[tls] Poison initial key derivation function master secret

A freshly initialised key derivation function master secret has the
"keyed" flag clear and so cannot accidentally be used to establish a
full TLS connection.

As an additional layer of defence in depth, poison the initial key
derivation master secret so that a predictable all-zero secret can
never be used accidentally in future.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

diff --git a/src/net/tls.c b/src/net/tls.c
index f0cad9f..9c2f4cd 100644
--- a/src/net/tls.c
+++ b/src/net/tls.c
@@ -549,6 +549,9 @@
 	/* Initialise handshake context */
 	digest_init ( digest, key->handshake );
 
+	/* Poison key derivation function master secret */
+	tls_ephemeral_label ( tls, "kdf poison", key->kdf, kdfsize );
+
 	/* Sanity checks */
 	assert ( ! key->keyed );
 	assert ( ! key->bound );