vatine: books-related stuff (books)
[personal profile] vatine

Second of the Spatterjay (sub-)series in Asher's Polity universe. Takes place about ten years after the end of the previous book. We do, again, follow several different viewpoint characters, on all sorts of moral sides of any situation that may happen in the book. Some are returning characters, some are new.

I'd definitely recommend starting with the first book in the series, but all in all an eminently readable book.
[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.
vatine: books-related stuff (books)
[personal profile] vatine
Previously unread.

This is the, what, ninth? eighth? book in Stross's The Laundry Files and the wheels on the hand-basket are truly on their way out, along a radial trajectory.

This book sees the return of many faces from previous books, as we slowly see things unwind around Bob. I am trying real hard to not let anything slip here, you see, as I feel that approaching the book spoiler-free is the most, ah, enjoyable? way of reading it. Surprising at least.

Anyway, Laundry Files, if you've read some of them before, you know what to expect. If you haven't, might I humbly suggest that this is perhaps not the best starting point (although it may well work as an intro novel). We do a fair bit of POV shifting in this book, even if it's primarily a "Bob" book (we also follow Mo, Mhari and Cassie, as well as the occasional follow-the-baddies).

All in all, a gripping read. I shall blame technology (and not being completely done with the previous book in time for the release) for taking this abysmally long to finish off something that was released a whole 4 days ago.

LJ account compromised?, deleted

Jul. 15th, 2017 06:03 pm
damerell: NetHack. (normal)
[personal profile] damerell
Today, I got an email from LJ to the effect that my LJ account had been logged into from, which is J. Random IP Address in France. Mysteriously, although this was some hours ago, I don't seem to have embarked on a spree of Viagra posts/comments or anything. Hence I've ended the unknown login session, changed password, deleted account (weirdly, all of which I could do without agreeing to the evil new T&Cs).

I imagine this is a manifestation of the downfall of LJ, but:
worth checking yourself ( ) if you ain't already deleted your account?
let me know, please, if I suddenly go spammy anywhere else...

2017 - #72, "The Skinner", Neal Asher

Jul. 15th, 2017 01:55 pm
vatine: books-related stuff (books)
[personal profile] vatine

This is the first book in the Sptterjay series, set in Asher's Polity world.

Time-wie, the Spatterjay books fall well after the rest of the series (bar, possibly, Transformation), but as the first two books takes place entirely (or almost entirely) on the planet of Spatterjay (see how the planet meshes with the name of the series...), it's not massively important exactly how it lines up timewise.

We follow a couple of different viewpoint characters. Ehrlin is a Hooper (that is, someone who's been infected by the leech virus, present in most (if not all) lifeforms on Spatterjay), who's been away from Spatterjay for a while, having adventures. Janer is employed by a sentient hornet hive, that he (some decades ago) spent two years indentured to, for killing one of its bodies at a football match. Sable Keech is a reif (basically a cyber-enhanced walking corpse), and ECS monitor. Sniper, a war drone. And Windcatcher, which I shall say nothing about. And a few more, who get walk-on POV roles.

Fundamentally, this is a story about loss and revenge. And how these things change, as time passes. I guess there's some talk about life and what immortality may mean for the human condition.

Again, this is a Polity book so it's kinda grimdark, in places.
vatine: books-related stuff (books)
[personal profile] vatine

First book in Jansen's Ambassador series. I must've confused this with another "interstellar ambassador" book, since my distinct memory was "ugh, do not like". This, as it turns out, is wrong.

Where was I? Ah, yes, Cory Wilson, our intrepid viewpoint character, is just about to start his job as ambassador for Earth to the gamra assembly, part of the interstellar crowd that run The Exchange (basically, what enables interstellar travel). And as part of the upcoming travel, he's talking to the president of the UN (well, it's called something else, and it seems to be an actual ruling body, and, you know...) when the unthinkable happens. And the unthinkable is taht there's a direct attack on said president, while our POV character is in the office.

And from there, things start unravelling at a frightening pace.

All in all, eminently readable. Now I'm vaguely interested in chasing down my previous post about this one.



July 2017


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