[merge to M97] sdp: reject large number of channels

the maximum used in practice is multiopus with
6 or 8 channels. 24 is the maximum number of channels
supported in the audio decoder.

BUG=chromium:1265806
(cherry picked from commit d58ac5a)

No-Try: True
Change-Id: Iba8e3185a1f235b846fed9c154e66fb3983664ed
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/238980
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@nvidia.com>
Cr-Original-Commit-Position: refs/heads/main@{#35440}
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/240180
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/branch-heads/4692@{webrtc-sdk#2}
Cr-Branched-From: c276aee-refs/heads/main@{#35313}

Author: fippo

pc/webrtc_sdp.cc

@@ -256,6 +256,9 @@ static const char kDefaultSctpmapProtocol[] = "webrtc-datachannel";
 // types.
 const int kWildcardPayloadType = -1;
 
+// Maximum number of channels allowed.
+static const size_t kMaxNumberOfChannels = 24;
+
 struct SsrcInfo {
   uint32_t ssrc_id;
   std::string cname;
@@ -3627,6 +3630,10 @@ bool ParseRtpmapAttribute(const std::string& line,
         return false;
       }
     }
+    if (channels > kMaxNumberOfChannels) {
+      return ParseFailed(line, "At most 24 channels are supported.", error);
+    }
+
     AudioContentDescription* audio_desc = media_desc->as_audio();
     UpdateCodec(payload_type, encoding_name, clock_rate, 0, channels,
                 audio_desc);

pc/webrtc_sdp_unittest.cc

@@ -4694,3 +4694,15 @@ TEST_F(WebRtcSdpTest, IllegalMidCharacterValue) {
   Replace("a=mid:", "a=mid:[]", &sdp);
   ExpectParseFailure(std::string(sdp), "a=mid:[]");
 }
+
+TEST_F(WebRtcSdpTest, MaxChannels) {
+  std::string sdp =
+      "v=0\r\n"
+      "o=- 11 22 IN IP4 127.0.0.1\r\n"
+      "s=-\r\n"
+      "t=0 0\r\n"
+      "m=audio 49232 RTP/AVP 108\r\n"
+      "a=rtpmap:108 ISAC/16000/512\r\n";
+
+  ExpectParseFailure(sdp, "a=rtpmap:108 ISAC/16000/512");
+}