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[personal profile] emperor
Fans of the coffee stall on the Cambridge market (link to my previous post on opening hours) may be interested to know that he doesn't seem to be open on Thursdays any more - AFAICT he's now Mon-Wed, Fri, Sat.

Not entirely co-incidentally, my coffee supplies are now rather low :(

Catch up

Jul. 18th, 2017 01:42 pm
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[personal profile] lnr
Done since Jun 14th:
  • Test rode an Onderwater tandem, which has the child stoker seat at front - Matthew loved it
  • Second parents evening for Matthew's school, nice to see teachers again and get more idea of school plans
  • Rainbow Sponsored Trike Ride - I ended up riding Matthew's bike as a balance bike since he didn't want to join in
  • Blood tests: my calcium, parathyroid hormone and vitamin D levels are all normal, but keep taking the vitamin D for now
  • We did HBA1C as well, which is average blood sugar levels, also fine - I'm at slight risk due to Type 2 diabetes in family and current weight)
  • Picnic lunch and playdate with Kirsten/Andre/Judith/Colin and Lammas Land - lots of fun
  • Shelford Feast - Matthew enjoyed all the stalls and mini steam train and bouncy castles, I helped out on the Rainbow stall
  • Eye Test for Matthew this morning: doing great, patching is helping his eyes work well together, ordered new lenses for his current glasses, next appointment in October half term
  • Work appointed one interim head, who only stayed 2 days, and are now appointing again
  • The "implementation" phase of Organisational Change is officially complete and we all now in theory have new jobs - but almost no management so not much actual change at the moment
  • Total resignations now at 4 (Patrick, James, Stephen, Andrew) with possibility of more to come

Plus assorted bike rides, visits to the park, dyeing hair purple again and so on - and lots of lego :)

Coming up in the near future:
  • Collect Matthew's school uniform (I see the school's admin at Pre-School and she's kindly said she'll bring it along for me)
  • Early start tomorrow for Rainbow Leavers Trip to Wandlebury
  • Rainbow end of term staff party tomorrow evening: as part of the committee I'm involved in helping host it
  • Rainbow leaving party on Friday morning - last day of pre-school!
  • A week in the lake district starting on Saturday
  • Test riding a Circe Helios tandem when we get back
  • Folk Festival on Sunday 30th - possibly with Matthew, possibly without
  • New Interim Head of IT Group starts (phased in) on 1st August (Hi Julian)
  • A week in Devon with family from 4th August - staying at Wortham Manor

In between the two weeks away Matthew will have a week at Hania's - and then when we get back he's got three weeks of holiday club before granny and grandad come to visit the first week in September, and then school starts on the 11th.

I think I know why I'm exhausted :)


[personal profile] mjg59
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.

Much music

Jul. 9th, 2017 07:58 pm
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[personal profile] emperor
It's getting to the end of the choir year, when some of the older children will be leaving us to go to university. Tradition is that they get to suggest music to do for the last few services of term. So while evensong did rather over-run this evening, we did have some lovely music - A 12 part magnificat by Victoria, Holst's 8-part setting of the Nunc Dimittis, and the anthem was also by Holst, Psalm 148 [links are to youtube recordings of other people singing these pieces].

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jorallan

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