[personal profile] mjg59
Yesterday I wrote about an alleged attack on the Windows 8 secure boot implementation. As I later clarified, it turns out that the story was, to put it charitably, entirely wrong. The attack is a boot kit targeted towards BIOS-based boots. It lives in the MBR. It'll never be executed on any UEFI systems, let alone secure boot ones. In fact, this is precisely the kind of attack that secure boot is intended to protect against. So, context.

The MBR contains code that's executed by the BIOS at boot time. This code is unverifiable - it's permitted to have arbitrary functionality. There's only 440 bytes, but that's enough to jump to somewhere else and read code from elsewhere. There's no way for the BIOS to know that this code is malicious. And one thing this code can obviously do is load the normal boot code and modify it to behave differently. Any self-validation code in the loader can be patched out at this point. The modified loader will then load the kernel, and potentially also modify it. At this point, you've lost. Any attempts to validate the code can be redirected to the original code and so everything will look fine, up until the point where the user runs a specific application and suddenly your kernel is sending all your keystrokes over UDP to someone in Nigeria.

These attacks exist now. They're in the wild. In a normal UEFI world you'd do the same thing by just replacing the UEFI bootloader. But with secure boot you'll be able to validate that the bootloader is appropriately signed and if someone's modified it you'll drop into some remediation mode that recovers your files, from install media if necessary.

Obviously, this protection is based on all the components of secure boot (ie, everything that runs before ExitBootServices() is called) being perfect. As I said, if any of them accept untrusted input and misinterpret it in such a way that they can be tricked into running arbitrary code, you'll still have problems. But when discussing the pros and cons of secure boot, it's important to make sure that we're talking about reality rather than making provably false assertions.


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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