udf: fix silent AED tagLocation corruption

When extending a file, udf_do_extend_file() may enter following empty
indirect extent. At the end of udf_do_extend_file() we revert prev_epos
to point to the last written extent. However if we end up not adding any
further extent in udf_do_extend_file(), the reverting points prev_epos
into the header area of the AED and following updates of the extents
(in udf_update_extents()) will corrupt the header.

Make sure that we do not follow indirect extent if we are not going to
add any more extents so that returning back to the last written extent
works correctly.

Link: https://lore.kernel.org/r/20210107234116.6190-2-magnani@ieee.orgSigned-off-by: NSteven J. Magnani <magnani@ieee.org>
Signed-off-by: NJan Kara <jack@suse.cz>

fs/udf/inode.c

@@ -544,11 +544,14 @@ static int udf_do_extend_file(struct inode *inode,
 
 		udf_write_aext(inode, last_pos, &last_ext->extLocation,
 				last_ext->extLength, 1);
+
 		/*
-		 * We've rewritten the last extent but there may be empty
-		 * indirect extent after it - enter it.
+		 * We've rewritten the last extent. If we are going to add
+		 * more extents, we may need to enter possible following
+		 * empty indirect extent.
 		 */
-		udf_next_aext(inode, last_pos, &tmploc, &tmplen, 0);
+		if (new_block_bytes || prealloc_len)
+			udf_next_aext(inode, last_pos, &tmploc, &tmplen, 0);
 	}
 
 	/* Managed to do everything necessary? */