ANDROID: KVM: arm64: Issue CMOs when tearing down shadow pages

On the guest teardown path, pKVM will zero the pages used to back the
guest shadow data structures before returning them to the host as they
may contain secrets (e.g. in the vCPU registers). However, the zeroing
is done using a cacheable alias, and CMOs are missing, hence giving the
host a potential opportunity to read the original content of the shadow
structs from memory.

Fix this by issuing CMOs after zeroing the pages.

[ qperret@: moved the CMOs to __unmap_donated_memory() to cover all
  callers, including the __pkvm_init_vm() error path ]

Bug: 259551298
Change-Id: Id696d47d16e4c3fd870cb70b792eeb7f2282fc78
Signed-off-by: Quentin Perret <qperret@google.com>

arch/arm64/kvm/hyp/nvhe/pkvm.c

@@ -626,6 +626,7 @@ static void *map_donated_memory(unsigned long host_va, size_t size)
 
 static void __unmap_donated_memory(void *va, size_t size)
 {
+	kvm_flush_dcache_to_poc(va, size);
 	WARN_ON(__pkvm_hyp_donate_host(hyp_virt_to_pfn(va),
 				       PAGE_ALIGN(size) >> PAGE_SHIFT));
 }