wgengine/netstack: add a missing refcount decrement after packet injection

Fixes #3762
Updates #3745 (probably fixes?)

Change-Id: I1d3f0590fd5b8adfbc9110bc45ff717bb9e79aae
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/netstack.go

@@ -520,6 +520,7 @@ func (ns *Impl) injectInbound(p *packet.Parsed, t *tstun.Wrapper) filter.Respons
 		Data: vv,
 	})
 	ns.linkEP.InjectInbound(pn, packetBuf)
+	packetBuf.DecRef()
 
 	// We've now delivered this to netstack, so we're done.
 	// Instead of returning a filter.Accept here (which would also

wgengine/netstack/netstack_test.go

@@ -0,0 +1,76 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package netstack
+
+import (
+	"runtime"
+	"testing"
+
+	"inet.af/netaddr"
+	"tailscale.com/net/packet"
+	"tailscale.com/net/tsdial"
+	"tailscale.com/net/tstun"
+	"tailscale.com/wgengine"
+	"tailscale.com/wgengine/filter"
+)
+
+// TestInjectInboundLeak tests that injectInbound doesn't leak memory.
+// See https://github.com/tailscale/tailscale/issues/3762
+func TestInjectInboundLeak(t *testing.T) {
+	tunDev := tstun.NewFake()
+	dialer := new(tsdial.Dialer)
+	logf := func(format string, args ...interface{}) {
+		if !t.Failed() {
+			t.Logf(format, args...)
+		}
+	}
+	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
+		Tun:    tunDev,
+		Dialer: dialer,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer eng.Close()
+	ig, ok := eng.(wgengine.InternalsGetter)
+	if !ok {
+		t.Fatal("not an InternalsGetter")
+	}
+	tunWrap, magicSock, ok := ig.GetInternals()
+	if !ok {
+		t.Fatal("failed to get internals")
+	}
+
+	ns, err := Create(logf, tunWrap, eng, magicSock, dialer)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ns.Close()
+	ns.ProcessLocalIPs = true
+	if err := ns.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	ns.atomicIsLocalIPFunc.Store(func(netaddr.IP) bool { return true })
+
+	pkt := &packet.Parsed{}
+	const N = 10_000
+	ms0 := getMemStats()
+	for i := 0; i < N; i++ {
+		outcome := ns.injectInbound(pkt, tunWrap)
+		if outcome != filter.DropSilently {
+			t.Fatalf("got outcome %v; want DropSilently", outcome)
+		}
+	}
+	ms1 := getMemStats()
+	if grew := int64(ms1.HeapObjects) - int64(ms0.HeapObjects); grew >= N {
+		t.Fatalf("grew by %v (which is too much and >= the %v packets we sent)", grew, N)
+	}
+}
+
+func getMemStats() (ms runtime.MemStats) {
+	runtime.GC()
+	runtime.ReadMemStats(&ms)
+	return
+}