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enable Linux kernel gpg verification in grub and/or enable Secure Boot by default

grub2 feature request - DRAFT

To be posted against grub2 upstream as well as against Debian.

Please comment on / rewrite / improve this draft.

grub-pc check_signatures=enforce support (BIOS) (non-EFI)

Could you please make it possible to do signature verification with grub-pc too?

Rationale:

We, the maintainers of Linux distributions that primarily run inside VMs (Whonix; Kicksecure) would like to implement verified boot. Not necessarily Secure Boot.

At the moment, there are no tools that can create VM images (with Debian Linux) which support EFI booting. Also, support by virtualizers such as KVM, Xen, VirtualBox for Secure Boot is either non-existing or undocumented.

Another reason is, that inside VMs we don’t necessarily need the complexity of EFI.

Instead we could boot unverified (usual virtual BIOS legacy boot) from a virtual, read-only (write protected) boot medium (such as ISO). That boot loader on the initial boot disk (grub2) could then verify and chainload the boot loader (grub2) on the main disk. In result, we would have a verified boot sequence.

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Debian feature request: grub-PC check_signatures=enforce support (non-EFI)

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debian-kernel mailing list: Guaranteeing initramfs integrity during Secure Boot

[1] Note that this doesn’t do much against an adversary with a kernel 0day.
It’s not meant to.

This should be effective against an adversary that gains physical access to a
device, yet cannot tamper with the live system (by plugging in a device that
exploits a buggy driver, by messing with the memory bus or a DMA-capable
interface, …) and cannot replace the firmware.

As you can see, this does not outright prevent evil-maid style attacks:
the goal here is to make such attacks harder/less practical.

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Not much Ubuntu specificity and general principles might be learned from this software:

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The security of Secure Boot

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An interesting comment which I don’t agree fully with but it raises and interesting point about initrd making this a kinda pointless exercise:

Lets say it would help to secure a system with enabled encryption. This might help when there is no way to get a custom signed binary. Then maybe bitkeeper would be a tiny bit more secure. I doubt that for Linux solutions as the logic is in the initrd. You could still modify that even if you could not load a modified kernel module (i still want to see that working). It is very unlikely that you can sign your initrd or you have to store the public key for that unencrypted somewhere. So what did you gain this time? Maybe a tiny bit in the case that you could not sign your own binaries. If you can do and use an initrd, that will be the weak point (and it was the weak point before as well).

So what can you do? Rely on hardware encryption if you need full security. Forget secure boot, it will not be more secure. All you can use it for is that you can not boot other systems that easyly (just like on the arm plattform).

Date: 2012-07-25 02:50 pm (UTC) From: [personal profile] mjg59

You’re completely right, secure boot does not prevent attacks that it is not intended to prevent.

grub bug #56887 grub-PC check_signatures=enforce support (non-EFI).

Hash Check all Files at Boot

Higher security level as Secure Boot.

Talking about VMs only in this concept.

We could boot from a virtual, read-only (write protected) boot medium such as another virtual HDD or ISO. Such a boot medium which runs a minimal linux distribution which then compares against checksums from Debian repository on the main boot drive:

  • The MBR (master boot record)
  • The VBR (volume boot record)
  • [A] the booloader
  • [B] the partition table
  • [C] the kernel
  • [D] the initrd
  • [E] all files shipped by all packages

There are tools that can help with checking all files on the hard drive such as debsums . However, while debsums is more popular, it is unsuitable. [2]

A tool such as debcheckroot might be more suitable for this task.

During development of Verifiable Builds experiences were made with verification of MBR, VBR, bootloader, partition table, kernel and initrd. Source code was created to analyze such files. [3]

Extraneous files would be reported, with option to delete them, to move them to quarantaine and/or to view them.

Initrd is by Debian default, auto generated on the local system. Hence, there is nothing to compare with from Debian repository. However, after verification of everything (all files from all packages) it would be secure to chroot into the verified system and to re-generate the initrd. Then to compare both versions. This might not be required if initrd can be extracted and compared against files on the root disk.

That boot medium (such as IOS) could be shipped on Whonix Host through a deb package /usr/share/verified-boot/check.iso .

Disadvantage of this concept might be that it might be slower than dm-verity. On the other hand the advantage of this concept is that this does not require a OEM image. Also it might be more secure since it does not verify against an OEM image but would verify the individual files. Another advantage is that users are free to install any package and not limited by a readonly root image. Users do not have to wait for the vendor to update the OEM image.

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Absolutely brilliant. I dont think we should judge performace just yet without having tried it.

How about splitting the process so that the most lowlevel essiential components are checked before boot and the rest can be done later after the important components are given the green light - during system run?

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It’s not entirely based on theory.

Using debsums (which that actual implementation should not use as explained in the concept) (also since it uses md5sums) - which is good enough for a quick performance test…

time sudo debsums -s

real 1m37.632s
user 0m27.825s
sys 0m11.242s

To add to that time:

  • time to boot into the verification system
  • some other stuff (initrd, bootloader, …) but maybe these are negligible
  • time to boot into the actual system

Any code executed could fake the results of verification. Seems hard to make the boot only really execute any files which are already verified. A lot more complex.

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Some interesting projects that may help:

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A lot to research.

https://docs.puri.sm/PureBoot.html

https://docs.puri.sm/PureBoot/Heads.html

https://trmm.net/Heads_33c3

https://trmm.net/Heads_FAQ

https://trmm.net/Heads_threat_model

https://trmm.net/Bootguard

https://www.youtube.com/watch?v=2kNnTsgujIA

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“PureBoot” is mainly just coreboot and heads along with a few other things.

Coreboot and Heads are BIOS replacements and probably not able to be used in VMs.

I wouldn’t really use Purism as a good source either.

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Provided feedback about debcheckroot.

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Possible to use check_signatures=enforce on non-EFI such as amd64?
https://lists.gnu.org/archive/html/help-grub/2019-12/msg00006.html

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