Re: [PATCH 1/4] xen/xsm: Add XSM_HW_PRIV

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.06.2025 05:13, Jason Andryuk wrote:
> On 2025-06-11 09:02, Jan Beulich wrote:
>> On 11.06.2025 00:57, Jason Andryuk wrote:
>>> Xen includes disctinct concepts of a control domain (privileged) and a
>>> hardware domain, but there is only a single XSM_PRIV check.  For dom0
>>> this is not an issue as they are one and the same.
>>>
>>> With hyperlaunch and its build capabilities, a non-privileged hwdom and a
>>> privileged control domain should be possible.  Today the hwdom fails the
>>> XSM_PRIV checks for hardware-related hooks which it should be allowed
>>> access to.
>>>
>>> Introduce XSM_HW_PRIV, and use it to mark many of the physdev_op and
>>> platform_op.  The hwdom is allowed access for XSM_HW_PRIV.
>>>
>>> Make XSM_HW_PRIV a new privilege level that is given to the hardware
>>> domain, but is not exclusive.  The control domain can still execute
>>> XSM_HW_PRIV commands.  This is a little questionable since it's unclear
>>> how the control domain can meaningfully execute them.  But this approach
>>> is chosen to maintain the increasing privileges and keep control domain
>>> fully privileged.
>>
>> I consider this conceptually wrong. "Control" aiui refers to software
>> (e.g. VMs or system-wide settings), but there ought to be a (pretty?)
>> clear boundary between control and hardware domains, imo. As to
>> "pretty" - should any overlap be necessary (xms_machine_memory_map()
>> comes to mind), such would need handling specially then, I think. At
>> the same time: The more of an overlap there is, the less clear it is
>> why the two want/need separating in the first place.
> 
> So you are in favor of splitting control and hardware into distinct 
> sets?  I am okay with this.  I implemented that originally, but I 
> started doubting it.  Mainly, should control be denied any permission?

Yes, imo: Fundamentally for anything the hardware domain is supposed to
be doing. Yet as indicated in other replies to this series - boundaries
aren't always as clear as they ought to be for a clean separation.

> We aren't using the toolstack to build domains - dom0less or Hyperlaunch 
> handles that.  This avoids issues that might arise from running the 
> toolstack.

IOW you don't have a control domain there in the first place?

Jan