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.
