Re: [PATCH 14/23] xsm/dummy: Allow XS_PRIV to call get_hvm_param

[Juergen Gross <jgross@xxxxxxxx>]

On 17.03.25 15:50, Jason Andryuk wrote:
> On 2025-03-17 10:18, Jan Beulich wrote:
> > On 06.03.2025 23:03, Jason Andryuk wrote:
> > > This is useful for a combined hardware/xenstore domain that will run
> > > init-dom0less and xenstored.  init-dom0less calls xc_hvm_param_get() to
> > > retrieve the xenstore event channel and pfn to configure xenstore for a
> > > guest.  With a hypervisor-allocated event channel and page, the
> > > set_hvm_param is not needed, and the normal domid permissions will allow
> > > xenstored to connect.
> > >
> > > Similarly, a hyperlaunch-ed xenstore stubdom needs to read a domain's
> > > xenstore event channel out of hvm_param.
> > >
> > > This allows reading but not modifying the guest, so allow the permission.
> > >
> > > Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> > Since this is exposing the entire param space to Xenstore, what I'm missing
> > is a security discussion for existing as well as potential future params.
> > There could well be some that better wouldn't be available for Xenstrore to
> > fetch.
> I can't speak for future parameters, but existing HVM_PARAMs didn't seem 
> sensitive to me.  The safest choice is to just pass the index to xsm_hvm_param() 
> and allow just HVM_PARAM_STORE_EVTCHN (and HVM_PARAM_STORE_PFN) for the xenstore 
> domain.
> This works for ARM and x86 HVM/PVH.  PV doesn't have a way to determine a 
> domain's event channel port, FWICT.
For what are you needing HVM_PARAM_STORE_PFN? The GNTTAB_RESERVED_XENSTORE
grant id should be enough to map the guest's Xenstore page?

And with that I'd rather suggest to expand struct xenstore_domain_interface
with the event channel port and let the component doing the seeding of the
grant table write the port into the struct.

This would enable Xenstore to just map the guest's Xenstore page and read
the event channel port from it. No additional hypercall permission needed.
And this would even work with PV domains. And as the Xenstore page is zeroed
initially, any event channel port != 0 could be regarded to be valid (a guest
choosing to write a bogus value there would just shoot itself in the foot by
harming its own Xenstore connection).


Juergen

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