tcp: fix Fast Open key endianness

Fast Open key could be stored in different endian based on the CPU.
Previously hosts in different endianness in a server farm using
the same key config (sysctl value) would produce different cookies.
This patch fixes it by always storing it as little endian to keep
same API for LE hosts.
Reported-by: Daniele Iamartino <danielei@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/sysctl_net_ipv4.c

@@ -265,8 +265,9 @@ static int proc_tcp_fastopen_key(struct ctl_table *table, int write,
 	    ipv4.sysctl_tcp_fastopen);
 	struct ctl_table tbl = { .maxlen = (TCP_FASTOPEN_KEY_LENGTH * 2 + 10) };
 	struct tcp_fastopen_context *ctxt;
-	int ret;
 	u32  user_key[4]; /* 16 bytes, matching TCP_FASTOPEN_KEY_LENGTH */
+	__le32 key[4];
+	int ret, i;
 
 	tbl.data = kmalloc(tbl.maxlen, GFP_KERNEL);
 	if (!tbl.data)
@@ -275,11 +276,14 @@ static int proc_tcp_fastopen_key(struct ctl_table *table, int write,
 	rcu_read_lock();
 	ctxt = rcu_dereference(net->ipv4.tcp_fastopen_ctx);
 	if (ctxt)
-		memcpy(user_key, ctxt->key, TCP_FASTOPEN_KEY_LENGTH);
+		memcpy(key, ctxt->key, TCP_FASTOPEN_KEY_LENGTH);
 	else
-		memset(user_key, 0, sizeof(user_key));
+		memset(key, 0, sizeof(key));
 	rcu_read_unlock();
 
+	for (i = 0; i < ARRAY_SIZE(key); i++)
+		user_key[i] = le32_to_cpu(key[i]);
+
 	snprintf(tbl.data, tbl.maxlen, "%08x-%08x-%08x-%08x",
 		user_key[0], user_key[1], user_key[2], user_key[3]);
 	ret = proc_dostring(&tbl, write, buffer, lenp, ppos);
@@ -290,7 +294,11 @@ static int proc_tcp_fastopen_key(struct ctl_table *table, int write,
 			ret = -EINVAL;
 			goto bad_key;
 		}
-		tcp_fastopen_reset_cipher(net, NULL, user_key,
+
+		for (i = 0; i < ARRAY_SIZE(user_key); i++)
+			key[i] = cpu_to_le32(user_key[i]);
+
+		tcp_fastopen_reset_cipher(net, NULL, key,
 					  TCP_FASTOPEN_KEY_LENGTH);
 	}