Re: [edk2-rfc] [edk2-devel] CPU hotplug using SMM with QEMU+OVMF

["Laszlo Ersek" <lersek@redhat.com>]

On 09/02/19 10:45, Igor Mammedov wrote:
> On Fri, 30 Aug 2019 20:46:14 +0200
> Laszlo Ersek <lersek@redhat.com> wrote:
> 
>> On 08/30/19 16:48, Igor Mammedov wrote:
>>
>>> (01) On boot firmware maps and initializes SMI handler at default SMBASE (30000)
>>>      (using dedicated SMRAM at 30000 would allow us to avoid save/restore
>>>       steps and make SMM handler pointer not vulnerable to DMA attacks)
>>>
>>> (02) QEMU hotplugs a new CPU in reset-ed state and sends SCI
>>>
>>> (03) on receiving SCI, host CPU calls GPE cpu hotplug handler
>>>       which writes to IO port 0xB2 (broadcast SMI)
>>>
>>> (04) firmware waits for all existing CPUs rendezvous in SMM mode,
>>>      new CPU(s) have SMI pending but does nothing yet
>>>
>>> (05) host CPU wakes up one new CPU (INIT-INIT-SIPI)
>>>      SIPI vector points to RO flash HLT loop.
>>>      (how host CPU will know which new CPUs to relocate?
>>>       possibly reuse QEMU CPU hotplug MMIO interface???)
>>>
>>> (06) new CPU does relocation.
>>>      (in case of attacker sends SIPI to several new CPUs,
>>>       open question how to detect collision of several CPUs at the same default SMBASE)
>>>
>>> (07) once new CPU relocated host CPU completes initialization, returns
>>>      from IO port write and executes the rest of GPE handler, telling OS
>>>      to online new CPU.  
>>
>> In step (03), it is the OS that handles the SCI; it transfers control to
>> ACPI. The AML can write to IO port 0xB2 only because the OS allows it.
>>
>> If the OS decides to omit that step, and sends an INIT-SIPI-SIPI
>> directly to the new CPU, can it steal the CPU?
> It sure can but this way it won't get access to privileged SMRAM
> so OS can't subvert firmware.
> The next time SMI broadcast is sent the CPU will use SMI handler at
> default 30000 SMBASE. It's up to us to define behavior here (for example
> relocation handler can put such CPU in shutdown state).
> 
> It's in the best interest of OS to cooperate and execute AML
> provided by firmware, if it does not follow proper cpu hotplug flow
> we can't guarantee that stolen CPU will work.

This sounds convincing enough, for the hotplugged CPU; thanks.

So now my concern is with step (01). While preparing for the initial
relocation (of cold-plugged CPUs), the code assumes the memory at the
default SMBASE (0x30000) is normal RAM.

Is it not a problem that the area is written initially while running in
normal 32-bit or 64-bit mode, but then executed (in response to the
first, synchronous, SMI) as SMRAM?

Basically I'm confused by the alias.

TSEG (and presumably, A/B seg) work like this:
- when open, looks like RAM to normal mode and SMM
- when closed, looks like black-hole to normal mode, and like RAM to SMM

The generic edk2 code knows this, and manages the SMRAM areas accordingly.

The area at 0x30000 is different:
- looks like RAM to both normal mode and SMM

If we set up the alias at 0x30000 into A/B seg,
- will that *permanently* hide the normal RAM at 0x30000?
- will 0x30000 start behaving like A/B seg?

Basically my concern is that the universal code in edk2 might or might
not keep A/B seg open while initially populating the area at the default
SMBASE. Specifically, I can imagine two issues:

- if the alias into A/B seg is inactive during the initial population,
then the initial writes go to RAM, but the execution (the first SMBASE
relocation) will occur from A/B seg through the alias

- alternatively, if the alias is always active, but A/B seg is closed
during initial population (which happens in normal mode), then the
initial writes go to the black hole, and execution will occur from a
"blank" A/B seg.

Am I seeing things? (Sorry, I keep feeling dumber and dumber in this
thread.)

Anyway, I guess we could try and see if OVMF still boots with the alias...

Thanks
Laszlo