Re: [PATCH v4 04/14] Documentation/x86: Secure Launch kernel documentation

[Ross Philipson <ross.philipson@oracle.com>]

On 12/3/21 11:03, Robin Murphy wrote:
> On 2021-12-03 15:47, Ross Philipson wrote:
>> On 12/2/21 12:26, Robin Murphy wrote:
>>> On 2021-08-27 14:28, Ross Philipson wrote:
>>> [...]
>>>> +IOMMU Configuration
>>>> +-------------------
>>>> +
>>>> +When doing a Secure Launch, the IOMMU should always be enabled and
>>>> the drivers
>>>> +loaded. However, IOMMU passthrough mode should never be used. This
>>>> leaves the
>>>> +MLE completely exposed to DMA after the PMR's [2]_ are disabled.
>>>> First, IOMMU
>>>> +passthrough should be disabled by default in the build configuration::
>>>> +
>>>> +  "Device Drivers" -->
>>>> +      "IOMMU Hardware Support" -->
>>>> +          "IOMMU passthrough by default [ ]"
>>>> +
>>>> +This unset the Kconfig value CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
>>>
>>> Note that the config structure has now changed, and if set, passthrough
>>> is deselected by choosing a different default domain type.
>>
>> Thanks for the heads up. We will have to modify this for how things
>> exist today.
>>
>>>
>>>> +In addition, passthrough must be disabled on the kernel command line
>>>> when doing
>>>> +a Secure Launch as follows::
>>>> +
>>>> +  iommu=nopt iommu.passthrough=0
>>>
>>> This part is a bit silly - those options are literally aliases for the
>>> exact same thing, and furthermore if the config is already set as
>>> required then the sole effect either of them will have is to cause "(set
>>> by kernel command line)" to be printed. There is no value in explicitly
>>> overriding the default value with the default value - if anyone can
>>> append an additional "iommu.passthrough=1" (or "iommu=pt") to the end of
>>> the command line they'll still win.
>>
>> I feel like when we worked on this, it was still important to set those
>> values. This could have been in an older kernel version. We will go back
>> and verify what you are saying here and adjust the documentation
>> accordingly.
>>
>> As to anyone just adding values to the command line, that is why the
>> command line is part of the DRTM measurements.
> 
> Yeah, I had a vague memory that that was the case - basically if you can
> trust the command line as much as the config then it's definitely
> redundant to pass an option for this (see iommu_subsys_init() - it's now
> all plumbed through iommu_def_domain_type), and if you can't then
> passing them is futile anyway.

Thanks you for your feedback.

Ross

> 
> Cheers,
> Robin.