static int ext4_journal_check_start(struct super_block *sb)
{
	journal_t *journal;

	might_sleep();
	if (sb->s_flags & MS_RDONLY)
		return -EROFS;
	WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);
	journal = EXT4_SB(sb)->s_journal;
	/*
	 * Special case here: if the journal has aborted behind our
	 * backs (eg. EIO in the commit thread), then we still need to
	 * take the FS itself readonly cleanly.
	 */
	if (journal && is_journal_aborted(journal)) {
		ext4_abort(sb, "Detected aborted journal");
		return -EROFS;
	}
	return 0;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.