Add support to opportunistic TLS

A lot of protocols provide the feature to upgrade their plain text
connections to an encrypted one, via some kind of "STARTTLS" command.

Add generic code to support this extension, and allow dissection of the
entire TLS handshake.

As examples, SMTP, POP, IMAP and FTP dissectors have been updated.

Since this feature requires to process more packets per flow, add the
possibility to disable it.

Fix some log messages.

Slight improvement on TCP sequence number tracking.

As a side effect, this commit fix also a memory leak found by
oss-fuzzer
```
==108966==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 22 byte(s) in 1 object(s) allocated from:
    #0 0x55f8b367a0be in malloc (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader_with_main+0x5480be) (BuildId: 94debacb4a6784c30420ab748c8bf3cc59621063)
    #1 0x55f8b36e1345 in ndpi_malloc_wrapper /home/ivan/svnrepos/nDPI/example/reader_util.c:321:10
    #2 0x55f8b379c7d2 in ndpi_malloc /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:212:25
    #3 0x55f8b379cb18 in ndpi_strdup /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:279:13
    #4 0x55f8b386ce46 in processClientServerHello /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2153:34
    #5 0x55f8b385ebf7 in processTLSBlock /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:867:5
    #6 0x55f8b39e708c in ndpi_extra_search_mail_smtp_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/mail_smtp.c:422:9
    #7 0x55f8b37e636c in ndpi_process_extra_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5884:9
    #8 0x55f8b37edc05 in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6276:5
    #9 0x55f8b3701ffc in packet_processing /home/ivan/svnrepos/nDPI/example/reader_util.c:1619:31
    #10 0x55f8b36faf14 in ndpi_workflow_process_packet /home/ivan/svnrepos/nDPI/example/reader_util.c:2189:10
    #11 0x55f8b36b6a50 in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:107:7

```
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=50765

1 files changed, 15 insertions, 12 deletions

diff --git a/src/lib/protocols/mail_imap.c b/src/lib/protocols/mail_imap.c
index a6809b454..2ae04f24b 100644
--- a/src/lib/protocols/mail_imap.c
+++ b/src/lib/protocols/mail_imap.c
@@ -30,6 +30,9 @@
 
 /* #define IMAP_DEBUG 1*/
 
+extern void switch_extra_dissection_to_tls(struct ndpi_detection_module_struct *ndpi_struct,
+					   struct ndpi_flow_struct *flow);
+
 static void ndpi_int_mail_imap_add_connection(struct ndpi_detection_module_struct *ndpi_struct, struct ndpi_flow_struct *flow,
 					      u_int16_t protocol) {
   flow->guessed_protocol_id = NDPI_PROTOCOL_UNKNOWN; /* Avoid IMAPS to be used s sub-protocol */
@@ -51,13 +54,6 @@ void ndpi_search_mail_imap_tcp(struct ndpi_detection_module_struct *ndpi_struct,
   printf("%s() [%.*s]\n", __FUNCTION__, packet->payload_packet_len, packet->payload);
 #endif
 
-  if(flow->l4.tcp.mail_imap_starttls == 2) {
-    NDPI_LOG_DBG2(ndpi_struct, "starttls detected\n");
-    NDPI_ADD_PROTOCOL_TO_BITMASK(flow->excluded_protocol_bitmask, NDPI_PROTOCOL_MAIL_IMAP);
-    NDPI_DEL_PROTOCOL_FROM_BITMASK(flow->excluded_protocol_bitmask, NDPI_PROTOCOL_TLS);
-    return;
-  }
-
   if(packet->payload_packet_len >= 4 && ntohs(get_u_int16_t(packet->payload, packet->payload_packet_len - 2)) == 0x0d0a) {
     // the DONE command appears without a tag
     if(packet->payload_packet_len == 6 && ((packet->payload[0] == 'D' || packet->payload[0] == 'd')
@@ -113,8 +109,17 @@ void ndpi_search_mail_imap_tcp(struct ndpi_detection_module_struct *ndpi_struct,
 	    && (packet->payload[command_start + 1] == 'K' || packet->payload[command_start + 1] == 'k')
 	    && packet->payload[command_start + 2] == ' ') {
 	  flow->l4.tcp.mail_imap_stage += 1;
-	  if(flow->l4.tcp.mail_imap_starttls == 1)
-	    flow->l4.tcp.mail_imap_starttls = 2;
+	  if(flow->l4.tcp.mail_imap_starttls == 1) {
+	    NDPI_LOG_DBG2(ndpi_struct, "starttls detected\n");
+	    ndpi_int_mail_imap_add_connection(ndpi_struct, flow, NDPI_PROTOCOL_MAIL_IMAPS);
+	    if(ndpi_struct->opportunistic_tls_imap_enabled) {
+	      NDPI_LOG_DBG(ndpi_struct, "Switching to [%d/%d]\n",
+			   flow->detected_protocol_stack[0], flow->detected_protocol_stack[1]);
+	      /* We are done (in IMAP dissector): delegating TLS... */
+	      switch_extra_dissection_to_tls(ndpi_struct, flow);
+	      return;
+	    }
+	  }
 	  saw_command = 1;
 	} else if((packet->payload[command_start] == 'U' || packet->payload[command_start] == 'u')
 		   && (packet->payload[command_start + 1] == 'I' || packet->payload[command_start + 1] == 'i')
@@ -126,7 +131,7 @@ void ndpi_search_mail_imap_tcp(struct ndpi_detection_module_struct *ndpi_struct,
 	    && packet->payload[command_start + 2] == ' ') {
 	  flow->l4.tcp.mail_imap_stage += 1;
 	  if(flow->l4.tcp.mail_imap_starttls == 1)
-	    flow->l4.tcp.mail_imap_starttls = 2;
+	    flow->l4.tcp.mail_imap_starttls = 0;
 	  saw_command = 1;
 	}
       }
@@ -156,7 +161,6 @@ void ndpi_search_mail_imap_tcp(struct ndpi_detection_module_struct *ndpi_struct,
 	    && (packet->payload[command_start + 7] == 'S' || packet->payload[command_start + 7] == 's')) {
         flow->l4.tcp.mail_imap_stage += 1;
         flow->l4.tcp.mail_imap_starttls = 1;
-        ndpi_int_mail_imap_add_connection(ndpi_struct, flow, NDPI_PROTOCOL_MAIL_IMAPS);
         saw_command = 1;
 	}
       }
@@ -242,7 +246,6 @@ void ndpi_search_mail_imap_tcp(struct ndpi_detection_module_struct *ndpi_struct,
 	  flow->l4.tcp.mail_imap_stage += 1;
 	  /* Authenticate phase may have multiple messages. Ignore them since they are
 	     somehow encrypted anyway. */
-	  flow->l4.tcp.mail_imap_starttls = 2;
           ndpi_int_mail_imap_add_connection(ndpi_struct, flow, NDPI_PROTOCOL_MAIL_IMAPS);
 	  saw_command = 1;
 	}