Commit message: net: __skb_flow_dissect() must cap its return value

After Tom patch, thoff field could point past the end of the buffer,
this could fool some callers.

If an skb was provided, skb->len should be the upper limit.
If not, hlen is supposed to be the upper limit.

Fixes: a6e544b0a88b ("flow_dissector: Jump to exit code in __skb_flow_dissect")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Yibin Yang <yibyang@cisco.com
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
CVE: CVE-2017-13715
CWE: CWE-665
Vulnerability description: The __skb_flow_dissect function in net/core/flow_dissector.c in the Linux kernel before 4.3 does not ensure that n_proto, ip_proto, and thoff are initialized, which allows remote attackers to cause a denial of service (system crash) or possibly execute arbitrary code via a single crafted MPLS packet.

Project: linux
File: flow_dissector.c

bool __skb_flow_dissect(const struct sk_buff *skb,
			struct flow_dissector *flow_dissector,
			void *target_container,
			void *data, __be16 proto, int nhoff, int hlen,
			unsigned int flags)
{
	struct flow_dissector_key_control *key_control;
	struct flow_dissector_key_basic *key_basic;
	struct flow_dissector_key_addrs *key_addrs;
	struct flow_dissector_key_ports *key_ports;
	struct flow_dissector_key_tags *key_tags;
	struct flow_dissector_key_vlan *key_vlan;
	struct flow_dissector_key_keyid *key_keyid;
	bool skip_vlan = false;
	u8 ip_proto = 0;
	bool ret = false;

	if (!data) {
		data = skb->data;
		proto = skb_vlan_tag_present(skb) ?
			 skb->vlan_proto : skb->protocol;
		nhoff = skb_network_offset(skb);
		hlen = skb_headlen(skb);
	}

	/* It is ensured by skb_flow_dissector_init() that control key will
	 * be always present.
	 */
	key_control = skb_flow_dissector_target(flow_dissector,
						FLOW_DISSECTOR_KEY_CONTROL,
						target_container);

	/* It is ensured by skb_flow_dissector_init() that basic key will
	 * be always present.
	 */
	key_basic = skb_flow_dissector_target(flow_dissector,
					      FLOW_DISSECTOR_KEY_BASIC,
					      target_container);

	if (dissector_uses_key(flow_dissector,
			       FLOW_DISSECTOR_KEY_ETH_ADDRS)) {
		struct ethhdr *eth = eth_hdr(skb);
		struct flow_dissector_key_eth_addrs *key_eth_addrs;

		key_eth_addrs = skb_flow_dissector_target(flow_dissector,
							  FLOW_DISSECTOR_KEY_ETH_ADDRS,
							  target_container);
		memcpy(key_eth_addrs, &eth->h_dest, sizeof(*key_eth_addrs));
	}

again:
	switch (proto) {
	case htons(ETH_P_IP): {
		const struct iphdr *iph;
		struct iphdr _iph;
ip:
		iph = __skb_header_pointer(skb, nhoff, sizeof(_iph), data, hlen, &_iph);
		if (!iph || iph->ihl < 5)
			goto out_bad;
		nhoff += iph->ihl * 4;

		ip_proto = iph->protocol;

		if (dissector_uses_key(flow_dissector,
				       FLOW_DISSECTOR_KEY_IPV4_ADDRS)) {
			key_addrs = skb_flow_dissector_target(flow_dissector,
							      FLOW_DISSECTOR_KEY_IPV4_ADDRS,
							      target_container);

			memcpy(&key_addrs->v4addrs, &iph->saddr,
			       sizeof(key_addrs->v4addrs));
			key_control->addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
		}

		if (ip_is_fragment(iph)) {
			key_control->flags |= FLOW_DIS_IS_FRAGMENT;

			if (iph->frag_off & htons(IP_OFFSET)) {
				goto out_good;
			} else {
				key_control->flags |= FLOW_DIS_FIRST_FRAG;
				if (!(flags & FLOW_DISSECTOR_F_PARSE_1ST_FRAG))
					goto out_good;
			}
		}

		if (flags & FLOW_DISSECTOR_F_STOP_AT_L3)
			goto out_good;

