netfilter: seqadj: re-load tcp header pointer after possible head reallocation

When adjusting sack block sequence numbers, skb_make_writable() gets
called to make sure tcp options are all in the linear area, and buffer
is not shared.

This can cause tcp header pointer to get reallocated, so we must
reaload it to avoid memory corruption.

This bug pre-dates git history.
Reported-by: Neel Mehta <nmehta@google.com>
Reported-by: Shane Huntley <shuntley@google.com>
Reported-by: Heather Adkins <argv@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_seqadj.c

@@ -115,12 +115,12 @@ static void nf_ct_sack_block_adjust(struct sk_buff *skb,
 /* TCP SACK sequence number adjustment */
 static unsigned int nf_ct_sack_adjust(struct sk_buff *skb,
 				      unsigned int protoff,
-				      struct tcphdr *tcph,
 				      struct nf_conn *ct,
 				      enum ip_conntrack_info ctinfo)
 {
-	unsigned int dir, optoff, optend;
+	struct tcphdr *tcph = (void *)skb->data + protoff;
 	struct nf_conn_seqadj *seqadj = nfct_seqadj(ct);
+	unsigned int dir, optoff, optend;
 
 	optoff = protoff + sizeof(struct tcphdr);
 	optend = protoff + tcph->doff * 4;
@@ -128,6 +128,7 @@ static unsigned int nf_ct_sack_adjust(struct sk_buff *skb,
 	if (!skb_make_writable(skb, optend))
 		return 0;
 
+	tcph = (void *)skb->data + protoff;
 	dir = CTINFO2DIR(ctinfo);
 
 	while (optoff < optend) {
@@ -207,7 +208,7 @@ int nf_ct_seq_adjust(struct sk_buff *skb,
 		 ntohl(newack));
 	tcph->ack_seq = newack;
 
-	res = nf_ct_sack_adjust(skb, protoff, tcph, ct, ctinfo);
+	res = nf_ct_sack_adjust(skb, protoff, ct, ctinfo);
 out:
 	spin_unlock_bh(&ct->lock);