From a804b42fe503df227600da0a8c62741af854c2e6 Mon Sep 17 00:00:00 2001
From: Tamir Duberstein <tamird@google.com>
Date: Mon, 12 Apr 2021 13:12:41 -0700
Subject: Drop locks before calling waiterQueue.Notify

Holding this lock can cause the user's callback to deadlock if it
attempts to inspect the accept queue.

PiperOrigin-RevId: 368068334
---
 pkg/tcpip/transport/tcp/accept.go   | 37 +++++++++------
 pkg/tcpip/transport/tcp/tcp_test.go | 92 +++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+), 15 deletions(-)

(limited to 'pkg/tcpip/transport/tcp')

diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index e2bd57ebf..7acc7e7b0 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -398,25 +398,32 @@ func (e *endpoint) deliverAccepted(n *endpoint, withSynCookie bool) {
 	e.mu.Unlock()
 	defer e.pendingAccepted.Done()
 
-	e.acceptMu.Lock()
-	for {
-		if e.accepted == (accepted{}) {
-			n.notifyProtocolGoroutine(notifyReset)
-			break
-		}
-		if e.accepted.endpoints.Len() == e.accepted.cap {
-			e.acceptCond.Wait()
-			continue
-		}
+	// Drop the lock before notifying to avoid deadlock in user-specified
+	// callbacks.
+	delivered := func() bool {
+		e.acceptMu.Lock()
+		defer e.acceptMu.Unlock()
+		for {
+			if e.accepted == (accepted{}) {
+				return false
+			}
+			if e.accepted.endpoints.Len() == e.accepted.cap {
+				e.acceptCond.Wait()
+				continue
+			}
 
-		e.accepted.endpoints.PushBack(n)
-		if !withSynCookie {
-			atomic.AddInt32(&e.synRcvdCount, -1)
+			e.accepted.endpoints.PushBack(n)
+			if !withSynCookie {
+				atomic.AddInt32(&e.synRcvdCount, -1)
+			}
+			return true
 		}
+	}()
+	if delivered {
 		e.waiterQueue.Notify(waiter.ReadableEvents)
-		break
+	} else {
+		n.notifyProtocolGoroutine(notifyReset)
 	}
-	e.acceptMu.Unlock()
 }
 
 // propagateInheritableOptionsLocked propagates any options set on the listening
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 5605a4390..cda3ecbcd 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -1291,6 +1291,98 @@ func TestListenShutdown(t *testing.T) {
 		))
 }
 
+var _ waiter.EntryCallback = (callback)(nil)
+
+type callback func(*waiter.Entry, waiter.EventMask)
+
+func (cb callback) Callback(entry *waiter.Entry, mask waiter.EventMask) {
+	cb(entry, mask)
+}
+
+func TestListenerReadinessOnEvent(t *testing.T) {
+	s := stack.New(stack.Options{
+		TransportProtocols: []stack.TransportProtocolFactory{tcp.NewProtocol},
+		NetworkProtocols:   []stack.NetworkProtocolFactory{ipv4.NewProtocol},
+	})
+	{
+		ep := loopback.New()
+		if testing.Verbose() {
+			ep = sniffer.New(ep)
+		}
+		const id = 1
+		if err := s.CreateNIC(id, ep); err != nil {
+			t.Fatalf("CreateNIC(%d, %T): %s", id, ep, err)
+		}
+		if err := s.AddAddress(id, ipv4.ProtocolNumber, context.StackAddr); err != nil {
+			t.Fatalf("AddAddress(%d, ipv4.ProtocolNumber, %s): %s", id, context.StackAddr, err)
+		}
+		s.SetRouteTable([]tcpip.Route{
+			{Destination: header.IPv4EmptySubnet, NIC: id},
+		})
+	}
+
+	var wq waiter.Queue
+	ep, err := s.NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
+	if err != nil {
+		t.Fatalf("NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, _): %s", err)
+	}
+	defer ep.Close()
+
+	if err := ep.Bind(tcpip.FullAddress{Addr: context.StackAddr}); err != nil {
+		t.Fatalf("Bind(%s): %s", context.StackAddr, err)
+	}
+	const backlog = 1
+	if err := ep.Listen(backlog); err != nil {
+		t.Fatalf("Listen(%d): %s", backlog, err)
+	}
+
+	address, err := ep.GetLocalAddress()
+	if err != nil {
+		t.Fatalf("GetLocalAddress(): %s", err)
+	}
+
+	conn, err := s.NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
+	if err != nil {
+		t.Fatalf("NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, _): %s", err)
+	}
+	defer conn.Close()
+
+	events := make(chan waiter.EventMask)
+	// Scope `entry` to allow a binding of the same name below.
+	{
+		entry := waiter.Entry{Callback: callback(func(_ *waiter.Entry, mask waiter.EventMask) {
+			events <- ep.Readiness(mask)
+		})}
+		wq.EventRegister(&entry, waiter.EventIn)
+		defer wq.EventUnregister(&entry)
+	}
+
+	entry, ch := waiter.NewChannelEntry(nil)
+	wq.EventRegister(&entry, waiter.EventOut)
+	defer wq.EventUnregister(&entry)
+
+	switch err := conn.Connect(address).(type) {
+	case *tcpip.ErrConnectStarted:
+	default:
+		t.Fatalf("Connect(%#v): %v", address, err)
+	}
+
+	// Read at least one event.
+	got := <-events
+	for {
+		select {
+		case event := <-events:
+			got |= event
+			continue
+		case <-ch:
+			if want := waiter.ReadableEvents; got != want {
+				t.Errorf("observed events = %b, want %b", got, want)
+			}
+		}
+		break
+	}
+}
+
 // TestListenCloseWhileConnect tests for the listening endpoint to
 // drain the accept-queue when closed. This should reset all of the
 // pending connections that are waiting to be accepted.
-- 
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