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

[Paolo Bonzini]

On 21/08/19 17:48, Kinney, Michael D wrote:
> Perhaps there is a way to avoid the 3000:8000 startup
> vector.
> 
> If a CPU is added after a cold reset, it is already in a
> different state because one of the active CPUs needs to
> release it by interacting with the hot plug controller.
> 
> Can the SMRR for CPUs in that state be pre-programmed to
> match the SMRR in the rest of the active CPUs?
> 
> For OVMF we expect all the active CPUs to use the same
> SMRR value, so a check can be made to verify that all 
> the active CPUs have the same SMRR value.  If they do,
> then any CPU released through the hot plug controller 
> can have its SMRR pre-programmed and the initial SMI
> will start within TSEG.
> 
> We just need to decide what to do in the unexpected 
> case where all the active CPUs do not have the same
> SMRR value.
> 
> This should also reduce the total number of steps.

The problem is not the SMRR but the SMBASE.  If the SMBASE area is
outside TSEG, it is vulnerable to DMA attacks independent of the SMRR.
SMBASE is also different for all CPUs, so it cannot be preprogrammed.

(As an aside, virt platforms are also immune to cache poisoning so they
don't have SMRR yet - we could use them for SMM_CODE_CHK_EN and block
execution outside SMRR but we never got round to it).

An even simpler alternative would be to make A0000h the initial SMBASE.
 However, I would like to understand what hardware platforms plan to do,
if anything.

Paolo

> Mike
> 
>> -----Original Message-----
>> From: address@hidden [mailto:address@hidden] On
>> Behalf Of Yao, Jiewen
>> Sent: Sunday, August 18, 2019 4:01 PM
>> To: Paolo Bonzini <address@hidden>
>> Cc: Alex Williamson <address@hidden>; Laszlo
>> Ersek <address@hidden>; address@hidden; edk2-
>> rfc-groups-io <address@hidden>; qemu devel list
>> <address@hidden>; Igor Mammedov
>> <address@hidden>; Chen, Yingwen
>> <address@hidden>; Nakajima, Jun
>> <address@hidden>; Boris Ostrovsky
>> <address@hidden>; Joao Marcal Lemos Martins
>> <address@hidden>; Phillip Goerl
>> <address@hidden>
>> Subject: Re: [edk2-rfc] [edk2-devel] CPU hotplug using
>> SMM with QEMU+OVMF
>>
>> in real world, we deprecate AB-seg usage because they
>> are vulnerable to smm cache poison attack.
>> I assume cache poison is out of scope in the virtual
>> world, or there is a way to prevent ABseg cache poison.
>>
>> thank you!
>> Yao, Jiewen
>>
>>
>>> 在 2019年8月19日，上午3:50，Paolo Bonzini
>> <address@hidden> 写道：
>>>
>>>> On 17/08/19 02:20, Yao, Jiewen wrote:
>>>> [Jiewen] That is OK. Then we MUST add the third
>> adversary.
>>>> -- Adversary: Simple hardware attacker, who can use
>> device to perform DMA attack in the virtual world.
>>>> NOTE: The DMA attack in the real world is out of
>> scope. That is be handled by IOMMU in the real world,
>> such as VTd. -- Please do clarify if this is TRUE.
>>>>
>>>> In the real world:
>>>> #1: the SMM MUST be non-DMA capable region.
>>>> #2: the MMIO MUST be non-DMA capable region.
>>>> #3: the stolen memory MIGHT be DMA capable region or
>> non-DMA capable
>>>> region. It depends upon the silicon design.
>>>> #4: the normal OS accessible memory - including ACPI
>> reclaim, ACPI
>>>> NVS, and reserved memory not included by #3 - MUST be
>> DMA capable region.
>>>> As such, IOMMU protection is NOT required for #1 and
>> #2. IOMMU
>>>> protection MIGHT be required for #3 and MUST be
>> required for #4.
>>>> I assume the virtual environment is designed in the
>> same way. Please
>>>> correct me if I am wrong.
>>>>
>>>
>>> Correct.  The 0x30000...0x3ffff area is the only
>> problematic one;
>>> Igor's idea (or a variant, for example optionally
>> remapping
>>> 0xa0000..0xaffff SMRAM to 0x30000) is becoming more
>> and more attractive.
>>>
>>> Paolo
>>
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