		break;
	}
	case htons(ETH_P_IPV6): {
		const struct ipv6hdr *iph;
		struct ipv6hdr _iph;

ipv6:
		iph = __skb_header_pointer(skb, nhoff, sizeof(_iph), data, hlen, &_iph);
		if (!iph)
			goto out_bad;

		ip_proto = iph->nexthdr;
		nhoff += sizeof(struct ipv6hdr);

		if (dissector_uses_key(flow_dissector,
				       FLOW_DISSECTOR_KEY_IPV6_ADDRS)) {
			key_addrs = skb_flow_dissector_target(flow_dissector,
							      FLOW_DISSECTOR_KEY_IPV6_ADDRS,
							      target_container);

			memcpy(&key_addrs->v6addrs, &iph->saddr,
			       sizeof(key_addrs->v6addrs));
			key_control->addr_type = FLOW_DISSECTOR_KEY_IPV6_ADDRS;
		}

		if ((dissector_uses_key(flow_dissector,
					FLOW_DISSECTOR_KEY_FLOW_LABEL) ||
		     (flags & FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL)) &&
		    ip6_flowlabel(iph)) {
			__be32 flow_label = ip6_flowlabel(iph);

			if (dissector_uses_key(flow_dissector,
					       FLOW_DISSECTOR_KEY_FLOW_LABEL)) {
				key_tags = skb_flow_dissector_target(flow_dissector,
								     FLOW_DISSECTOR_KEY_FLOW_LABEL,
								     target_container);
				key_tags->flow_label = ntohl(flow_label);
			}
			if (flags & FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL)
				goto out_good;
		}

		if (flags & FLOW_DISSECTOR_F_STOP_AT_L3)
			goto out_good;

		break;
	}
	case htons(ETH_P_8021AD):
	case htons(ETH_P_8021Q): {
		const struct vlan_hdr *vlan;
		struct vlan_hdr _vlan;
		bool vlan_tag_present = skb && skb_vlan_tag_present(skb);

		if (vlan_tag_present)
			proto = skb->protocol;

		if (!vlan_tag_present || eth_type_vlan(skb->protocol)) {
			vlan = __skb_header_pointer(skb, nhoff, sizeof(_vlan),
						    data, hlen, &_vlan);
			if (!vlan)
				goto out_bad;
			proto = vlan->h_vlan_encapsulated_proto;
			nhoff += sizeof(*vlan);
			if (skip_vlan)
				goto again;
		}

		skip_vlan = true;
		if (dissector_uses_key(flow_dissector,
				       FLOW_DISSECTOR_KEY_VLAN)) {
			key_vlan = skb_flow_dissector_target(flow_dissector,
							     FLOW_DISSECTOR_KEY_VLAN,
							     target_container);

			if (vlan_tag_present) {
				key_vlan->vlan_id = skb_vlan_tag_get_id(skb);
				key_vlan->vlan_priority =
					(skb_vlan_tag_get_prio(skb) >> VLAN_PRIO_SHIFT);
			} else {
				key_vlan->vlan_id = ntohs(vlan->h_vlan_TCI) &
					VLAN_VID_MASK;
				key_vlan->vlan_priority =
					(ntohs(vlan->h_vlan_TCI) &
					 VLAN_PRIO_MASK) >> VLAN_PRIO_SHIFT;
			}
		}

		goto again;
	}
	case htons(ETH_P_PPP_SES): {
		struct {
			struct pppoe_hdr hdr;
			__be16 proto;
		} *hdr, _hdr;
		hdr = __skb_header_pointer(skb, nhoff, sizeof(_hdr), data, hlen, &_hdr);
		if (!hdr)
			goto out_bad;
		proto = hdr->proto;
		nhoff += PPPOE_SES_HLEN;
		switch (proto) {
		case htons(PPP_IP):
			goto ip;
		case htons(PPP_IPV6):
			goto ipv6;
		default:
			goto out_bad;
		}
	}
	case htons(ETH_P_TIPC): {
		struct {
			__be32 pre[3];
			__be32 srcnode;
		} *hdr, _hdr;
		hdr = __skb_header_pointer(skb, nhoff, sizeof(_hdr), data, hlen, &_hdr);
		if (!hdr)
			goto out_bad;

