chan_iax2: Fix crash due to negative length frame lengths.

chan_iax2 has several code paths where a frame's data length
is calculated by subtraction. On some paths, there is a check
for negative length. One of these paths is missing this check,
and on this path, it is possible for the result to be negative,
leading to a crash as a result of memory operations using the
bogus length.

Add a check to capture this off-nominal case. This will log
the appropriate warnings as in other cases and prevent a crash.
Also update the log messages to be clearer.

Resolves: #1707

channels/chan_iax2.c

@@ -10145,7 +10145,7 @@ static int socket_process_meta(int packet_len, struct ast_iax2_meta_hdr *meta, s
 						iaxs[fr->callno]->last = fr->ts;
 				}
 			} else {
-				ast_log(LOG_WARNING, "Datalen < 0?\n");
+				ast_log(LOG_ERROR, "Dropping malformed frame (datalen %d?)\n", f.datalen);
 			}
 		}
 		ast_mutex_unlock(&iaxsl[fr->callno]);
@@ -12017,6 +12017,12 @@ immediatedial:
 			return 1;
 		}
 		f.datalen = res - sizeof(*vh);
+		if (f.datalen < 0) {
+			ast_log(LOG_ERROR, "Dropping malformed frame (datalen %d?)\n", f.datalen);
+			ast_variables_destroy(ies.vars);
+			ast_mutex_unlock(&iaxsl[fr->callno]);
+			return 1;
+		}
 		if (f.datalen)
 			f.data.ptr = thread->buf + sizeof(*vh);
 		else
@@ -12046,7 +12052,7 @@ immediatedial:
 		}
 		f.datalen = res - sizeof(struct ast_iax2_mini_hdr);
 		if (f.datalen < 0) {
-			ast_log(LOG_WARNING, "Datalen < 0?\n");
+			ast_log(LOG_ERROR, "Dropping malformed frame (datalen %d?)\n", f.datalen);
 			ast_variables_destroy(ies.vars);
 			ast_mutex_unlock(&iaxsl[fr->callno]);
 			return 1;
@@ -12150,6 +12156,7 @@ immediatedial:
 			ast_frame_byteswap_be(&f);
 	} else
 		f.samples = 0;
+
 	iax_frame_wrap(fr, &f);
 
 	/* If this is our most recent packet, use it as our basis for timestamping */

channels/iax2/parser.c

@@ -1210,6 +1210,7 @@ void iax_frame_wrap(struct iax_frame *fr, struct ast_frame *f)
 	if (fr->af.datalen) {
 		size_t copy_len = fr->af.datalen;
 		if (copy_len > fr->afdatalen) {
+			ast_assert(fr->af.datalen >= 0); /* Length should never be negative */
 			ast_log(LOG_ERROR, "Losing frame data because destination buffer size '%d' bytes not big enough for '%d' bytes in the frame\n",
 				(int) fr->afdatalen, (int) fr->af.datalen);
 			copy_len = fr->afdatalen;
@@ -1230,6 +1231,8 @@ struct iax_frame *iax_frame_new(int direction, int datalen, unsigned int cacheab
 {
 	struct iax_frame *fr;
 
+	ast_assert(datalen >= 0); /* Length should never be negative */
+
 #if !defined(NO_FRAME_CACHE)
 	if (cacheable) {
 		struct iax_frames *iax_frames;