ovl: fix IOCB_DIRECT if underlying fs doesn't support direct IO

Normally the check at open time suffices, but e.g loop device does set
IOCB_DIRECT after doing its own checks (which are not sufficent for
overlayfs).

Make sure we don't call the underlying filesystem read/write method with
the IOCB_DIRECT if it's not supported.

Reported-by: Huang Jianan <huangjianan@oppo.com>
Fixes: 16914e6 ("ovl: add ovl_read_iter()")
Cc: <stable@vger.kernel.org> # v4.19
Tested-by: Huang Jianan <huangjianan@oppo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/file.c

@@ -296,6 +296,12 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	if (ret)
 		return ret;
 
+	ret = -EINVAL;
+	if (iocb->ki_flags & IOCB_DIRECT &&
+	    (!real.file->f_mapping->a_ops ||
+	     !real.file->f_mapping->a_ops->direct_IO))
+		goto out_fdput;
+
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	if (is_sync_kiocb(iocb)) {
 		ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
@@ -320,7 +326,7 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 out:
 	revert_creds(old_cred);
 	ovl_file_accessed(file);
-
+out_fdput:
 	fdput(real);
 
 	return ret;
@@ -349,6 +355,12 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	if (ret)
 		goto out_unlock;
 
+	ret = -EINVAL;
+	if (iocb->ki_flags & IOCB_DIRECT &&
+	    (!real.file->f_mapping->a_ops ||
+	     !real.file->f_mapping->a_ops->direct_IO))
+		goto out_fdput;
+
 	if (!ovl_should_sync(OVL_FS(inode->i_sb)))
 		ifl &= ~(IOCB_DSYNC | IOCB_SYNC);
 
@@ -384,6 +396,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	}
 out:
 	revert_creds(old_cred);
+out_fdput:
 	fdput(real);
 
 out_unlock: