Oct. 11th, 2012

I've been continuing to work on Shim this week, and it's now getting pretty close to feature complete. The biggest change has been migrating the MokList variable from a custom format to the one used by the UEFI spec databases, which means it's easier for the kernel to import the MOK entries and use them for validating module signatures.

The other benefit of this format is that it supports hashes as well as certificates, and so I've added support for enrolling hashes through the MokManager UI. This means that distributions can now ship with a signed copy of Shim without having to sign any other components of their distribution. When the user attempts to boot off the media they'll be faced with a menu and a countdown. If the countdown reaches 0, the system will simply fall back to the next entry in the bootlist. If they hit a key, they can choose to enrol a hash. The user then navigates the filesystem explorer, chooses the bootloader, confirms that they want to enrol it and then exits. Shim then verifies the bootloader against the hash and boots successfully.

The big advantage of this over the Linux Foundation approach is that once a hash has been enrolled the need for physical end-user presence is removed - ie, if you enrol the hash, you don't need to hit a key every time you boot. This is still slightly sub-optimal in that if you update your bootloader you'll need to enrol a new hash, but that can be partially automated by calling MokUtil in the postinst - the user then simply needs to confirm that they want to enrol the hash, rather than having to choose it manually. Completely transparent updates are going to require a signed bootloader and an enrolled signing key.

A couple of people have asked whether we're planning on implementing the Linux Foundation approach of simply asking the user whether they want to boot an unsigned file. We've considered it, but at the moment are leaning towards "no" - it's simply too easy to use to trick naive users into running untrusted code. Users are trained to click through pretty much any security prompt that they see, and if an attacker replaces a legitimate bootloader with one that asks them to press "y" to make their computer work, they'll press "y". If that bootloader then launches a trojaned Windows bootloader that launches a trojaned Windows kernel, that's kind of a problem. This could be somewhat mitigated by limiting this feature to removable media, and we're seriously considering that, but there are still some risks associated. We might just end up writing the code but disabling it at build time, and then anyone who wants to distribute with that policy can do so at their own risk.

Meanwhile, Peter Jones is working on tidying up the code we're using for the actual signing and will be publishing that once the last couple of kinks are worked out. We're using hardware cryptography, so even if someone compromises the build systems they won't be able to obtain the private key that we use. However, should something disastrous and unanticipated happen, we do have a plan in place for migrating to new keys with minimal user impact. We'll document the code and infrastructure we're using in order to make it as easy as possible for other distributions to implement equivalent functionality.

As I've mentioned before, our goal is to make it as easy as possible for distributions to implement whatever level of Secure Boot policy they want without having to engage with Microsoft themselves. Shim allows distributions to ship an OS that has no signed binaries at all, or alternatively to ship an OS that uses filesystem-level cryptography to ensure that even userspace is completely signed. I'm expecting to see a range of options available, and I hope that the majority of users will find something to suit their needs.


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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