net/dccp: fix use-after-free in dccp_invalid_packet

pskb_may_pull() can reallocate skb->head, we need to reload dh pointer
in dccp_invalid_packet() or risk use after free.

Bug found by Andrey Konovalov using syzkaller.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ipv4.c

@@ -700,6 +700,7 @@ int dccp_invalid_packet(struct sk_buff *skb)
 {
 	const struct dccp_hdr *dh;
 	unsigned int cscov;
+	u8 dccph_doff;
 
 	if (skb->pkt_type != PACKET_HOST)
 		return 1;
@@ -721,18 +722,19 @@ int dccp_invalid_packet(struct sk_buff *skb)
 	/*
 	 * If P.Data Offset is too small for packet type, drop packet and return
 	 */
-	if (dh->dccph_doff < dccp_hdr_len(skb) / sizeof(u32)) {
-		DCCP_WARN("P.Data Offset(%u) too small\n", dh->dccph_doff);
+	dccph_doff = dh->dccph_doff;
+	if (dccph_doff < dccp_hdr_len(skb) / sizeof(u32)) {
+		DCCP_WARN("P.Data Offset(%u) too small\n", dccph_doff);
 		return 1;
 	}
 	/*
 	 * If P.Data Offset is too too large for packet, drop packet and return
 	 */
-	if (!pskb_may_pull(skb, dh->dccph_doff * sizeof(u32))) {
-		DCCP_WARN("P.Data Offset(%u) too large\n", dh->dccph_doff);
+	if (!pskb_may_pull(skb, dccph_doff * sizeof(u32))) {
+		DCCP_WARN("P.Data Offset(%u) too large\n", dccph_doff);
 		return 1;
 	}
-
+	dh = dccp_hdr(skb);
 	/*
 	 * If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
 	 * has short sequence numbers), drop packet and return