Fix MFLN probe and allow returning whether MFLN succeeded or not after a connection. (#6000)

Fixes #5996

* Add extensions to probe message for EC, others

probeMFLN was failing on some connection attempts to servers which only
supported EC based ciphers because it did not include the proper TLS
handshake extensions to list what kinds of ECs it supported.

Add those to the probeMFLN ClientHello message to make probes pass.

* Add client.getMFLNStatus method, returns MFLN state

After a connection it is useful to check whether MFLN negotiation
succeeded.  getMFLNStatus returns a bool (valid only after
client.connect() succeeds, of course) indicating whether the requested
buffer sizes were negotiated successfully.

Author: earlephilhower

doc/esp8266wifi/bearssl-client-secure-class.rst

@@ -180,6 +180,11 @@ Once you have verified (or know beforehand) that MFLN is supported you can use t
 
 In certain applications where the TLS server does not support MFLN (not many do as of this writing as it is relatively new to OpenSSL), but you control both the ESP8266 and the server to which it is communicating, you may still be able to `setBufferSizes()` smaller if you guarantee no chunk of data will overflow those buffers.
 
+bool getMFLNStatus()
+^^^^^^^^^^^^^^^^^^^^
+
+After a successful connection, this method returns whether or not MFLN negotiation succeeded or not.  If it did not succeed, and you reduced the receive buffer with `setBufferSizes` then you may experience reception errors if the server attempts to send messages larger than your receive buffer.
+
 Sessions (Resuming connections fast)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

libraries/ESP8266WiFi/examples/BearSSL_MaxFragmentLength/BearSSL_MaxFragmentLength.ino

@@ -87,6 +87,7 @@ int fetchMaxFragmentLength() {
   }
   client.connect("tls.mbed.org", 443);
   if (client.connected()) {
+    Serial.printf("MFLN status: %s\n", client.getMFLNStatus() ? "true" : "false");
     Serial.printf("Memory used: %d\n", ret - ESP.getFreeHeap());
     ret -= ESP.getFreeHeap();
     fetch(&client);

libraries/ESP8266WiFi/keywords.txt

@@ -183,6 +183,7 @@ setBufferSizes	KEYWORD2
 getLastSSLError	KEYWORD2
 setCertStore	KEYWORD2
 probeMaxFragmentLength	KEYWORD2
+getMFLNStatus	KEYWORD2
 
 #WiFiServerBearSSL
 setRSACert	KEYWORD2

libraries/ESP8266WiFi/src/WiFiClientSecureBearSSL.cpp

@@ -965,6 +965,7 @@ bool WiFiClientSecure::_connectSSL(const char* hostName) {
     return false;
   }
   br_ssl_engine_set_buffers_bidi(_eng, _iobuf_in.get(), _iobuf_in_size, _iobuf_out.get(), _iobuf_out_size);
+
   // Apply any client certificates, if supplied.
   if (_sk && _sk->isRSA()) {
     br_ssl_client_set_single_rsa(_sc.get(), _chain ? _chain->getX509Certs() : nullptr, _chain ? _chain->getCount() : 0,
@@ -1257,7 +1258,13 @@ bool WiFiClientSecure::probeMaxFragmentLength(IPAddress ip, uint16_t port, uint1
   //      0xc0, 0x13, // BR_TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
   static const uint8_t clientHelloTail_P[] PROGMEM = {
     0x01, 0x00, // No compression
-    0x00, 0x05, // Extension length
+    0x00, 26 + 14 + 6 +  5, // Extension length
+    0x00, 0x0d, 0x00, 0x16, 0x00, 0x14, 0x04, 0x03, 0x03, 0x03, 0x05, 0x03,
+          0x06, 0x03, 0x02, 0x03, 0x04, 0x01, 0x03, 0x01, 0x05, 0x01, 0x06,
+	  0x01, 0x02, 0x01, // Supported signature algorithms
+    0x00, 0x0a, 0x00, 0x0a, 0x00, 0x08, 0x00, 0x17, 0x00, 0x18, 0x00, 0x19,
+          0x00, 0x1d, // Supported groups
+    0x00, 0x0b, 0x00, 0x02, 0x01, 0x00, // Supported EC formats
     0x00, 0x01, // Max Frag Len
     0x00, 0x01, // len of MaxFragLen
   };
@@ -1322,6 +1329,8 @@ bool WiFiClientSecure::probeMaxFragmentLength(IPAddress ip, uint16_t port, uint1
   uint8_t sessionLen;
   uint8_t cipher[2];
   uint8_t comp;
+  uint8_t extBytes[2];
+  uint16_t extLen;
 
