Re: [Xen-devel] [PATCH v4 4/6] xen/x86: Allow stubdom access to irq created for msi.

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Feb 07, 2019 at 10:57:19AM +0100, Roger Pau Monné wrote:
> On Thu, Feb 07, 2019 at 01:07:47AM +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.
> > 
> > This is not needed for PCI INTx, because 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).
> > 
> > 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>
> > ---
> > Changes in v3:
> >  - extend commit message
> > Changes in v4:
> >  - add missing destroy_irq on error path
> > 
> > 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).
> > ---
> >  xen/arch/x86/irq.c     | 24 ++++++++++++++++++++++++
> >  xen/arch/x86/physdev.c |  9 +++++++++
> >  2 files changed, 33 insertions(+)
> > 
> > diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
> > index 8b44d6c..5e5dcac 100644
> > --- a/xen/arch/x86/irq.c
> > +++ b/xen/arch/x86/irq.c
> > @@ -2674,6 +2674,22 @@ 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) &&
> > +                    current->domain->target == d )
> > +            {
> > +                ret = irq_permit_access(current->domain, irq);
> > +                if ( ret ) {
> > +                    dprintk(XENLOG_G_ERR,
> > +                            "dom%d: can't grant it's stubdom (%d) access 
> > to "
> > +                            "irq %d for msi: %d!\n",
> > +                            d->domain_id,
> > +                            current->domain->domain_id,
> > +                            irq,
> > +                            ret);
> > +                    destroy_irq(irq);
> > +                    return ret;
> 
> I'm afraid his won't work for devices that support multiple MSI vectors.
> Note that map_domain_pirq also has a call to create_irq, and you are
> not adding the sutbdom permissions there.
> 
> IMO, the safer way to fix this would be to modify create_irq and
> destroy_irq so that you give permissions to the subtdomain in the same
> place that hardware domain permissions are given. Note that you will
> have to change the function to take an extra domain parameter
> AFAICT.

That may be a good idea, I'll try.

> Alternatively the permissions could be granted/revoked in
> {un}map_domain_pirq, which already contains a call to
> irq_access_permitted/irq_deny_access, I think I've suggested this in a
> previous version already [0]. This seems less intrusive that modifying
> create_irq/destroy_irq if viable.

I've already tried that. And as responded there, it won't fly, as the
first thing map_domain_pirq does is checking irq permission, which would
fail for irq allocated by allocate_and_map_msi_pirq.

> Thanks, Roger.
> 
> [0] https://lists.xenproject.org/archives/html/xen-devel/2019-01/msg01240.html

-- 
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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