KVM: Don't WARN if memory is dirtied without a vCPU when the VM is dying

In the Linux kernel, the following vulnerability has been resolved:

KVM: Don't WARN if memory is dirtied without a vCPU when the VM is dying

When marking a page dirty, complain about not having a running/loaded vCPU

if and only if the VM is still alive, i.e. its refcount is non-zero. This

will allow fixing a memory leak for x86 SEV-ES guests without hitting what

is effectively a false positive on the WARN.

For some SEV-ES VM-Exits, KVM keeps a writable mapping of a guest page

across an exit to userspace, and typically unmaps the page on the next

KVM_RUN. But if userspace never calls KVM_RUN after such an exit, then KVM

needs to unmap the page when the vCPU is destroyed, which in turn triggers

the WARN about not having a running vCPU.

Alternatively, SEV-ES could temporarily load the vCPU to suppress the WARN,

as is done in nested_vmx_free_vcpu() (but for completely unrelated reasons;

suppressing WARN from nested_put_vmcs12_pages() is pure happenstance). But

loading a vCPU during destruction is gross (ideally nVMX code would be

cleaned up), risks complicating the SEV-ES code (KVM would need to ensure

the temporarily load()+put() only runs when the vCPU isn't already loaded),

and is ultimately pointless.

The motivation for the WARN is to guard against KVM dirtying guest memory

without pushing the corresponding GFN to the active vCPU's dirty ring, e.g.

to ensure userspace doesn't miss a dirty page. But for the VM's refcount

to reach zero, there can't be _any_ userspace mappings to the dirty ring,

as mapping the dirty ring requires doing mmap() on the vCPU FD. I.e. if

userspace had a valid mapping for the dirty ring, then the vCPU file and

thus the owning VM would still be alive. And so since userspace can't

possibly reach the dirty ring, whether or not KVM technically "misses" a

push to the dirty ring is irrelevant.