ovl: check whiteout in ovl_create_over_whiteout()

Kaixuxia repors that it's possible to crash overlayfs by removing the
whiteout on the upper layer before creating a directory over it.  This is a
reproducer:

 mkdir lower upper work merge
 touch lower/file
 mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merge
 rm merge/file
 ls -al merge/file
 rm upper/file
 ls -al merge/
 mkdir merge/file

Before commencing with a vfs_rename(..., RENAME_EXCHANGE) verify that the
lookup of "upper" is positive and is a whiteout, and return ESTALE
otherwise.

Reported by: kaixuxia <xiakaixu1987@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: e9be9d5e ("overlay filesystem")
Cc: <stable@vger.kernel.org> # v3.18

fs/overlayfs/dir.c

@@ -462,6 +462,10 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
 	if (IS_ERR(upper))
 		goto out_unlock;
 
+	err = -ESTALE;
+	if (d_is_negative(upper) || !IS_WHITEOUT(d_inode(upper)))
+		goto out_dput;
+
 	newdentry = ovl_create_temp(workdir, cattr);
 	err = PTR_ERR(newdentry);
 	if (IS_ERR(newdentry))