tcp: md5: add missing memory barriers in tcp_md5_do_add()/tcp_md5_hash_key()

MD5 keys are read with RCU protection, and tcp_md5_do_add()
might update in-place a prior key.

Normally, typical RCU updates would allocate a new piece
of memory. In this case only key->key and key->keylen might
be updated, and we do not care if an incoming packet could
see the old key, the new one, or some intermediate value,
since changing the key on a live flow is known to be problematic
anyway.

We only want to make sure that in the case key->keylen
is changed, cpus in tcp_md5_hash_key() wont try to use
uninitialized data, or crash because key->keylen was
read twice to feed sg_init_one() and ahash_request_set_crypt()

Fixes: 9ea88a15 ("tcp: md5: check md5 signature without socket lock")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -4033,10 +4033,13 @@ EXPORT_SYMBOL(tcp_md5_hash_skb_data);
 
 int tcp_md5_hash_key(struct tcp_md5sig_pool *hp, const struct tcp_md5sig_key *key)
 {
+	u8 keylen = key->keylen;
 	struct scatterlist sg;
 
-	sg_init_one(&sg, key->key, key->keylen);
-	ahash_request_set_crypt(hp->md5_req, &sg, NULL, key->keylen);
+	smp_rmb(); /* paired with smp_wmb() in tcp_md5_do_add() */
+
+	sg_init_one(&sg, key->key, keylen);
+	ahash_request_set_crypt(hp->md5_req, &sg, NULL, keylen);
 	return crypto_ahash_update(hp->md5_req);
 }
 EXPORT_SYMBOL(tcp_md5_hash_key);

net/ipv4/tcp_ipv4.c

@@ -1113,6 +1113,9 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
 	if (key) {
 		/* Pre-existing entry - just update that one. */
 		memcpy(key->key, newkey, newkeylen);
+
+		smp_wmb(); /* pairs with smp_rmb() in tcp_md5_hash_key() */
+
 		key->keylen = newkeylen;
 		return 0;
 	}