to create/modify fishy USB packets (i.e. try to understand how a webcam communicates and set funny values for resolution on purpose),
  • try to exploit an Operating System (probably best to start off with a self-broken USB driver or application)
• I’ll try to have a roadmap by the beginning of the next week.