tcp: md5: dont write skb head in tcp_md5_hash_header()

tcp_md5_hash_header() writes into skb header a temporary zero value,
this might confuse other users of this area.

Since tcphdr is small (20 bytes), copy it in a temporary variable and
make the change in the copy.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1209,7 +1209,7 @@ extern void tcp_free_md5sig_pool(void);
 extern struct tcp_md5sig_pool	*tcp_get_md5sig_pool(void);
 extern void tcp_put_md5sig_pool(void);
 
-extern int tcp_md5_hash_header(struct tcp_md5sig_pool *, struct tcphdr *);
+extern int tcp_md5_hash_header(struct tcp_md5sig_pool *, const struct tcphdr *);
 extern int tcp_md5_hash_skb_data(struct tcp_md5sig_pool *, const struct sk_buff *,
 				 unsigned header_len);
 extern int tcp_md5_hash_key(struct tcp_md5sig_pool *hp,

net/ipv4/tcp.c

@@ -2994,17 +2994,19 @@ void tcp_put_md5sig_pool(void)
 EXPORT_SYMBOL(tcp_put_md5sig_pool);
 
 int tcp_md5_hash_header(struct tcp_md5sig_pool *hp,
-			struct tcphdr *th)
+			const struct tcphdr *th)
 {
 	struct scatterlist sg;
+	struct tcphdr hdr;
 	int err;
 
-	__sum16 old_checksum = th->check;
-	th->check = 0;
+	/* We are not allowed to change tcphdr, make a local copy */
+	memcpy(&hdr, th, sizeof(hdr));
+	hdr.check = 0;
+
 	/* options aren't included in the hash */
-	sg_init_one(&sg, th, sizeof(struct tcphdr));
-	err = crypto_hash_update(&hp->md5_desc, &sg, sizeof(struct tcphdr));
-	th->check = old_checksum;
+	sg_init_one(&sg, &hdr, sizeof(hdr));
+	err = crypto_hash_update(&hp->md5_desc, &sg, sizeof(hdr));
 	return err;
 }
 EXPORT_SYMBOL(tcp_md5_hash_header);