Hi Nico,
Sorry for this late report. I am still fiddling with the coreboot code to setup the SMM_BWP bit you suggested but with not much progress.
Beside the SMM_BWP, I followed your recommendation and was able to successfully turn on the PR0 & FLOCKDN protections. I can confirm these by running chipsec against the compiled image. [chipsec_main@Controller_locked.txt]
I think I am fine at the moment and am somehow confident to put my X1 Carbon onto the internet. I will further explore coreboot codes for additional flash memory write protections on my own.
I just want to express my gratitude for your help and towards the people who make this project even possible and freely available to the whole world, Thank you!!
My warmest regards,
Alan
Hello Nico,
Just a quick update that the platform I rely on (Debian 11 testing live) for Coreboot compilation messed up itself after package updates and my progress is retarded.
[Debian.jpg]
I had some readings on the PRx registers and now I understand that those are registers accepting base, limit & range of address in memory to protect from read/write and is provided and enforced by the SPI controller. I completely agree with you that the protection bits I asked in my previous email are not effective security measures since SMM code is privileged enough to freely change those bits anytime and as you mentioned there is presently not yet any secure SMM implementation (I think that the UEFI spec makes everyones' life more difficult.....).
I will report back in next week on the menu makeconfig options you mentioned in your previous email.
Thanks for your help,
Alan
Hello Nico,
Thank you very much for your reply, it is informative and helpful!
I will read more regarding the PRx registers and try to compile with the menuconfig options you mentioned. Will also scrutinize the southbridge C code and see if I can comprehend it. I will report back again within the next few days.
Hope this email finds you are having a wonderful day,
Alan
On Sun, May 31, 2020 at 6:01 PM Nico Huber <
nico.h@gmx.de> wrote:
Hello Alan,
On 31.05.20 06:20, Alan K.L. Mok wrote:
> 1. Can anyone please tell let me know how can I achieve the captioned
> objectives? I looked into ifdtool & uefitool but found nothing related to
> my goal. I also tried the "lock ME/TXE" option during make menuconfig but
> Intel chipsec is still reporting the captioned bits not set on my
> Coreboot-flashed X1 Carbon
this option sets access permissions in the Flash Descriptor (IFD). They
act like the PR0-5 registers but on top of them.
The default settings (only valid ones as by Intel for production
machines) will write-protect the IFD, read- and write-protect the
ME region.
> 2. Is it correct to say that once the PR0-5 bits are set and Coreboot
> flashed into the machine, the values of the PR registers will be configured
> accordingly after machine boot up (when OS is having control)?
I'm not sure if I follow. Why do you call PR0-5 bits? those are
registers that may or may not be filled with a flash range to
protect. It is the host firmware's (coreboot's) responsibility
to write these registers. But what values are to be written is
up to you.
Chipsec is a nice tool to check what things are configured and how.
But the user still has to know how these things work. Otherwise they
might make wrong assumptions about security. One good example is the
FLOCKDN (flash lockdown) bit. When it's set, one cannot change the
flash controller's configuration (including PR0-5 ranges) anymore
until the next reboot. But it doesn't tell you if the locked confi-
guration is secure, only that it can't be changed.
> BIOSWE, BLE, SMM_BWP & PR0-5 protection bits
BIOSWE, BLE, and SMM_BWP shift the blame from the OS to SMM (which is
supposed to be controlled by the firmware). However, to my knowledge,
nobody has yet achieved a secure SMM implementation. So many people
ignore this. And IMHO, any good security concept shouldn't treat SMM
more privileged than the OS.
If you want to set these bits anyway, have a look at your south-
bridge's code (src/southbridge/intel/bd82x6x/lpc.c), around
pch_disable_smm_only_flashing(). This function clears SMM_BWP,
I guess.
The PR0-5 ranges can protect your flash efficiently, AFAIK. Not even
SMM should be able to get around these. If you just want to write
protect your whole flash chip (you won't be able to flash internally
anymore), have a look at "Boot media protection mechanism" in the
"Security" config menu. If set to "Lock boot media using the con-
troller", it should use one of the PR0-5 ranges.
FLOCKDN should be set when "Lock down chipset in coreboot" ("Chipset"
menu) is enabled.
Hope that helps,
Nico