mm: thp: KVM: Explicitly check for THP when populating secondary MMU

Add a helper, is_transparent_hugepage(), to explicitly check whether a
compound page is a THP and use it when populating KVM's secondary MMU.
The explicit check fixes a bug where a remapped compound page, e.g. for
an XDP Rx socket, is mapped into a KVM guest and is mistaken for a THP,
which results in KVM incorrectly creating a huge page in its secondary
MMU.

Fixes: 936a5fe6e6 ("thp: kvm mmu transparent hugepage support")
Reported-by: syzbot+c9d1fb51ac9d0d10c39d@syzkaller.appspotmail.com
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

mm/huge_memory.c

@@ -527,6 +527,17 @@ void prep_transhuge_page(struct page *page)
 	set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
 }
 
+bool is_transparent_hugepage(struct page *page)
+{
+	if (!PageCompound(page))
+		return 0;
+
+	page = compound_head(page);
+	return is_huge_zero_page(page) ||
+	       page[1].compound_dtor == TRANSHUGE_PAGE_DTOR;
+}
+EXPORT_SYMBOL_GPL(is_transparent_hugepage);
+
 static unsigned long __thp_get_unmapped_area(struct file *filp, unsigned long len,
 		loff_t off, unsigned long flags, unsigned long size)
 {