		if (dissector_uses_key(flow_dissector,
				       FLOW_DISSECTOR_KEY_TIPC_ADDRS)) {
			key_addrs = skb_flow_dissector_target(flow_dissector,
							      FLOW_DISSECTOR_KEY_TIPC_ADDRS,
							      target_container);
			key_addrs->tipcaddrs.srcnode = hdr->srcnode;
			key_control->addr_type = FLOW_DISSECTOR_KEY_TIPC_ADDRS;
		}
		goto out_good;
	}

	case htons(ETH_P_MPLS_UC):
	case htons(ETH_P_MPLS_MC): {
		struct mpls_label *hdr, _hdr[2];
mpls:
		hdr = __skb_header_pointer(skb, nhoff, sizeof(_hdr), data,
					   hlen, &_hdr);
		if (!hdr)
			goto out_bad;

		if ((ntohl(hdr[0].entry) & MPLS_LS_LABEL_MASK) >>
		     MPLS_LS_LABEL_SHIFT == MPLS_LABEL_ENTROPY) {
			if (dissector_uses_key(flow_dissector,
					       FLOW_DISSECTOR_KEY_MPLS_ENTROPY)) {
				key_keyid = skb_flow_dissector_target(flow_dissector,
								      FLOW_DISSECTOR_KEY_MPLS_ENTROPY,
								      target_container);
				key_keyid->keyid = hdr[1].entry &
					htonl(MPLS_LS_LABEL_MASK);
			}

			goto out_good;
		}

		goto out_good;
	}

	case htons(ETH_P_FCOE):
		if ((hlen - nhoff) < FCOE_HEADER_LEN)
			goto out_bad;

		nhoff += FCOE_HEADER_LEN;
		goto out_good;
	default:
		goto out_bad;
	}

ip_proto_again:
	switch (ip_proto) {
	case IPPROTO_GRE: {
		struct gre_base_hdr *hdr, _hdr;
		u16 gre_ver;
		int offset = 0;

		hdr = __skb_header_pointer(skb, nhoff, sizeof(_hdr), data, hlen, &_hdr);
		if (!hdr)
			goto out_bad;

		/* Only look inside GRE without routing */
		if (hdr->flags & GRE_ROUTING)
			break;

		/* Only look inside GRE for version 0 and 1 */
		gre_ver = ntohs(hdr->flags & GRE_VERSION);
		if (gre_ver > 1)
			break;

		proto = hdr->protocol;
		if (gre_ver) {
			/* Version1 must be PPTP, and check the flags */
			if (!(proto == GRE_PROTO_PPP && (hdr->flags & GRE_KEY)))
				break;
		}

		offset += sizeof(struct gre_base_hdr);

		if (hdr->flags & GRE_CSUM)
			offset += sizeof(((struct gre_full_hdr *)0)->csum) +
				  sizeof(((struct gre_full_hdr *)0)->reserved1);

		if (hdr->flags & GRE_KEY) {
			const __be32 *keyid;
			__be32 _keyid;

			keyid = __skb_header_pointer(skb, nhoff + offset, sizeof(_keyid),
						     data, hlen, &_keyid);
			if (!keyid)
				goto out_bad;

			if (dissector_uses_key(flow_dissector,
					       FLOW_DISSECTOR_KEY_GRE_KEYID)) {
				key_keyid = skb_flow_dissector_target(flow_dissector,
								      FLOW_DISSECTOR_KEY_GRE_KEYID,
								      target_container);
				if (gre_ver == 0)
					key_keyid->keyid = *keyid;
				else
					key_keyid->keyid = *keyid & GRE_PPTP_KEY_MASK;
			}
			offset += sizeof(((struct gre_full_hdr *)0)->key);
		}

		if (hdr->flags & GRE_SEQ)
			offset += sizeof(((struct pptp_gre_header *)0)->seq);

		if (gre_ver == 0) {
			if (proto == htons(ETH_P_TEB)) {
				const struct ethhdr *eth;
				struct ethhdr _eth;

				eth = __skb_header_pointer(skb, nhoff + offset,
							   sizeof(_eth),
							   data, hlen, &_eth);
				if (!eth)
					goto out_bad;
				proto = eth->h_proto;
				offset += sizeof(*eth);

