psk_client_callback, 128-byte id bug.

Fix a bug in handling of 128 byte long PSK identity in
psk_client_callback.

OpenSSL supports PSK identities of up to (and including) 128 bytes in
length. PSK identity is obtained via the psk_client_callback,
implementors of which are expected to provide a NULL-terminated
identity. However, the callback is invoked with only 128 bytes of
storage thus making it impossible to return a 128 byte long identity and
the required additional NULL byte.

This CL fixes the issue by passing in a 129 byte long buffer into the
psk_client_callback. As a safety precaution, this CL also zeroes out the
buffer before passing it into the callback, uses strnlen for obtaining
the length of the identity returned by the callback, and aborts the
handshake if the identity (without the NULL terminator) is longer than
128 bytes.

(Original patch amended to achieve strnlen in a different way.)
Reviewed-by: NRich Salz <rsalz@openssl.org>

ssl/s3_clnt.c

@@ -3119,7 +3119,11 @@ int ssl3_send_client_key_exchange(SSL *s)
 #ifndef OPENSSL_NO_PSK
 		else if (alg_k & SSL_kPSK)
 			{
-			char identity[PSK_MAX_IDENTITY_LEN];
+			/* The callback needs PSK_MAX_IDENTITY_LEN + 1 bytes
+			 * to return a \0-terminated identity. The last byte
+			 * is for us for simulating strnlen. */
+			char identity[PSK_MAX_IDENTITY_LEN + 2];
+			size_t identity_len;
 			unsigned char *t = NULL;
 			unsigned char psk_or_pre_ms[PSK_MAX_PSK_LEN*2+4];
 			unsigned int pre_ms_len = 0, psk_len = 0;
@@ -3133,8 +3137,9 @@ int ssl3_send_client_key_exchange(SSL *s)
 				goto err;
 				}
 
+			memset(identity, 0, sizeof(identity));
 			psk_len = s->psk_client_callback(s, s->ctx->psk_identity_hint,
-				identity, PSK_MAX_IDENTITY_LEN,
+				identity, sizeof(identity) - 1,
 				psk_or_pre_ms, sizeof(psk_or_pre_ms));
 			if (psk_len > PSK_MAX_PSK_LEN)
 				{
@@ -3148,7 +3153,14 @@ int ssl3_send_client_key_exchange(SSL *s)
 					SSL_R_PSK_IDENTITY_NOT_FOUND);
 				goto psk_err;
 				}
-
+			identity[PSK_MAX_IDENTITY_LEN + 1] = '\0';
+			identity_len = strlen(identity);
+			if (identity_len > PSK_MAX_IDENTITY_LEN)
+				{
+				SSLerr(SSL_F_SSL3_SEND_CLIENT_KEY_EXCHANGE,
+					ERR_R_INTERNAL_ERROR);
+				goto psk_err;
+				}
 			/* create PSK pre_master_secret */
 			pre_ms_len = 2+psk_len+2+psk_len;
 			t = psk_or_pre_ms;
@@ -3183,13 +3195,12 @@ int ssl3_send_client_key_exchange(SSL *s)
 				s->method->ssl3_enc->generate_master_secret(s,
 					s->session->master_key,
 					psk_or_pre_ms, pre_ms_len);
-			n = strlen(identity);
-			s2n(n, p);
-			memcpy(p, identity, n);
-			n+=2;
+			s2n(identity_len, p);
+			memcpy(p, identity, identity_len);
+			n = 2 + identity_len;
 			psk_err = 0;
 		psk_err:
-			OPENSSL_cleanse(identity, PSK_MAX_IDENTITY_LEN);
+			OPENSSL_cleanse(identity, sizeof(identity));
 			OPENSSL_cleanse(psk_or_pre_ms, sizeof(psk_or_pre_ms));
 			if (psk_err != 0)
 				{