mirror of
https://github.com/linuxboot/heads.git
synced 2024-12-18 12:46:26 +00:00
f6232aa70f
TODO: - $(pcrs) call sometimes fail in DEBUG call, outputting too many chars to be inserted in kmesg. Call removed here since redundant (PCR6 already extended with LUKS header) - Notes added for TPM2 simplification over TPM1 in code as TODO Signed-off-by: Thierry Laurion <insurgo@riseup.net>
123 lines
4.1 KiB
Markdown
123 lines
4.1 KiB
Markdown
Frequently Asked Questions about Heads
|
|
===
|
|
|
|
Why replace UEFI with coreboot?
|
|
---
|
|
While Intel's edk2 tree that is the base of UEFI firmware is open source,
|
|
the firmware that vendors install on their machines is proprietary and
|
|
closed source. Updates for bugs fixes or security vulnerabilities
|
|
are at the vendor's convenience; user specific enhancements are likely not
|
|
possible; and the code is not auditable.
|
|
|
|
UEFI is much more complex than the BIOS that it replaced. It consists of
|
|
millions of lines of code and is an entire operating system,
|
|
with network device drivers, graphics, USB, TCP, https, etc, etc, etc.
|
|
All of these features represents increased "surface area" for attacks,
|
|
as well as unnecessary complexity in the boot process.
|
|
|
|
coreboot is open source and focuses on just the code necessary to bring
|
|
the system up from reset. This minimal code base has a much smaller
|
|
surface area and is possible to audit. Additionally, self-help is
|
|
possible if custom features are required or if a security vulnerability
|
|
needs to be patched.
|
|
|
|
|
|
What's wrong with UEFI Secure Boot?
|
|
---
|
|
Can't audit it, signing keys are controlled by vendors,
|
|
doesn't handle hand off in all cases, depends on possible leaked keys.
|
|
|
|
|
|
Why use Linux instead of vboot2?
|
|
---
|
|
vboot2 is part of the coreboot tree and is used by Google in the
|
|
Chromebook system to provide boot time security by verifying the
|
|
hashes on the coreboot payload. This works well for the specialized
|
|
Chrome OS on the Chromebook, but is not as flexible as a measured
|
|
boot solution.
|
|
|
|
By moving the verification into the boot scripts we're able to have
|
|
a much flexible verification system and use more common tools like PGP
|
|
to sign firmware stages.
|
|
|
|
|
|
What about Trusted GRUB?
|
|
---
|
|
The mainline grub doesn't have support for TPM and signed kernels, but
|
|
there is a Trusted grub fork that does. Due to philosophical differences
|
|
the code might not be merged into the mainline. And due to problems
|
|
with secure boot (which Trusted Grub builds on), many distributions have
|
|
signed insecure kernels that bypass all of the protections secure
|
|
boot promised.
|
|
|
|
Additionally, grub is closer to UEFI in that it must have device
|
|
drivers for all the different boot devices, as well as filesystems.
|
|
This duplicates the code that exists in the Linux kernel and has its
|
|
own attack surface.
|
|
|
|
Using coreboot and Linux as a boot loader allows us to restrict
|
|
the signature validation to keys that we control. We also have one code
|
|
base for the device drivers in the Linux-as-a-boot-loader as well
|
|
as Linux in the operating system.
|
|
|
|
|
|
What is the concern with the Intel Management Engine?
|
|
---
|
|
"Rootkit in your chipset", "x86 considered harmful", etc
|
|
|
|
|
|
How about the other embedded devices in the system?
|
|
---
|
|
#goodbios, funtenna, etc.
|
|
|
|
|
|
Should we be concerned about the binary blobs?
|
|
---
|
|
Maybe. x230 has very few (MRC) since it has native vga init.
|
|
|
|
|
|
Why use ancient Thinkpads instead of modern Macbooks?
|
|
---
|
|
coreboot support, TPM, nice keyboards, cheap to experiment on.
|
|
|
|
How likely are physical presence attacks vs remote software attacks?
|
|
---
|
|
Who knows.
|
|
|
|
|
|
Defense in depth vs single layers
|
|
---
|
|
Yes.
|
|
|
|
is it worth doing the hardware modifications?
|
|
---
|
|
Depends on your threat model.
|
|
|
|
|
|
Should I validate the TPMTOTP on every boot?
|
|
---
|
|
Probably. I want to make it also do it at S3.
|
|
|
|
|
|
suspend vs shutdown?
|
|
---
|
|
S3 is subject to cold boot attacks, although they are harder to
|
|
pull off on a Heads system since the boot devices are constrained.
|
|
|
|
However, without tpmtotp in s3 it is hard to know if the system is in
|
|
a safe state when the xscreensaver lock screen comes up. Is it a fake
|
|
to deceive you and steal your login password? Maybe! It wouldn't get
|
|
your disk password, which is perhaps an improvement.
|
|
|
|
|
|
Disk key in TPM (LUKS TPM Disk Unlock Key) or user passphrase?
|
|
---
|
|
Depends on your threat model. With the Disk Unlock Key in the TPM an
|
|
attacker would need to have the entire machine (or a backdoor in the TPM)
|
|
to get the key and their attempts to unlock it can be rate limited
|
|
by the TPM hardware.
|
|
|
|
However, this ties the disk to that one machine (without having to
|
|
recover and type in the master key), which might be an unacceptable risk
|
|
for some users.
|