10 Aug 2012 mjg59   » (Master)

Thoughts on the SUSE Secure Boot implementation

There's a post here describing SUSE's approach to implementing Secure Boot support. In summary, it's pretty similar to the approach we're taking in Fedora - a first stage shim loader is signed with a key in db, it loads a second stage bootloader (grub 2) that's signed with a key that's in shim, the second stage bootloader loads a signed kernel. The main difference between the approaches is the use of a separate key database in shim, whereas we are currently planning on using a built-in key and the contents of the firmware key database.

The main concern about using a separate key database is that applications are able to modify variable content at runtime. The Secure Boot databases are protected by requiring that any updates be signed, but this is less practical for scenarios where you want the user to be able to modify the database themselves while still protecting them from untrusted applications installing their own keys. SUSE have solved this problem by not setting the runtime access flag on the variable, meaning that it's inaccessible once ExitBootServices() has been called early in the init sequence. Since we only start running any untrusted code after ExitBootServices(), the variable can only be accessed by trusted code.

It's a wonderfully elegant solution. We've been planning on supporting user keys by trusting the contents of db, and the Windows 8 requirements specify that it must be possible for a physically present user to add keys to it. The problem there has been that different vendors offer different UI for this, in some cases even requiring that the keys be in different formats. Using an entirely separate database and offering support for enrolment in the early boot phase means that the UI and formats can be kept consistent, which makes it much easier for users to manage their own keys.

I suspect that we'll adopt this approach in Fedora as well - it doesn't allow anything that our solution wouldn't have, but it does make some of them easier. Full marks to SUSE on this.

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Syndicated 2012-08-10 05:02:25 from Matthew Garrett

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