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

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Date:   Fri, 3 Dec 2021 16:03:42 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Ross Philipson <ross.philipson@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        iommu@...ts.linux-foundation.org, linux-integrity@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc:     dpsmith@...rtussolutions.com, luto@...capital.net,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        kanth.ghatraju@...cle.com, tglx@...utronix.de,
        trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v4 04/14] Documentation/x86: Secure Launch kernel
 documentation

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.

Cheers,
Robin.