In measured boot, each component of the boot process is "measured" (ie, hashed and that hash recorded) in a register in the Trusted Platform Module (TPM) build into the system. The TPM has several different registers (Platform Configuration Registers, or PCRs) which are typically used for different purposes - for instance, PCR0 contains measurements of various system firmware components, PCR2 contains any option ROMs, PCR4 contains information about the partition table and the bootloader. The allocation of these is defined by the
PC Client working group of the Trusted Computing Group. However, once the boot loader takes over, we're outside the spec[1].
One important thing to note here is that the TPM doesn't actually have any ability to directly interfere with the boot process. If you try to boot modified code on a system, the TPM will contain different measurements but boot will still succeed. What the TPM
can do is refuse to hand over secrets unless the measurements are correct. This allows for configurations where your disk encryption key can be stored in the TPM and then handed over automatically if the measurements are unaltered. If anybody interferes with your boot process then the measurements will be different, the TPM will refuse to hand over the key, your disk will remain encrypted and whoever's trying to compromise your machine will be sad.
The problem here is that a
lot of things can affect the measurements. Upgrading your bootloader or kernel will do so. At that point if you reboot your disk fails to unlock and you become unhappy. To get around this your update system needs to notice that a new component is about to be installed, generate the new expected hashes and re-seal the secret to the TPM using the new hashes. If there are several different points in the update where this can happen, this can quite easily go wrong. And if it goes wrong, you're back to being unhappy.
Is there a way to improve this? Surprisingly, the answer is "yes" and the people to thank are Microsoft.
Appendix A of a basically entirely unrelated spec defines a mechanism for storing the UEFI Secure Boot policy and used keys in PCR 7 of the TPM. The idea here is that you trust your OS vendor (since otherwise they could just backdoor your system anyway), so anything signed by your OS vendor is acceptable. If someone tries to boot something signed by a different vendor then PCR 7 will be different. If someone disables secure boot, PCR 7 will be different. If you upgrade your bootloader or kernel, PCR 7 will be the same. This simplifies things significantly.
I've put together a (not well-tested) patchset for
Shim that adds support for including Shim's measurements in PCR 7. In conjunction with appropriate firmware, it should then be straightforward to seal secrets to PCR 7 and not worry about things breaking over system updates. This makes tying things like disk encryption keys to the TPM much more reasonable.
However, there's still one pretty major problem, which is that the initramfs (ie, the component responsible for setting up the disk encryption in the first place) isn't signed and isn't included in PCR 7[2]. An attacker can simply modify it to stash any TPM-backed secrets or mount the encrypted filesystem and then drop to a root prompt. This, uh, reduces the utility of the entire exercise.
The simplest solution to this that I've come up with depends on how Linux implements initramfs files. In its simplest form, an initramfs is just a
cpio archive. In its slightly more complicated form, it's a compressed cpio archive. And in its peak form of evolution, it's a series of compressed cpio archives concatenated together. As the kernel reads each one in turn, it extracts it over the previous ones. That means that any files in the final archive will overwrite files of the same name in previous archives.
My proposal is to generate a small initramfs whose sole job is to get secrets from the TPM and stash them in the kernel keyring, and then measure an additional value into PCR 7 in order to ensure that the secrets can't be obtained again. Later disk encryption setup will then be able to set up dm-crypt using the secret already stored within the kernel. This small initramfs will be built into the signed kernel image, and the bootloader will be responsible for appending it to the end of any user-provided initramfs. This means that the TPM will only grant access to the secrets while trustworthy code is running - once the secret is in the kernel it will only be available for in-kernel use, and once PCR 7 has been modified the TPM won't give it to anyone else. A similar approach for some kernel command-line arguments (the kernel, module-init-tools and systemd all interpret the kernel command line left-to-right, with later arguments overriding earlier ones) would make it possible to ensure that certain kernel configuration options (such as the iommu) weren't overridable by an attacker.
There's obviously a few things that have to be done here (standardise how to embed such an initramfs in the kernel image, ensure that luks knows how to use the kernel keyring, teach all relevant bootloaders how to handle these images), but overall this should make it practical to use PCR 7 as a mechanism for supporting TPM-backed disk encryption secrets on Linux without introducing a hug support burden in the process.
[1] The
patchset I've posted to add measured boot support to
Grub use PCRs 8 and 9 to measure various components during the boot process, but other bootloaders may have different policies.
[2] This is because most Linux systems generate the initramfs locally rather than shipping it pre-built. It may also get rebuilt on various userspace updates, even if the kernel hasn't changed. Including it in PCR 7 would entirely break the fragility guarantees and defeat the point of all of this.
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Like all good blog posts, this one starts with an apology about not blogging for ages in my case it looks like it s been about 7 years which is definitely a new personal best (perhaps the equally or more remarkable thing is that I have diligently kept WordPress running in the meantime). In that time, as you might expect, a few things have happened, like I met a wonderful woman and fell in love and we have two wonderful children. I also decided to part ways with my first baby and leave my role as CTO & co-founder of Collabora. This was obviously a very tough decision it s a fantastic team where I met and made many life-long friends, and they are still going strong and doing awesome things with Open Source. However, shortly after that, in February last year, I was lucky enough to land what is basically a dream job working at Endless Computers as the VP of Deployment.
As I m sure most readers know, Endless is building an OS to bring personal computing to millions of new users across the world. It s based on Free Software like GNOME, Debian, ostree and Flatpak, and the more successful Endless is, the more people who get access to education, technology and opportunity and the more FOSS users and developers there will be in the world. But in my role, I get to help define the product, understand our users and how technology might help them, take Open Source out to new people, solve commercial problems to get Endless OS out into the world, manage a fantastic team and work with bright people, learn from great managers and mentors, and still find time to squash bugs, review and write more code than I used to. Like any startup, we have a lot to do and not enough time to do it, so although there aren t quite enough days in the week, I m really happy!
In any case, the main point of this blog post is that I m at GUADEC in Manchester right now, and I d like to blog about Flathub, but I thought it would be weird to just show up and say that after 7 years of silence without saying hello again. 
In the past there used to be a puppet-labs project called 
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Some month ago I upgrade my (European) fenix3 to a (Japanese) 
.. because you will never wake up the next day, since the alarm was deleted due to an unattended (so-called) sync operation. Happened to me, several times, worst was when I was with clients working as guide.
So the problem is well known, see 
This post attempts to chart my journey towards getting usefully started with Ansible to manage my system configurations. It s a high level discussion of how I went about doing so and what I got out of it, rather than including any actual config snippets - there are plenty of great resources out there that handle the actual practicalities of getting started much better than I could.
I ve been convinced about the merits of configuration management for machines for a while now; I remember conversations about producing an appropriate set of recipes to reproduce our haphazard development environment reliably over 4 years ago. That never really got dealt with before I left, and as managing systems hasn t been part of my day job since then I never got around to doing more than working my way through the 
In a few days, I ll be attending 
Having publicly running web application always brings challenges in terms of security and in generally in handling untrusted data. Security wise 
This is about he same you pay for Netflix, or some other streaming service. If you remain on the free side, what remains for you to do is SNS-like stuff, and uploading your papers so that academia.edu can make money from it.
What I am really surprised that they can pull this of at a .edu domain. The registry requirements state
I recently raised a 
In a case of blatant self-promotion, I am going to add the