Re: [Xen-devel] [PATCH v2 4/4] xen/x86: Allow stubdom access to irq created for msi.

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Wed, Jan 16, 2019 at 11:52:21AM +0100, Marek Marczykowski-Górecki wrote:
> On Wed, Jan 16, 2019 at 10:21:29AM +0100, Roger Pau Monné wrote:
> > On Tue, Jan 15, 2019 at 04:36:31PM +0100, Marek Marczykowski-Górecki wrote:
> > > From: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
> > > 
> > > Stubdomains need to be given sufficient privilege over the guest which it
> > > provides emulation for in order for PCI passthrough to work correctly.
> > > When a HVM domain try to enable MSI, QEMU in stubdomain calls
> > > PHYSDEVOP_map_pirq, but later it needs to call XEN_DOMCTL_bind_pt_irq as
> > > part of xc_domain_update_msi_irq. Allow for that as part of
> > > PHYSDEVOP_map_pirq.
> > 
> > I see, that's not a problem AFAICT for PCI INTx because the IRQ in
> > that case is known beforehand, and the stubdomain is given permissions
> > over this IRQ by libxl__device_pci_add (there's a do_pci_add against
> > the stubdomain).
> 
> Exactly.
> 
> > > 
> > > Based on 
> > > https://github.com/OpenXT/xenclient-oe/blob/5e0e7304a5a3c75ef01240a1e3673665b2aaf05e/recipes-extended/xen/files/stubdomain-msi-irq-access.patch
> > >  by Eric Chanudet <chanudete@xxxxxxxxxxxx>.
> > > 
> > > Signed-off-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Marek Marczykowski-Górecki 
> > > <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > ---
(...)
> > > diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
> > > index 8b44d6c..123ca69 100644
> > > --- a/xen/arch/x86/irq.c
> > > +++ b/xen/arch/x86/irq.c
> > > @@ -2674,6 +2674,21 @@ int allocate_and_map_msi_pirq(struct domain *d, 
> > > int index, int *pirq_p,
> > >          {
> > >      case MAP_PIRQ_TYPE_MULTI_MSI:
> > >              irq = create_irq(NUMA_NO_NODE);
> > > +            if ( !(irq < nr_irqs_gsi || irq >= nr_irqs) &&
> > 
> > This check is already performed below, maybe you could re-arrange the
> > code as:
> > 
> > case MAP_PIRQ_TYPE_MULTI_MSI:
> >         irq = create_irq(NUMA_NO_NODE);
> >     }
> > 
> >     if ( irq < nr_irqs_gsi || irq >= nr_irqs )
> >     {
> >         dprintk(XENLOG_G_ERR, "dom%d: can't create irq for msi!\n",
> >                 d->domain_id);
> >         return -EINVAL;
> >     }
> >     if ( current->domain->target == d )
> >         ...
> > 
> > But I wonder whether it would be better to place the irq_permit_access
> > in map_domain_pirq, together with the existing irq_permit_access that
> > grant the target domain permissions over the irq.
> 
> That may be a good idea. Let me try that in v3. But I'll wait for a
> feedback on libxl patches first.

That doesn't work, as map_domain_pirq() check irq access earlier.
Which bring be to a question, what should be rules guarding stubdomain
access to PHYSDEVOP_map_pirq? With this patch, stubdomain will be able
to create and map multiple irq (DoS possibility?), as only target domain
is validated in practice. Is that ok? If not, what additional limits
could be applied here?
In INTx case the problem doesn't apply, because toolstack grant access
to particular IRQ and no allocation happen on stubdomain request. But in
MSI case, it isn't that easy as IRQ number isn't known before (as
explained in the commit message).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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