[personal profile] mjg59
(Edit: It's been suggested that the title of this could give the wrong impression. "Don't like Secure Boot? That's not a reason to buy a Chromebook" may have been better)

People are, unsurprisingly, upset that Microsoft have imposed UEFI Secure Boot on the x86 market. A situation in which one company gets to determine which software will boot on systems by default is obviously open to abuse. What's more surprising is that many of the people who are upset about this are completely fine with encouraging people to buy Chromebooks.

Out of the box, Chromebooks are even more locked down than Windows 8 machines. The Chromebook firmware validates the kernel, and the kernel verifies the filesystem. Want to run a version of Chrome you've built yourself? Denied. Thankfully, Google have provided a way around this - you can (depending on the machine) either flip a physical switch or perform a special keystroke in the firmware to disable the validation. Doing so deletes all your data in the process, in order to avoid the situation where a physically present attacker wants to steal your data or backdoor your system unnoticed, but after that it'll boot any OS you want. The downside is that you've lost the security that you previously had. If a remote attacker manages to replace your kernel with a backdoored one, the firmware will boot it anyway. Want the same level of security as the stock firmware? You can't. There's no way for you to install your own signing keys, and Google won't sign third party binaries. Chromebooks are either secure and running Google's software, or insecure and running your software.

Much like Chromebooks, Windows 8 certified systems are required to permit the user to disable Secure Boot. In contrast to Chromebooks, Windows 8 certified systems are required to permit the user to install their own keys. And, unlike Google, Microsoft will sign alternative operating systems. Windows 8 certified systems provide greater user freedom than Chromebooks.

Some people don't like Secure Boot because they don't trust Microsoft. If you trust Google more, then a Chromebook is a reasonable choice. But some people don't like Secure Boot because they see it as an attack on user freedom, and those people should be willing to criticise Google's stance. Unlike Microsoft, Chromebooks force the user to choose between security and freedom. Nobody should be forced to make that choice.

(Updated to add that some Chromebooks have a software interface for disabling validation)

Re: Lot of chromebooks could be secured.

Date: 2013-02-05 07:45 am (UTC)
From: (Anonymous)
"Unfortunately not, it's more of a suggestion. The switch doesn't actually do anything to prevent writes to the card."

In fact it is more than a simple suggestion. On good quality SD it disconnects the flash memory write pins. Yes cheaper just do suggestion. This suggestion is picked up by the controller as well that also blocks writing.

I know this from the Raspberry PI. The write protect wire is not connected on the Raspberry PI. So the cards that the write protect is a simple suggestion it will write. I have some SD cards the Raspberry PI cannot write to when the SD switch is set to write protect. Yes some cards have physical lock in card.

So yes a lot of Chromebooks SD can be set read only by the suggestion to controller that a software attack cannot override and will prevent all writes. Because the controller obeys the suggestion.

Final method to secure a SD card against writing is order a batch of custom Read only cards these don't contain flash at all. Something larger groups can do.

There is a reason why I said a Lot not all. There are some hardware varation that require some careful handling like the right brands SD cards with working write protect switches. Other chromebooks a SD card with switch set is enough because controller is picking up the load. Those will not notice if you use a better quality SD with real write switch.


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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