net: qdisc_pkt_len_init() should be more robust

Without proper validation of DODGY packets, we might very well
feed qdisc_pkt_len_init() with invalid GSO packets.

tcp_hdrlen() might access out-of-bound data, so let's use
skb_header_pointer() and proper checks.

Whole story is described in commit d0c081b49137 ("flow_dissector:
properly cap thoff field")

We have the goal of validating DODGY packets earlier in the stack,
so we might very well revert this fix in the future.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Jason Wang <jasowang@redhat.com>
Reported-by: syzbot+9da69ebac7dddd804552@syzkaller.appspotmail.com
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3151,10 +3151,21 @@ static void qdisc_pkt_len_init(struct sk_buff *skb)
 		hdr_len = skb_transport_header(skb) - skb_mac_header(skb);
 
 		/* + transport layer */
-		if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)))
-			hdr_len += tcp_hdrlen(skb);
-		else
-			hdr_len += sizeof(struct udphdr);
+		if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))) {
+			const struct tcphdr *th;
+			struct tcphdr _tcphdr;
+
+			th = skb_header_pointer(skb, skb_transport_offset(skb),
+						sizeof(_tcphdr), &_tcphdr);
+			if (likely(th))
+				hdr_len += __tcp_hdrlen(th);
+		} else {
+			struct udphdr _udphdr;
+
+			if (skb_header_pointer(skb, skb_transport_offset(skb),
+					       sizeof(_udphdr), &_udphdr))
+				hdr_len += sizeof(struct udphdr);
+		}
 
 		if (shinfo->gso_type & SKB_GSO_DODGY)
 			gso_segs = DIV_ROUND_UP(skb->len - hdr_len,