Commit message: net_sched: cls_route: remove the right filter from hashtable

route4_change() allocates a new filter and copies values from
the old one. After the new filter is inserted into the hash
table, the old filter should be removed and freed, as the final
step of the update.

However, the current code mistakenly removes the new one. This
looks apparently wrong to me, and it causes double "free" and
use-after-free too, as reported by syzbot.

Reported-and-tested-by: syzbot+f9b32aaacd60305d9687@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+2f8c233f131943d6056d@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+9c2df9fd5e9445b74e01@syzkaller.appspotmail.com
Fixes: 1109c00547fc ("net: sched: RCU cls_route")
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
CVE: CVE-2021-3715
CWE: CWE-200
Vulnerability description: A flaw was found in the "Routing decision" classifier in the Linux kernel's Traffic Control networking subsystem in the way it handled changing of classification filters, leading to a use-after-free condition. This flaw allows unprivileged local users to escalate their privileges on the system. The highest threat from this vulnerability is to confidentiality, integrity, as well as system availability.

Project: linux
File: cls_route.c

static int route4_change(struct net *net, struct sk_buff *in_skb,
			 struct tcf_proto *tp, unsigned long base, u32 handle,
			 struct nlattr **tca, void **arg, bool ovr,
			 bool rtnl_held, struct netlink_ext_ack *extack)
{
	struct route4_head *head = rtnl_dereference(tp->root);
	struct route4_filter __rcu **fp;
	struct route4_filter *fold, *f1, *pfp, *f = NULL;
	struct route4_bucket *b;
	struct nlattr *opt = tca[TCA_OPTIONS];
	struct nlattr *tb[TCA_ROUTE4_MAX + 1];
	unsigned int h, th;
	int err;
	bool new = true;

	if (opt == NULL)
		return handle ? -EINVAL : 0;

	err = nla_parse_nested_deprecated(tb, TCA_ROUTE4_MAX, opt,
					  route4_policy, NULL);
	if (err < 0)
		return err;

	fold = *arg;
	if (fold && handle && fold->handle != handle)
			return -EINVAL;

	err = -ENOBUFS;
	f = kzalloc(sizeof(struct route4_filter), GFP_KERNEL);
	if (!f)
		goto errout;

	err = tcf_exts_init(&f->exts, net, TCA_ROUTE4_ACT, TCA_ROUTE4_POLICE);
	if (err < 0)
		goto errout;

	if (fold) {
		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;
	}

	err = route4_set_parms(net, tp, base, f, handle, head, tb,
			       tca[TCA_RATE], new, ovr, extack);
	if (err < 0)
		goto errout;

	h = from_hash(f->handle >> 16);
	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);

	if (fold && fold->handle && f->handle != fold->handle) {
		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 == f) {
					*fp = f->next;
					break;
				}
			}
		}
	}

	route4_reset_fastmap(head);
	*arg = f;
	if (fold) {
		tcf_unbind_filter(tp, &fold->res);
		tcf_exts_get_net(&fold->exts);
		tcf_queue_work(&fold->rwork, route4_delete_filter_work);
	}
	return 0;

errout:
	if (f)
		tcf_exts_destroy(&f->exts);
	kfree(f);
	return err;
}