Commit message: netfilter: add back stackpointer size checks

The rationale for removing the check is only correct for rulesets
generated by ip(6)tables.

In iptables, a jump can only occur to a user-defined chain, i.e.
because we size the stack based on number of user-defined chains we
cannot exceed stack size.

However, the underlying binary format has no such restriction,
and the validation step only ensures that the jump target is a
valid rule start point.

IOW, its possible to build a rule blob that has no user-defined
chains but does contain a jump.

If this happens, no jump stack gets allocated and crash occurs
because no jumpstack was allocated.

Fixes: 7814b6ec6d0d6 ("netfilter: xtables: don't save/restore jumpstack offset")
Reported-by: syzbot+e783f671527912cd9403@syzkaller.appspotmail.com
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
CVE: CVE-2018-1065
CWE: CWE-476
Vulnerability description: The netfilter subsystem in the Linux kernel through 4.15.7 mishandles the case of a rule blob that contains a jump but lacks a user-defined chain, which allows local users to cause a denial of service (NULL pointer dereference) by leveraging the CAP_NET_RAW or CAP_NET_ADMIN capability, related to arpt_do_table in net/ipv4/netfilter/arp_tables.c, ipt_do_table in net/ipv4/netfilter/ip_tables.c, and ip6t_do_table in net/ipv6/netfilter/ip6_tables.c.

Project: linux

ipt_do_table(struct sk_buff *skb,
	     const struct nf_hook_state *state,
	     struct xt_table *table)
{
	unsigned int hook = state->hook;
	static const char nulldevname[IFNAMSIZ] __attribute__((aligned(sizeof(long))));
	const struct iphdr *ip;
	/* Initializing verdict to NF_DROP keeps gcc happy. */
	unsigned int verdict = NF_DROP;
	const char *indev, *outdev;
	const void *table_base;
	struct ipt_entry *e, **jumpstack;
	unsigned int stackidx, cpu;
	const struct xt_table_info *private;
	struct xt_action_param acpar;
	unsigned int addend;

	/* Initialization */
	stackidx = 0;
	ip = ip_hdr(skb);
	indev = state->in ? state->in->name : nulldevname;
	outdev = state->out ? state->out->name : nulldevname;
	/* We handle fragments by dealing with the first fragment as
	 * if it was a normal packet.  All other fragments are treated
	 * normally, except that they will NEVER match rules that ask
	 * things we don't know, ie. tcp syn flag or ports).  If the
	 * rule is also a fragment-specific rule, non-fragments won't
	 * match it. */
	acpar.fragoff = ntohs(ip->frag_off) & IP_OFFSET;
	acpar.thoff   = ip_hdrlen(skb);
	acpar.hotdrop = false;
	acpar.state   = state;

	WARN_ON(!(table->valid_hooks & (1 << hook)));
	local_bh_disable();
	addend = xt_write_recseq_begin();
	private = READ_ONCE(table->private); /* Address dependency. */
	cpu        = smp_processor_id();
	table_base = private->entries;
	jumpstack  = (struct ipt_entry **)private->jumpstack[cpu];

	/* Switch to alternate jumpstack if we're being invoked via TEE.
	 * TEE issues XT_CONTINUE verdict on original skb so we must not
	 * clobber the jumpstack.
	 *
	 * For recursion via REJECT or SYNPROXY the stack will be clobbered
	 * but it is no problem since absolute verdict is issued by these.
	 */
	if (static_key_false(&xt_tee_enabled))
		jumpstack += private->stacksize * __this_cpu_read(nf_skb_duplicated);

	e = get_entry(table_base, private->hook_entry[hook]);

	do {
		const struct xt_entry_target *t;
		const struct xt_entry_match *ematch;
		struct xt_counters *counter;

		WARN_ON(!e);
		if (!ip_packet_match(ip, indev, outdev,
		    &e->ip, acpar.fragoff)) {
 no_match:
			e = ipt_next_entry(e);
			continue;
		}

		xt_ematch_foreach(ematch, e) {
			acpar.match     = ematch->u.kernel.match;
			acpar.matchinfo = ematch->data;
			if (!acpar.match->match(skb, &acpar))
				goto no_match;
		}

		counter = xt_get_this_cpu_counter(&e->counters);
		ADD_COUNTER(*counter, skb->len, 1);

		t = ipt_get_target(e);
		WARN_ON(!t->u.kernel.target);

#if IS_ENABLED(CONFIG_NETFILTER_XT_TARGET_TRACE)
		/* The packet is traced: log it */
		if (unlikely(skb->nf_trace))
			trace_packet(state->net, skb, hook, state->in,
				     state->out, table->name, private, e);
#endif
		/* Standard target? */
		if (!t->u.kernel.target->target) {
			int v;

			v = ((struct xt_standard_target *)t)->verdict;
			if (v < 0) {
				/* Pop from stack? */
				if (v != XT_RETURN) {
					verdict = (unsigned int)(-v) - 1;
					break;
				}
				if (stackidx == 0) {
					e = get_entry(table_base,
					    private->underflow[hook]);
				} else {
					e = jumpstack[--stackidx];
					e = ipt_next_entry(e);
				}
 				continue;
 			}
 			if (table_base + v != ipt_next_entry(e) &&
			    !(e->ip.flags & IPT_F_GOTO))
 				jumpstack[stackidx++] = e;
 
 			e = get_entry(table_base, v);
 			continue;
		}

		acpar.target   = t->u.kernel.target;
		acpar.targinfo = t->data;

		verdict = t->u.kernel.target->target(skb, &acpar);
		if (verdict == XT_CONTINUE) {
			/* Target might have changed stuff. */
			ip = ip_hdr(skb);
			e = ipt_next_entry(e);
		} else {
			/* Verdict */
			break;
		}
	} while (!acpar.hotdrop);

	xt_write_recseq_end(addend);
	local_bh_enable();

	if (acpar.hotdrop)
		return NF_DROP;
	else return verdict;
}