Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!
This page explains how coreboot can help with various security aspects of your system, compared to proprietary/closed-source boot firmware implementations(BIOS/EFI/UEFI). It doesn't however address issues such as the Intel Management Engine or AMD PSP.
Note that while coreboot itself is free software, many boards still use blobs. Some however don't require any. If so they are typically supported by Libreboot, a 100% free software coreboot distribution.
Fixes can take months before being available on non-free firmwares, if you are lucky enough to have them. With free software boot fimrwares, security issues can be fixed, and in coreboot many are.
Examples:
Security fixes are usually mentioned in coreboot ChangeLog on the blog.
Using proprietary software parts in coreboot, such as proprietary RAM initialization, will make you dependent on the producer of that software to fix security issues affecting it.
Because the boot firmware is the first code that executes on the main CPU, it's an interesting target for rootkits:
Given the above, being able to know what your boot firmware does is very important.
coreboot has reproducible builds. That permits to verify that a given binary corresponds to a given source code. This should work out of the box.
There is even a way to verify that the same code produces the same binary regardless of the git revision with:
$ make BUILD_TIMELESS=1
However reproducible builds isn't sufficient to verify that you are really running the binary you flashed:
Dumping the flash chip externally is strongly advised for that, since some chipsets makes it too easy for the SMM code to give back (to Flashrom) a binary than differs from the one in the flash chip. The same chipset mechanism also makes it too easy to write a modified version to the flash.
If you do not trust the computer dumping the flash externally and can't setup proper flash write protection (patches for it were not merged yet at the time of writing) you can get around that by verifying the flash chip content with unrelated computers, while keeping the computer you read the flash from offline. This way no computer can predict if it will be the last to verify the flash chip, and so covertly modifying it will be risky since it can be detected.
Given that, with coreboot, the hardware initialization is separated from the boot logic, many security features only makes sense when implemented in payloads. Nevertheless, coreboot implement some security features.
| coreboot | GRUB | SeaBIOS | Depthcharge | Tianocore | |
|---|---|---|---|---|---|
| Password verification | No | Yes | No | ? | |
| Signature verification | Yes | Yes | No<ref>This can be achieved by configuring SeaBIOS to only load an IPXE option rom, that in turn verifies signatures. Loading other option rom has also to be avoided. This is possible on some laptops for instance.</ref> | Yes | |
| Can open encrypted partition | No | Yes | No | No |
If we, for now, start by assuming that coreboot and what resides in the boot flash is trusted, we then might still have other untrusted chips in the same device:
Some laptops, like the chromebooks, do have free software on the embedded controller. This is the way to go. However some laptops don't.
This chip usually handles:
Since coreboot has to talk to such chip, input comming from such chip, such as its firmware revision should be considered as untrusted.
The following storage mediums typically use non-free firmwares:
If we assume that the hard disk firmware is potentially malicious, we then can workaround it by making sure that the code stored on that disk is not modified on the fly by the hard disk firmware.
To do that we can take advantage of the fact that we run code from the boot flash first, which doesn't have such issue.
Code signing and encryption can be used to do that, however care must be taken not to be vulnerable to TOCTU(Time of check, time of use) attacks.
DMA is often understood as a way to access the RAM independently of the CPU.<ref> https://en.wikipedia.org/wiki/Direct_memory_access </ref> However, DMA is, in a more broad context, just a way to do "memory to memory" transfers. That might not involve the main CPU RAM at all, like with SATA's DMA.
Bus capable of accessing the main CPU's RAM:
Bus not capable of accessing the main CPU RAM:
This is not very useful: The most interesting time would be right before power-off, which could be implemented in SMM. Unfortunately a cautious attacker just pulls the plug.
To prevent reading data after a reboot, a payload could be adapted to clean out memory. Using applications that manage sensible data sensibly (ie. wipe after use) is still a better solution.
At boot, the RAM isn't initialized nor functional. This is the boot fimrware's task. Having a free software boot firmware gives us the opportunity to try to never leave the system RAM unprotected from such attacks. The idea would be to try to initialize the IOMMU before activating the RAM.
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