Re: CPU hotplug using SMM with QEMU+OVMF


Laszlo Ersek
 

On 08/15/19 18:21, Paolo Bonzini wrote:
On 15/08/19 17:00, Laszlo Ersek wrote:
On 08/14/19 16:04, Paolo Bonzini wrote:
On 14/08/19 15:20, Yao, Jiewen wrote:
- Does this part require a new branch somewhere in the OVMF SEC code?
How do we determine whether the CPU executing SEC is BSP or
hot-plugged AP?
[Jiewen] I think this is blocked from hardware perspective, since the first instruction.
There are some hardware specific registers can be used to determine if the CPU is new added.
I don’t think this must be same as the real hardware.
You are free to invent some registers in device model to be used in OVMF hot plug driver.
Yes, this would be a new operation mode for QEMU, that only applies to
hot-plugged CPUs. In this mode the AP doesn't reply to INIT or SMI, in
fact it doesn't reply to anything at all.

- How do we tell the hot-plugged AP where to start execution? (I.e. that
it should execute code at a particular pflash location.)
[Jiewen] Same real mode reset vector at FFFF:FFF0.
You do not need a reset vector or INIT/SIPI/SIPI sequence at all in
QEMU. The AP does not start execution at all when it is unplugged, so
no cache-as-RAM etc.

We only need to modify QEMU so that hot-plugged APIs do not reply to
INIT/SIPI/SMI.

I don’t think there is problem for real hardware, who always has CAR.
Can QEMU provide some CPU specific space, such as MMIO region?
Why is a CPU-specific region needed if every other processor is in SMM
and thus trusted.
I was going through the steps Jiewen and Yingwen recommended.

In step (02), the new CPU is expected to set up RAM access. In step
(03), the new CPU, executing code from flash, is expected to "send board
message to tell host CPU (GPIO->SCI) -- I am waiting for hot-add
message." For that action, the new CPU may need a stack (minimally if we
want to use C function calls).

Until step (03), there had been no word about any other (= pre-plugged)
CPUs (more precisely, Jiewen even confirmed "No impact to other
processors"), so I didn't assume that other CPUs had entered SMM.

Paolo, I've attempted to read Jiewen's response, and yours, as carefully
as I can. I'm still very confused. If you have a better understanding,
could you please write up the 15-step process from the thread starter
again, with all QEMU customizations applied? Such as, unnecessary steps
removed, and platform specifics filled in.
Sure.

(01a) QEMU: create new CPU. The CPU already exists, but it does not
start running code until unparked by the CPU hotplug controller.

(01b) QEMU: trigger SCI

(02-03) no equivalent

(04) Host CPU: (OS) execute GPE handler from DSDT

(05) Host CPU: (OS) Port 0xB2 write, all CPUs enter SMM (NOTE: New CPU
will not enter CPU because SMI is disabled)

(06) Host CPU: (SMM) Save 38000, Update 38000 -- fill simple SMM
rebase code.
(Could Intel open source code for this?)

(07a) Host CPU: (SMM) Write to CPU hotplug controller to enable
new CPU

(07b) Host CPU: (SMM) Send INIT/SIPI/SIPI to new CPU.

(08a) New CPU: (Low RAM) Enter protected mode.
PCI DMA attack might be relevant (but yes, I see you've mentioned that
too, down-thread)


(08b) New CPU: (Flash) Signals host CPU to proceed and enter cli;hlt loop.

(09) Host CPU: (SMM) Send SMI to the new CPU only.

(10) New CPU: (SMM) Run SMM code at 38000, and rebase SMBASE to
TSEG.
I wish we could simply wake the new CPU -- after step 07a -- with an
SMI. IOW, if we could excise steps 07b, 08a, 08b.

Our CPU hotplug controller, and the initial parked state in 01a for the
new CPU, are going to be home-brewed anyway.

On the other hand...

(11) Host CPU: (SMM) Restore 38000.

(12) Host CPU: (SMM) Update located data structure to add the new CPU
information. (This step will involve CPU_SERVICE protocol)

(13) New CPU: (Flash) do whatever other initialization is needed

(14) New CPU: (Flash) Deadloop, and wait for INIT-SIPI-SIPI.
basically step 08b is the environment to which the new CPU returns in
13/14, after the RSM.

Do we absolutely need low RAM for 08a (for entering protected mode)? we
could execute from pflash, no? OTOH we'd still need RAM for the stack,
and that could be attacked with PCI DMA similarly. I believe.

(15) Host CPU: (OS) Send INIT-SIPI-SIPI to pull new CPU in..


In other words, the cache-as-RAM phase of 02-03 is replaced by the
INIT-SIPI-SIPI sequence of 07b-08a-08b.


The QEMU DSDT could be modified (when secure boot is in effect) to OUT
to 0xB2 when hotplug happens. It could write a well-known value to
0xB2, to be read by an SMI handler in edk2.
I dislike involving QEMU's generated DSDT in anything SMM (even
injecting the SMI), because the AML interpreter runs in the OS.

If a malicious OS kernel is a bit too enlightened about the DSDT, it
could willfully diverge from the process that we design. If QEMU
broadcast the SMI internally, the guest OS could not interfere with that.

If the purpose of the SMI is specifically to force all CPUs into SMM
(and thereby force them into trusted state), then the OS would be
explicitly counter-interested in carrying out the AML operations from
QEMU's DSDT.
But since the hotplug controller would only be accessible from SMM,
there would be no other way to invoke it than to follow the DSDT's
instruction and write to 0xB2.
Right.

FWIW, real hardware also has plenty of
0xB2 writes in the DSDT or in APEI tables (e.g. for persistent store
access).
Thanks
Laszlo

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