KVM: Fix a race condition for usage of is_hwpoison_address()

is_hwpoison_address accesses the page table, so the caller must hold
current->mm->mmap_sem in read mode. So fix its usage in hva_to_pfn of
kvm accordingly.

Comment is_hwpoison_address to remind other users.
Reported-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

mm/memory-failure.c

@@ -1298,6 +1298,9 @@ int soft_offline_page(struct page *page, int flags)
 	return ret;
 }
 
+/*
+ * The caller must hold current->mm->mmap_sem in read mode.
+ */
 int is_hwpoison_address(unsigned long addr)
 {
 	pgd_t *pgdp;

virt/kvm/kvm_main.c

@@ -947,12 +947,13 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr)
 	if (unlikely(npages != 1)) {
 		struct vm_area_struct *vma;
 
+		down_read(&current->mm->mmap_sem);
 		if (is_hwpoison_address(addr)) {
+			up_read(&current->mm->mmap_sem);
 			get_page(hwpoison_page);
 			return page_to_pfn(hwpoison_page);
 		}
 
-		down_read(&current->mm->mmap_sem);
 		vma = find_vma(current->mm, addr);
 
 		if (vma == NULL || addr < vma->vm_start ||