kvm: mmu: Fix race in emulated page table writes

When a guest page table is updated via an emulated write,
kvm_mmu_pte_write() is called to update the shadow PTE using the just
written guest PTE value. But if two emulated guest PTE writes happened
concurrently, it is possible that the guest PTE and the shadow PTE end
up being out of sync. Emulated writes do not mark the shadow page as
unsync-ed, so this inconsistency will not be resolved even by a guest TLB
flush (unless the page was marked as unsync-ed at some other point).

This is fixed by re-reading the current value of the guest PTE after the
MMU lock has been acquired instead of just using the value that was
written prior to calling kvm_mmu_pte_write().

Signed-off-by: Junaid Shahid <junaids@google.com>
Reviewed-by: Wanpeng Li <wanpengli@tencent.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>