Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page

[Ankur Arora]

On 2020-12-01 5:26 p.m., David Woodhouse wrote
> On Tue, 2020-12-01 at 16:40 -0800, Ankur Arora wrote:
> > On 2020-12-01 5:07 a.m., David Woodhouse wrote:
> > > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
> > > > +static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
> > > > +{
> > > > +       struct shared_info *shared_info;
> > > > +       struct page *page;
> > > > +
> > > > +       page = gfn_to_page(kvm, gfn);
> > > > +       if (is_error_page(page))
> > > > +               return -EINVAL;
> > > > +
> > > > +       kvm->arch.xen.shinfo_addr = gfn;
> > > > +
> > > > +       shared_info = page_to_virt(page);
> > > > +       memset(shared_info, 0, sizeof(struct shared_info));
> > > > +       kvm->arch.xen.shinfo = shared_info;
> > > > +       return 0;
> > > > +}
> > > > +
> > >
> > > Hm.
> > >
> > > How come we get to pin the page and directly dereference it every time,
> > > while kvm_setup_pvclock_page() has to use kvm_write_guest_cached()
> > > instead?
> >
> > So looking at my WIP trees from the time, this is something that
> > we went back and forth on as well with using just a pinned page or a
> > persistent kvm_vcpu_map().
>
> OK, thanks.
>
> > I remember distinguishing shared_info/vcpu_info from kvm_setup_pvclock_page()
> > as shared_info is created early and is not expected to change during the
> > lifetime of the guest which didn't seem true for MSR_KVM_SYSTEM_TIME (or
> > MSR_KVM_STEAL_TIME) so that would either need to do a kvm_vcpu_map()
> > kvm_vcpu_unmap() dance or do some kind of synchronization.
> >
> > That said, I don't think this code explicitly disallows any updates
> > to shared_info.
>
> It needs to allow updates as well as disabling the shared_info pages.
> We're going to need that to implement SHUTDOWN_soft_reset for kexec.
True.

> > > If that was allowed, wouldn't it have been a much simpler fix for
> > > CVE-2019-3016? What am I missing?
> >
> > Agreed.
> >
> > Perhaps, Paolo can chime in with why KVM never uses pinned page
> > and always prefers to do cached mappings instead?
> >
> > > Should I rework these to use kvm_write_guest_cached()?
> >
> > kvm_vcpu_map() would be better. The event channel logic does RMW operations
> > on shared_info->vcpu_info.
>
> I've ported the shared_info/vcpu_info parts and made a test case, and
> was going back through to make it use kvm_write_guest_cached(). But I
> should probably plug on through the evtchn bits before I do that.
>
> I also don't see much locking on the cache, and can't convince myself
> that accessing the shared_info page from multiple CPUs with
> kvm_write_guest_cached() or kvm_map_gfn() is sane unless they each have
> their own cache.

I think you could get a VCPU specific cache with kvm_vcpu_map().

> What I have so far is at
>
> https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/xenpv

Thanks. Will take a look there.

Ankur

> I'll do the event channel support tomorrow and hook it up to my actual
> VMM to give it some more serious testing.