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SubjectRe: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page
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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().

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.

>
> 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.


Ankur

>
>

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