It’s been 4 months since my last post but I’ve been working on some fun stuff. Said work has progressed to the point where it’s actually worth talking about publically so I’m crawling out from under my favorite rock and putting it “out there”.
My last few bits of writing were about some random OpenEmbedded stuff, basically outlining things I was learning while bumbling my way through the OE basics. I’ve been reading through the meta-selinux and meta-virtualization layers and they’re a great place to learn. Over the winter Holiday here I had some extra vacation time from my day job to burn so I finally got serious about a project I’ve been meaning to start for way too long.
Over the past year I’ve been thinking a lot about the “right way” to measure a software system. We’ve implemented a measurement architecture on XT but this has a few down sides: First a system as large as XT is very difficult to use as a teaching tool. It’s hard to explain and show someone the benefits of measuring a system when your example is large, complex and the relevant bits are spread throughout the whole system. Even our engineers who know our build system inside and out often get lost in the details. Second the code belongs to Citrix and closed source software isn’t very useful to anyone except the people selling it.
So after reading through the meta-selinux and meta-xen layers a bunch and learning a good bit about writing recipes I’ve started work on a reference image for a “measured system”. I’m keeping the recipes that make up this work in a layer I call ‘meta-measured’. For this first post on the topic of measured systems I’ll stick to discussing the basic mechanics of it’s construction. This includes some data on the supporting recipes and some of the component parts necessary for booting it. Hopefully along the way I’ll be able to justify the work by discussing the potential benefits to system security but the theory and architecture discussions will be left for a later post.
get the source
If you’re interested in just building it and playing with the live image this is where you should start. Take a look and let me know what you think. Feedback would be much appreciated.
All of the work I’ve done to get this first bootable image working is up on my github. You can get there, from here: https://github.com/flihp. The ‘meta-measured’ layer is here: https://github.com/flihp/meta-measured.git. To automate setting up a build environment for this I’ve got another repo with a few scripts to checkout the necessary supporting software (bitbake / OE / meta-intel etc), a local.conf (which you may need to modify for your environment), and a script to build the ‘iso’ that can be written to a USB drive for booting a test system: https://github.com/flihp/measured-build-scripts.
The best way to build this currently is to checkout the measured-build-scripts repo:
git clone git://github.com/flihp/measured-build-scripts.git
run the ‘fetch.sh’ script to populate the required git submodules and to clone the meta-measured layer:
cd measured-build-scripts ./fetch.sh
build the iso
If you try to run the ./build.sh script next as you would think you should, the build will fail currently. It will do so while attempting to download the SINIT / ACM module for TXT / tboot because Intel hides the ACMs behind a legal terms wall with terms that must be accepted before the files can be downloaded. I’ve put the direct link to it in the recipe but the download fails unless you’ve got the right cookie in your browser so wget blows up. Download it yourself from here: http://software.intel.com/en-us/articles/intel-trusted-execution-technology, then drop the zip into your ‘download’ directory manually. I’ve got the local.conf with DL_DIR hardwired to /mnt/openembedded/downloads so you’ll likely want to change this to suit your environment.
Anyway I’ll sort out a way to fool the Intel lawyer wall eventually … I’m tempted to mirror these files since the legal notice seems to allow this but I don’t really have the bandwidth ATM. Once you’ve got this sorted, run the build.sh script. I typically tee the output to a file for debugging … this is some very ‘pre-alpha’ stuff so you should expect to debug the build a bit 🙂
./build.sh | tee build.log
This will build a few images from the measured-image-bootimg recipe (tarballs, cpios, and an iso). The local.conf I’ve got in my build directory is specific to my test hardware so if you’ve got an Intel SugarBay system to test on then you can dump the ISO directly to a USB stick and boot it. If you don’t have a SugarBay system then you’ll have to do some work to get it booting since this measured boot stuff is closely tied to the hardware, though the ACMs I’ve packaged work for 2nd and 3rd gen i5 and i7 hardware (Sandy and Ivy Bridge).
I’ve organized the recipes that make up this work into two categories: Those that are specific to the TPM and those that are specific to TXT / tboot. Each of these two technologies requires some kernel configs so those are separated out into fragments like I’ve found in other layers. My test hardware has USB 3.0 ports which the base OE layers don’t seem to have yet. I’ve included this config in my oe-measured distro just so I can use the ports on the front of my test system.
The TPM recipes automate building the Trousers daemon, libtspi and some user space tools that consume the TSS interface. Recipes for the TPM software are pretty straight forward as most are autotools projects. Some work was required to get the trousers project separated into packages for the daemon and library.
