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

[Robin Murphy]

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.