Jan. 1st, 2016

I gave a presentation at 32C3 this week. One of the things I said was "If any of you are doing seriously confidential work on Apple laptops, stop. For the love of god, please stop." I didn't really have time to go into the details of that at the time, but right now I'm sitting on a plane with a ridiculous sinus headache and the pseudoephedrine hasn't kicked in yet so here we go.

The basic premise of my presentation was that it's very difficult to determine whether your system is in a trustworthy state before you start typing your secrets (such as your disk decryption passphrase) into it. If it's easy for an attacker to modify your system such that it's not trustworthy at the point where you type in a password, it's easy for an attacker to obtain your password. So, if you actually care about your disk encryption being resistant to anybody who can get temporary physical possession of your laptop, you care about it being difficult for someone to compromise your early boot process without you noticing.

There's two approaches to this. The first is UEFI Secure Boot. If you cryptographically verify each component of the boot process, it's not possible for a user to compromise the boot process. The second is a measured boot. If you measure each component of the boot process into the TPM, and if you use these measurements to control access to a secret that allows the laptop to prove that it's trustworthy (such as Joanna Rutkowska's Anti Evil Maid or my variant on the theme), an attacker can compromise the boot process but you'll know that they've done so before you start typing.

So, how do current operating systems stack up here?

Windows: Supports UEFI Secure Boot in a meaningful way. Supports measured boot, but provides no mechanism for the system to attest that it hasn't been compromised. Good, but not perfect.

Linux: Supports UEFI Secure Boot[1], but doesn't verify signatures on the initrd[2]. This means that attacks such as Evil Abigail are still possible. Measured boot isn't in a good state, but it's possible to incorporate with a bunch of manual work. Vulnerable out of the box, but can be configured to be better than Windows.

Apple: Ha. Snare talked about attacking the Apple boot process in 2012 - basically everything he described then is still possible. Apple recently hired the people behind Legbacore, so there's hope - but right now all shipping Apple hardware has no firmware support for UEFI Secure Boot and no TPM. This makes it impossible to provide any kind of boot attestation, and there's no real way you can verify that your system hasn't been compromised.

Now, to be fair, there's attacks that even Windows and properly configured Linux will still be vulnerable to. Firmware defects that permit modification of System Management Mode code can still be used to circumvent these protections, and the Management Engine is in a position to just do whatever it wants and fuck all of you. But that's really not an excuse to just ignore everything else. Improving the current state of boot security makes it more difficult for adversaries to compromise a system, and if we ever do get to the point of systems which aren't running any hidden proprietary code we'll still need this functionality. It's worth doing, and it's worth doing now.

[1] Well, except Ubuntu's signed bootloader will happily boot unsigned kernels which kind of defeats the entire point of the exercise
[2] Initrds are built on the local machine, so we can't just ship signed images

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Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

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