[coreboot] Disabling Intel ME 11 via undocumented mode
kakaroto at kakaroto.homelinux.net
Thu Dec 14 08:20:43 CET 2017
>From the PT article you linked to, after the stage 5 of BUP execution :
"It is at this stage that we find HAP processing; in this mode, BUP
hangs instead of executing InitScript. This means that the remaining
sequence of actions in normal mode has nothing to do with HAP and will
not be considered. The main thing we would like to note is that in HAP
mode, BUP initializes the entire platform (ICC, Boot Guard) but does
not start the main ME processes."
As for the kernel, that's my mistake, I remembered that prior to ME
11.x, the KERNEL module was removed by me_cleaner, and BUP was the
first process loaded. I hadn't realized that the execution order
changed in ME 11.x, and that explains why the KERNEL module cannot be
removed by me_cleaner in ME 11.x.
On ME 10.x and lower, the BUP module was the first executed, and it
would then load the KERNEL, so if BUP is halted before it did that,
then the kernel doesn't run. In ME 11.x however, they changed the
order, the KERNEL module is first to be loaded, but it only starts the
BUP process :
"The first process that the kernel creates is BUP, which runs in its
own address space in ring-3. The kernel does not launch any other
processes itself; this is done by BUP itself"
So, you are right, on Skylake+ (ME 11.x), the kernel would be running
but the BUP process is the one halted and no other processes get
launched, but on ME 10.x and lower, the kernel wouldn't be running
since BUP is loaded first (this is true for me_cleaned ME 10.x, and
I'm not sure what exactly the MeAltDisable flag does, which is the
equivalent of HAP on previous versions, but there wasn't any specific
research done on that).
I hadn't realized that until now when researching an error-free
response to you, so thanks for helping me notice that mistake in my
On Wed, Dec 13, 2017 at 12:54 PM, Timothy Pearson
<tpearson at raptorengineering.com> wrote:
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> According to Positive Technologies, on Skylake and higher (like the
> Purism machines) the kernel loads the BUP, and the HAP bit only disables
> the normal userspace processes .
> What proof do you have that the kernel itself is halted?
>  http://blog.ptsecurity.com/2017/08/disabling-intel-me.html
> On 12/13/2017 11:34 AM, Youness Alaoui wrote:
>>> I guess I still disagree with the use of the word "disabled". If the ME
>>> wasn't required for boot, and was actually disabled within a few cycles
>>> of its CPU starting, the remaining attack surface simply wouldn't exist.
>>> This is not what happens though, and AFAIK even the ME kernel continues
>>> to run since the ME needs to continue handling platform power events.
>>> If this many holes are present in even the ROM code, then having the ME
>>> kernel running remains a massive security problem.
>> I'm just going to answer the bit about the use of the term "disabled".
>> I've explained it in my blog post before (here if you missed it :
>> https://puri.sm/posts/deep-dive-into-intel-me-disablement/) but I do
>> believe the ME is in this case Disabled. What you are thinking about
>> is what I called "Removed". The reason it's called disabled is because
>> the ME stops running, it's not actively doing anything, it doesn't
>> respond to HECI, it doesn't even boot into the kernel. You said that
>> "the ME kernel continues to run", but that's not the case. The entire
>> ME core stops execution during the bring-up phase, so it's technically
>> disabled because it stops itself at some point after boot.
>> Having the ME *removed* would be interesting because that would mean
>> that even the bring up phase wouldn't get executed and we could remove
>> the entire ME firmware from the flash. But that still wouldn't mean
>> that nothing runs on the ME core because there is still some small
>> code embeded in the ROM.
>> Anyways, that's my justification on why using the term "disabled" is
>> valid in this case when HAP is enabled. You are free to disagree if
>> that didn't convince you.
> - --
> Timothy Pearson
> Raptor Engineering
> +1 (415) 727-8645 (direct line)
> +1 (512) 690-0200 (switchboard)
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