From 9175fe2675264bbb08a9d6d8830e282c0b68f17c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Claus=20Lensb=C3=B8l?= <claus@tailscale.com>
Date: Fri, 17 Jul 2026 15:10:08 -0400
Subject: [PATCH] net/tstun: drop TSMP messages injected into TUN (#20511)
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Enforce that TSMP messages are only accepted for transmission over the
wireguard connection from within the client.

Updates tailscale/corp#45059

Signed-off-by: Claus Lensbøl <claus@tailscale.com>
---
 net/tstun/wrap.go      |  9 +++++++++
 net/tstun/wrap_test.go | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/net/tstun/wrap.go b/net/tstun/wrap.go
index bcc12f141..e63dccd49 100644
--- a/net/tstun/wrap.go
+++ b/net/tstun/wrap.go
@@ -761,6 +761,14 @@ func (t *Wrapper) filterPacketOutboundToWireGuard(p *packet.Parsed, pc *peerConf
 		}
 	}
 
+	// TSMP traffic should only originate from tailscaled, not from the host
+	// itself.
+	if p.IPProto == ipproto.TSMP {
+		t.limitedLogf("[unexpected] received TSMP out packet over tstun; dropping")
+		metricPacketOutDropTSMP.Add(1)
+		return filter.DropSilently, gro
+	}
+
 	// Issue 1526 workaround: if we sent disco packets over
 	// Tailscale from ourselves, then drop them, as that shouldn't
 	// happen unless a networking stack is confused, as it seems
@@ -1507,6 +1515,7 @@ func (t *Wrapper) SetConnectionCounter(fn netlogfunc.ConnectionCounter) {
 	metricPacketOutDrop          = clientmetric.NewCounter("tstun_out_to_wg_drop")
 	metricPacketOutDropFilter    = clientmetric.NewCounter("tstun_out_to_wg_drop_filter")
 	metricPacketOutDropSelfDisco = clientmetric.NewCounter("tstun_out_to_wg_drop_self_disco")
+	metricPacketOutDropTSMP      = clientmetric.NewCounter("tstun_out_to_wg_drop_tsmp")
 )
 
 func (t *Wrapper) InstallCaptureHook(cb packet.CaptureCallback) {
diff --git a/net/tstun/wrap_test.go b/net/tstun/wrap_test.go
index 3690ce907..76edf7563 100644
--- a/net/tstun/wrap_test.go
+++ b/net/tstun/wrap_test.go
@@ -1236,3 +1236,36 @@ func TestFilterDropEmptyTSMPDiscoAdvertInbound(t *testing.T) {
 		t.Errorf("got %v; want DropSilently", got)
 	}
 }
+
+// Drop TSMP packets from ourselves.
+func TestFilterDropTSMP(t *testing.T) {
+	var memLog tstest.MemLogger
+	tw := &Wrapper{logf: memLog.Logf, limitedLogf: memLog.Logf}
+	ipHdr := packet.IP4Header{
+		IPProto: ipproto.TSMP,
+		Src:     netaddr.IPv4(1, 2, 3, 4),
+		Dst:     netaddr.IPv4(5, 6, 7, 8),
+	}
+	tsmpPayload := make([]byte, 33)
+	tsmpPayload[0] = byte(packet.TSMPTypeDiscoAdvertisement)
+	pkt := make([]byte, ipHdr.Len()+len(tsmpPayload))
+	ipHdr.Marshal(pkt)
+	copy(pkt[ipHdr.Len():], tsmpPayload)
+
+	wantMetric := metricPacketOutDropTSMP.Value() + 1
+
+	pp := new(packet.Parsed)
+	pp.Decode(pkt)
+	got, _ := tw.filterPacketOutboundToWireGuard(pp, nil, nil)
+	if got != filter.DropSilently {
+		t.Errorf("got %v; want DropSilently", got)
+	}
+	if got, want := memLog.String(), "[unexpected] received TSMP out packet over tstun; dropping\n"; got != want {
+		t.Errorf("log output mismatch\n got: %q\nwant: %q\n", got, want)
+	}
+
+	if metricPacketOutDropTSMP.Value() != wantMetric {
+		t.Errorf("expected metric\n got: %d\nwant: %d\n",
+			metricPacketOutDropTSMP.Value(), wantMetric)
+	}
+}