Re: [PATCH 2/4] x86/P2M: relax guarding of MMIO entries

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 01/09/2021 14:08, Jan Beulich wrote:
>>>> Restricting execute permissions is something unique to virt.  It doesn't
>>>> exist in a non-virtualised system, as I and D side reads are
>>>> indistinguishable outside of the core.
>>>>
>>>> Furthermore, it is inexpressible on some systems/configurations.
>>>>
>>>> Introspection is the only technology which should be restricting execute
>>>> permissions in the p2m, and only when it takes responsibility for
>>>> dealing with the fallout.
>>> IOW are you saying that the calls to set_identity_p2m_entry()
>>> (pre-dating XSA-378) were wrong to use p2m_access_rw?
>> Yes.
>>
>>>  Because that's
>>> what's getting the way here.
>> On a real machine, you really can write some executable code into an
>> E820 reserved region and jump to it.  You can also execute code from
>> real BARs is you happen to know that they are prefetchable (or you're a
>> glutton for UC reads...)
>>
>> And there is the WPBT ACPI table which exists specifically to let
>> firmware inject drivers/applications into a windows environment, and may
>> come out of the SPI ROM in the first place.
>>
>>
>> Is it sensible to execute an E820 reserved region, or unmarked BAR? 
>> Probably not.
>>
>> Should it work, because that's how real hardware behaves?  Absolutely.
>>
>> Any restrictions beyond that want handling by some kind of introspection
>> agent which has a policy of what to do with legal-but-dodgy-looking actions.
> IOW you suggest we remove p2m_access_t parameters from various functions,
> going with just default access?

p2m_access_t was very obviously a bodge when introduced, and I doubt it
would pass today's review standards.

It is definitely a mis-design, given its ill-specified and overlapping
semantics with respect to the p2m type.

>  Of course, as pointed out in another
> reply, we'll need to split p2m_mmio_direct into multiple types then, at
> the very least to honor the potential r/o restriction of AMD IOMMU unity
> mapped regions. (FAOD all of this isn't a short term plan anyway, at least
> afaic.)

I don't think that will be necessary.

IVMDs are almost non-existent, and given how many other areas of the AMD
IOMMU spec are totally unused, I wouldn't be surprised if r/o unity
mappings were in that category too.  There's no obvious usecase for r/o,
as anything critical enough in the platform to have an IVMD in the first
place will also be non-trivial enough to require bidirectional
communication somehow.

The unity mapping only says "this device requires read-only access".  It
doesn't say "this must be mapped read-only", and it is legitimate to map
a r/o unity mapping as r/w.

If such a case actually exists, there's clearly one agent in the system
with r/w access into the r/o range, and mapping it r/w is equivalent to
the "IOMMU not enabled in the first place" case which is the default
case for most software for the past decade-and-a-bit.

In other words, I don't think the r/o unit maps on their own are a good
enough reasons to split the type.  If we gain other reasons then fine,
but this seems like chunk of complexity with no real users.

~Andrew