The tboot recipes were a bit more work because tboot packages a bunch of utilites in the main tboot source tree so they had to be separated out into different packages (this work is still on-going). Further tboot doesn’t use autotools and they squash most compiler flags that the OE environment passes in. The compler flags required by tboot are static which stands at odds with OE and a cross-compiled environment that wants to change the path to everything including the compiler.
I’ve no clue if tboot will build properly on anything other than an Intel system. Further the issue of Intel hiding the ACMs required for their chipssets behind an EULA wall is annoying as the default OE fetcher won’t work.
My first instinct is always to to describe a system by construction: from the bottom up. In this case I think going top-down is a better approach so we’ll start with the rootfs and work backwards. The TPM recipes includes two images based on the core-image from OE core. That’s one initramfs image and one rootfs. The rootfs is just the core-image with the TPM kernel drivers, trousers daemon, tpm-tools and the tpm-quote-tools. I haven’t done much with this rootfs other than booting it up and see if TXT and the TPM works as expected.
There’s also an initramfs with the TPM kernel drivers, trousers daemon and the tpm-tools but not the quote tools. This is a very minimal initramfs with the TSS daemon loaded manually in the initrd script. It’s not expected that users will be using the tpm-tools interactively here but that’s what I’ve been doing for initial testing. Only the
tpm_extendpcr tool (open source from Citrix) is used to extend a PCR with the sha1sum hash of the rootfs before the call to switch_root. This requires that the ‘coreutils’ package be included just for the one utility which bloats the initramfs unfortunately. Slimming this down should’t be too much work in the future. Anyway I think this is ‘the right way’ to extend the measurement chain from the initramfs up to the rootfs of the system.
The rest of the measruements we care about are taken care of by the components from the TXT recipes. There’s only one image in the TXT recipe group however. This is derived from the OE core live image and it’s intended to be ‘deployable’ in the lanugage of OE recipes. I think this means an hddimg or an ISO image, basically something you can ‘dd’ to disk and boot. Currently it’s the basis for a live image but could easily be used for something like an installer simply by switching out the rootfs.
This image is not a separate root filesystem but instead it’s an image created with the files necessary to boot the system: syslinux (configured with the mboot.c32 comboot module), tboot, acms and the initrd and the rootfs from the TPM recipes. tboot measures the bootloader config, all of the boot modules and a bunch of other stuff (see the README in the tboot sources for details). It stores these measurements in the TPM for us, creating the ‘dynamic root of trust for measurement’ (DRTM).
Once tboot has measured all of the modules, the initramfs takes over. The initramfs then measures the rootfs as described above before the switch to root. I’ve added a few kernel parameters to pass the name of the rootfs and the PCR where it’s measurement is to be stored.
If the rootfs is measured on each boot it must be mounted read-only to prevent its measurement from changing … yup even mounting a journaled file system read-write will modify the journal and change the filesystem. Creating a read-only image is a bit of work so for this first prototype I’ve used a bit of a short cut: I’ve mounted the rootfs read only, create a ramfs read write, then the two are combined in a unionfs. In this configuration when rootfs boots it looks like a read / write mount. Thus on each boot the measurements in the TPM are the same.
Measuring a system is all well and good but who cares? Measurements are only useful when they’re communicated to external parties. For now this image only takes measurements and these measurements are the same on each boot. That’s it. Where this can be most immediately useful is that these measurements can be predicted in the build.
The PCRs 0-7 are reserved for the BIOs and we have no way of predicting these values currently as they’re unique to the platform and that’s messy. The tboot PCRs however (17, 18 and 19 in the Legacy mapping currently used) can be calculated based on the hashing done by tboot (read their docs and http://firstname.lastname@example.org/msg00069.html). The PCR value containing the measurement of the rootfs can be calculated quite simply as well.
For a reference live image this is interesting only in an academic capacity. As I suggest above, this image can be used as a template for something like an installer which would give the predictability of PCR values much deeper meaning: Consider an installer architecture where the installer itself is a very small rootfs that downloads the install package from a remote server (basically Debian’s netboot iso or a PXE boot setup). Assuming we have a method for exchanging system measurements (more future work) it would be very useful for the remote server to be able to evaluate measurements from the installer before releasing the install package.
This is probably a good place to wrap up this post. The meta-measured layer I’ve described is still very new and the images I’ve built are still usefuly only for ‘tire-kicking’. My next post will hopefully discuss predicting measurement values in the build system and other fun stuffs.