Enforce a minimum Send MSS.

Without this enforcement, options larger than the specified MSS can cause
MaxPayloadSize to go negative and cause all sorts of problems.

PiperOrigin-RevId: 460224034

pkg/tcpip/header/tcp.go

@@ -222,6 +222,10 @@ const (
 	// same as the value TCP_MIN_MSS defined net/tcp.h.
 	TCPMinimumMSS = IPv4MaximumHeaderSize + TCPHeaderMaximumSize + MinIPFragmentPayloadSize - IPv4MinimumSize - TCPMinimumSize
 
+	// TCPMinimumSendMSS is the minimum value for MSS in a sender. This is the
+	// same as the value TCP_MIN_SND_MSS in net/tcp.h.
+	TCPMinimumSendMSS = TCPOptionsMaximumSize + MinIPFragmentPayloadSize
+
 	// TCPMaximumMSS is the maximum acceptable value for MSS.
 	TCPMaximumMSS = 0xffff
 
@@ -458,6 +462,9 @@ func ParseSynOptions(opts []byte, isAck bool) TCPSynOptions {
 				return synOpts
 			}
 			synOpts.MSS = mss
+			if mss < TCPMinimumSendMSS {
+				synOpts.MSS = TCPMinimumSendMSS
+			}
 			i += 4
 
 		case TCPOptionWS:

pkg/tcpip/transport/tcp/snd.go

@@ -861,11 +861,6 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 		if seg.payloadSize() > available {
 			// A negative value causes splitSeg to panic anyways, so just panic
 			// earlier to get more information about the cause.
-			// TOOD(b/236090764): Remove this panic once the cause of negative values
-			// of "available" is understood.
-			if available < 0 {
-				panic(fmt.Sprintf("got available=%d, want available>=0. limit %d, s.MaxPayloadSize %d, seg.payloadSize() %d, gso.MaxSize %d, gso.MSS %d", available, limit, s.MaxPayloadSize, seg.payloadSize(), s.ep.gso.MaxSize, s.ep.gso.MSS))
-			}
 			s.splitSeg(seg, available)
 		}
 

pkg/tcpip/transport/tcp/test/e2e/tcp_test.go

@@ -3573,6 +3573,28 @@ func TestSetTTL(t *testing.T) {
 	}
 }
 
+func TestSendMSSLessThanOptionsSize(t *testing.T) {
+	const mss = 10
+	const writeSize = 300
+	c := context.New(t, 65535)
+	defer c.Cleanup()
+
+	// The sizes of these options add up to 12.
+	c.CreateConnectedWithRawOptions(context.TestInitialSequenceNumber, 30000, -1 /* epRcvBuf */, []byte{
+		header.TCPOptionMSS, 4, byte(mss / 256), byte(mss % 256),
+		header.TCPOptionTS, header.TCPOptionTSLength, 1, 2, 3, 4, 5, 6, 7, 8,
+		header.TCPOptionSACKPermitted, header.TCPOptionSackPermittedLength,
+	})
+	e2e.CheckBrokenUpWrite(t, c, writeSize)
+
+	var r bytes.Reader
+	r.Reset(make([]byte, writeSize))
+	_, err := c.EP.Write(&r, tcpip.WriteOptions{})
+	if err != nil {
+		t.Fatalf("Write failed: %s", err)
+	}
+}
+
 func TestActiveSendMSSLessThanMTU(t *testing.T) {
 	const maxPayload = 100
 	c := context.New(t, 65535)