0ctf 2017 kernel pwn knote write up

UAF due to using hlist_add_behind() without checking.

There is a pair locker(mutex_lock) at delete_note(), but isn’t at edit_note_time().

And it doesn’t check the flag before hlist_add_behind() in insert_note().

    for(;;) {
        /* add before a larger epoch */
        iter = hlist_entry(node, struct note_t, next);
        if (iter->epoch > epoch) {
            hlist_add_before(&(note->next), node);
            flag = true;
            break;
        }

        if (node->next == NULL)
            break;

        node = node->next;
    }

    /* at behind the last node */
    // if (!flag)  <-- patch...
	// it can lead to hlist broken.
    hlist_add_behind(&(note->next), node);

Exploitation:

1. UaF 

　　First we could free arbitrary object (eg. tty_struct) via any vulnerabilities,
re-allocate fake object with evil functions or rop gadgets.
Finally we can call related function in user mode.

2. kernel info leak

　　should use the kzalloc() instead of kmalloc()