xfs: prevent swapext from operating on write-only files

This patch prevents user "foo" from using the SWAPEXT ioctl to swap
a write-only file owned by user "bar" into a file owned by "foo" and
subsequently reading it. It does so by checking that the file
descriptors passed to the ioctl are also opened for reading.

Signed-off-by: Dan Rosenberg <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>

fs/xfs/xfs_dfrag.c

···
 		goto out;
 	}
 
-	if (!(file->f_mode & FMODE_WRITE) || (file->f_flags & O_APPEND)) {
+	if (!(file->f_mode & FMODE_WRITE) ||
+	    !(file->f_mode & FMODE_READ) ||
+	    (file->f_flags & O_APPEND)) {
 		error = XFS_ERROR(EBADF);
 		goto out_put_file;
 	}
···
 	}
 
 	if (!(tmp_file->f_mode & FMODE_WRITE) ||
+	    !(tmp_file->f_mode & FMODE_READ) ||
 	    (tmp_file->f_flags & O_APPEND)) {
 		error = XFS_ERROR(EBADF);
 		goto out_put_tmp_file;