				/* Cap headers that we access via pointers at the
				 * end of the Ethernet header as our maximum alignment
				 * at that point is only 2 bytes.
				 */
				if (NET_IP_ALIGN)
					hlen = (nhoff + offset);
			}
		} else { /* version 1, must be PPTP */
			u8 _ppp_hdr[PPP_HDRLEN];
			u8 *ppp_hdr;

			if (hdr->flags & GRE_ACK)
				offset += sizeof(((struct pptp_gre_header *)0)->ack);

			ppp_hdr = skb_header_pointer(skb, nhoff + offset,
						     sizeof(_ppp_hdr), _ppp_hdr);
			if (!ppp_hdr)
				goto out_bad;

			switch (PPP_PROTOCOL(ppp_hdr)) {
			case PPP_IP:
				proto = htons(ETH_P_IP);
				break;
			case PPP_IPV6:
				proto = htons(ETH_P_IPV6);
				break;
			default:
				/* Could probably catch some more like MPLS */
				break;
			}

			offset += PPP_HDRLEN;
		}

		nhoff += offset;
		key_control->flags |= FLOW_DIS_ENCAPSULATION;
		if (flags & FLOW_DISSECTOR_F_STOP_AT_ENCAP)
			goto out_good;

		goto again;
	}
	case NEXTHDR_HOP:
	case NEXTHDR_ROUTING:
	case NEXTHDR_DEST: {
		u8 _opthdr[2], *opthdr;

		if (proto != htons(ETH_P_IPV6))
			break;

		opthdr = __skb_header_pointer(skb, nhoff, sizeof(_opthdr),
					      data, hlen, &_opthdr);
		if (!opthdr)
			goto out_bad;

		ip_proto = opthdr[0];
		nhoff += (opthdr[1] + 1) << 3;

		goto ip_proto_again;
	}
	case NEXTHDR_FRAGMENT: {
		struct frag_hdr _fh, *fh;

		if (proto != htons(ETH_P_IPV6))
			break;

		fh = __skb_header_pointer(skb, nhoff, sizeof(_fh),
					  data, hlen, &_fh);

		if (!fh)
			goto out_bad;

		key_control->flags |= FLOW_DIS_IS_FRAGMENT;

		nhoff += sizeof(_fh);
		ip_proto = fh->nexthdr;

		if (!(fh->frag_off & htons(IP6_OFFSET))) {
			key_control->flags |= FLOW_DIS_FIRST_FRAG;
			if (flags & FLOW_DISSECTOR_F_PARSE_1ST_FRAG)
				goto ip_proto_again;
		}
		goto out_good;
	}
	case IPPROTO_IPIP:
		proto = htons(ETH_P_IP);

		key_control->flags |= FLOW_DIS_ENCAPSULATION;
		if (flags & FLOW_DISSECTOR_F_STOP_AT_ENCAP)
			goto out_good;

		goto ip;
	case IPPROTO_IPV6:
		proto = htons(ETH_P_IPV6);

		key_control->flags |= FLOW_DIS_ENCAPSULATION;
		if (flags & FLOW_DISSECTOR_F_STOP_AT_ENCAP)
			goto out_good;

		goto ipv6;
	case IPPROTO_MPLS:
		proto = htons(ETH_P_MPLS_UC);
		goto mpls;
	default:
		break;
	}

	if (dissector_uses_key(flow_dissector,
			       FLOW_DISSECTOR_KEY_PORTS)) {
		key_ports = skb_flow_dissector_target(flow_dissector,
						      FLOW_DISSECTOR_KEY_PORTS,
						      target_container);
		key_ports->ports = __skb_flow_get_ports(skb, nhoff, ip_proto,
							data, hlen);
	}

out_good:
	ret = true;

out_bad:
	key_basic->n_proto = proto;
	key_basic->ip_proto = ip_proto;
	key_control->thoff = (u16)nhoff;

	return ret;
}