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Standard Windows bootloader already does that, it's part of supported deployment flow in highest security setting - in fact part of why MS requires that computers with certain Windows-related branding (the part about stickers "designed for windows" etc., I just don't remember which one) have to enable end users to replace platform keys.

Except the same keys have been used to sign some EFI loaders from other vendors, namely Kaspersky Rescue Disk 18, so it isn't really as locked down as they would have us believe.


There are two keys distributed by MS - one for Windows, one for third party applications. The latter one is used by Linux shim bootloader and the like. MS also requires that all devices that are supposed to be "full featured" support replacing all keys and loading user keys, and the highest security deployment setup for Windows Enterprise requires that you sign windows bootloader (or, more likely, sign the hash of the specific binary you are using) yourself.

> the highest security deployment setup for Windows Enterprise requires that you sign windows bootloader (or, more likely, sign the hash of the specific binary you are using) yourself.

To my knowledge that's not possible with the Microsoft bootloader, you need to use Microsoft's keys, hence why I am suggesting that this open source bootloader could be useful. Can you provide some more information about such a setup?

This document is aimed at OEMs, but they specifically say that high-security environments can generate Secure Boot keys and use them on their devices in conjunction with their own Windows image. (I’d imagine that you need the Windows SDK and some know how/MS consulting to get it working, though)


This document is about generating the platform key, but changing the platform key alone doesn't allow you to do what I'm suggesting. Even after setting your own platform key you would still need to trust the key for Microsoft's bootloader. See section 1.4.1:

> The Microsoft Windows Production PCA 2011 with a SHA-1 Cert Hash of 58 0a 6f 4c c4 e4 b6 69 b9 eb dc 1b 2b 3e 08 7b 80 d0 67 8d must be included in db in order to allow the Windows OS Loader to load.

For OEM to allow reinstall done "normally", yes.

But DB entries can also contain SHA-256 hashes of the image to load (with the image stripped of the signature, which btw allows you to also resign it).



> which btw allows you to also resign it

Are you sure? I havent tried, but this disagrees: https://docs.microsoft.com/en-us/previous-versions/windows/i...

"Windows boot components: BootMgr, WinLoad, Windows Kernel Startup. Windows boot components verify the signature on each component. Any non-trusted components will not be loaded and instead will trigger Secure Boot remediation. "

> DB entries can also contain SHA-256 hashes

OK, fair enough, but it still doesn't really solve the problem because an attacker could just copy your modified bootmgr to be able to steal and use your workstation. In order for this to work you'd also have to add some kind of additional checks which we can now do with an open source version of the bootloader.

The signatures are, IIRC, checked in order, so a resigned BootMgr won't impact verification of WinLoad, etc.

As for abusing signed bootloader, the full process depends on verifying that the signed bootloader appropriately handles TPM, which is then also coupled with an encrypted disk, which in turn works to prevent loading unsanctioned code.

Of course, it's possible to defeat this, but the idea is to frustrate the efforts and raise the cost of an attack, as well as give you a longer timeframe to deal with an attack.

Those mechanisms only prevent an attacker from decrypting the disk. The scenario I'm envisioning is where the attacker steals your workstation, wipes the disk and reinstalls Windows.

That's not part of the threat model - the encrypted data is lost, the point of the defences are to prevent damage to things accessible from that specific machine, not to prevent stealing the machine.

That's not part of Microsoft's threat model. Which is exactly the problem! It could be done, if only Microsoft supported that use case. But with an open source bootloader, we could make the necessary changes ourselves.

No, that's the threat model that most clients that require high security have (the machine is often nigh-infinitely cheaper than the data and access rights), and MS, surprise, explicitly insists (in difference to secure boot spec) that owner of the physical machine is, well, it's owner and can reinstall, resell, etc.

I don't see how any of this is relevant. All I said was that it's an interesting possibility which has now been made feasible. Who cares about the threat model that most high security clients have? If they don't want it then I'm not talking about those clients.

Besides: the prevalence of terrible software like Lojack/CompuTrace in the enterprise just goes to show that many clients actually do care about physical theft scenarios. Also consider how basically every modern mobile device now provides factory reset protection.

The basic procedure is as follows:

- Remove all keys (switch to Setup mode) - Setup your own PKI and platform keys - Sign hash of specific EFI files and load those signatures into EFI

The last part doesn't require modifying files themselves, as you're locking specific files. The firmware will make a hash of the file, and verify that it's on permitted list (the list is signed)

If I'm not mistaken it's actually a hash of the file's signature that you have to register, not a hash of the file itself. And Bootmgr will not allow you to change its signature. Thus any computer that is configured to boot Windows with secure boot will be able to boot any copy of Windows.

Not if you replace the keys with your own.

Sure, and I do that, but most users will not, they will select Secure Boot and think that their boot is now secure.

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