userfaultfd: avoid huge_zero_page in UFFDIO_MOVE

While testing UFFDIO_MOVE ioctl, syzbot triggered VM_BUG_ON_PAGE caused by
a call to PageAnonExclusive() with a huge_zero_page as a parameter. 
UFFDIO_MOVE does not yet handle zeropages and returns EBUSY when one is
encountered.  Add an early huge_zero_page check in the PMD move path to
avoid this situation.

Link: https://lkml.kernel.org/r/20240112013935.1474648-1-surenb@google.com
Fixes: adef4406 ("userfaultfd: UFFDIO_MOVE uABI")
Reported-by: syzbot+705209281e36404998f6@syzkaller.appspotmail.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/userfaultfd.c

@@ -1393,6 +1393,12 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
 				err = -ENOENT;
 				break;
 			}
+			/* Avoid moving zeropages for now */
+			if (is_huge_zero_pmd(*src_pmd)) {
+				spin_unlock(ptl);
+				err = -EBUSY;
+				break;
+			}
 
 			/* Check if we can move the pmd without splitting it. */
 			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||