tcp: sysctl interface leaks 16 bytes of kernel memory

If the rc_dereference of tcp_fastopen_ctx ever fails then we copy 16 bytes
of kernel stack into the proc result.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/sysctl_net_ipv4.c

@@ -248,6 +248,8 @@ int proc_tcp_fastopen_key(ctl_table *ctl, int write, void __user *buffer,
 	ctxt = rcu_dereference(tcp_fastopen_ctx);
 	if (ctxt)
 		memcpy(user_key, ctxt->key, TCP_FASTOPEN_KEY_LENGTH);
+	else
+		memset(user_key, 0, sizeof(user_key));
 	rcu_read_unlock();
 
 	snprintf(tbl.data, tbl.maxlen, "%08x-%08x-%08x-%08x",