   ret = probe.readBytes(fragResp, 5);
   if (!probe.connected() || (ret != 5) || (fragResp[0] != 0x16) || (fragResp[1] != 0x03) || (fragResp[2] != 0x03)) {
@@ -1388,10 +1397,40 @@ bool WiFiClientSecure::probeMaxFragmentLength(IPAddress ip, uint16_t port, uint1
     // short read or invalid compression
     return _SendAbort(probe, supportsLen);
   }
-  if (handLen > 0) {
-    // At this point, having an extension present means that the extension we
-    // sent was accepted.
-    supportsLen = true;
+
+  ret = probe.readBytes(extBytes, 2);
+  handLen -= ret;
+  extLen = extBytes[1] || (extBytes[0]<<8);
+  if ((extLen == 0) || (ret != 2)) {
+    return _SendAbort(probe, supportsLen);
+  }
+
+  while (handLen > 0) {
+    // Parse each extension and look for MFLN
+    uint8_t typeBytes[2];
+    ret = probe.readBytes(typeBytes, 2);
+    handLen -= 2;
+    if ((ret != 2) || (handLen <= 0) ) {
+      return _SendAbort(probe, supportsLen);
+    }
+    uint8_t lenBytes[2];
+    ret = probe.readBytes(lenBytes, 2);
+    handLen -= 2;
+    uint16_t extLen = lenBytes[1] | (lenBytes[0]<<8);
+    if ((ret != 2) || (handLen <= 0) || (extLen > 32) || (extLen > handLen) ) {
+      return _SendAbort(probe, supportsLen);
+    }
+    if ((typeBytes[0]==0x00) && (typeBytes[1]==0x01)) { // MFLN extension!
+      // If present and 1-byte in length, it's supported
+      return _SendAbort(probe, extLen==1 ? true : false);
+    }
+    // Skip the extension, move to next one
+    uint8_t junk[32];
+    ret = probe.readBytes(junk, extLen);
+    handLen -= extLen;
+    if (ret != extLen) {
+      return _SendAbort(probe, supportsLen);
+    }
   }
   return _SendAbort(probe, supportsLen);
 }

libraries/ESP8266WiFi/src/WiFiClientSecureBearSSL.h

@@ -103,6 +103,11 @@ class WiFiClientSecure : public WiFiClient {
     // Sets the requested buffer size for transmit and receive
     void setBufferSizes(int recv, int xmit);
 
+    // Returns whether MFLN negotiation for the above buffer sizes succeeded (after connection)
+    int getMFLNStatus() {
+      return connected() && br_ssl_engine_get_mfln_negotiated(_eng);
+    }
+
     // Return an error code and possibly a text string in a passed-in buffer with last SSL failure
     int getLastSSLError(char *dest = NULL, size_t len = 0);
 
@@ -117,7 +122,7 @@ class WiFiClientSecure : public WiFiClient {
     bool setCiphers(std::vector<uint16_t> list);
     bool setCiphersLessSecure(); // Only use the limited set of RSA ciphers without EC
 
-    // Check for Maximum Fragment Length support for given len
+    // Check for Maximum Fragment Length support for given len before connection (possibly insecure)
     static bool probeMaxFragmentLength(IPAddress ip, uint16_t port, uint16_t len);
     static bool probeMaxFragmentLength(const char *hostname, uint16_t port, uint16_t len);
     static bool probeMaxFragmentLength(const String& host, uint16_t port, uint16_t len);

tools/sdk/include/bearssl/bearssl_git.h

@@ -1,2 +1,2 @@
 // Do not edit -- Automatically generated by tools/sdk/ssl/bearssl/Makefile
-#define BEARSSL_GIT 6778687
+#define BEARSSL_GIT a143020

tools/sdk/include/bearssl/bearssl_hash.h

@@ -724,7 +724,7 @@ void br_sha256_update(br_sha256_context *ctx, const void *data, size_t len);
  */
 void br_sha256_out(const br_sha256_context *ctx, void *out);
 
-#if BR_DOXYGEN_IGNORE
+#ifdef BR_DOXYGEN_IGNORE
 /**
  * \brief Save SHA-256 running state.
  *
@@ -742,7 +742,7 @@ uint64_t br_sha256_state(const br_sha256_context *ctx, void *out);
 #define br_sha256_state       br_sha224_state
 #endif
 
-#if BR_DOXYGEN_IGNORE
+#ifdef BR_DOXYGEN_IGNORE
 /**
  * \brief Restore SHA-256 running state.
  *

tools/sdk/include/bearssl/bearssl_ssl.h

@@ -864,6 +864,7 @@ typedef struct {
 	 */
 	uint16_t max_frag_len;
 	unsigned char log_max_frag_len;
+	unsigned char max_frag_len_negotiated;
 	unsigned char peer_log_max_frag_len;
 
 	/*
@@ -1830,6 +1831,17 @@ void br_ssl_engine_set_buffer(br_ssl_engine_context *cc,
 void br_ssl_engine_set_buffers_bidi(br_ssl_engine_context *cc,
 	void *ibuf, size_t ibuf_len, void *obuf, size_t obuf_len);
 
+/**
+ * \brief Determine if MFLN negotiation was successful
+ *
+ * \param cc     SSL engine context.
+ */
+static inline uint8_t
+br_ssl_engine_get_mfln_negotiated(br_ssl_engine_context *cc)
+{
+        return cc->max_frag_len_negotiated;
+}
+
 /**
  * \brief Inject some "initial entropy" in the context.
  *