Correctly lock when removing neighbor entries

Fix a panic when two entries in Failed state are removed at the same time.

PiperOrigin-RevId: 344143777

pkg/tcpip/stack/neighbor_cache.go

@@ -233,6 +233,8 @@ func (n *neighborCache) addStaticEntry(addr tcpip.Address, linkAddr tcpip.LinkAd
 }
 
 // removeEntryLocked removes the specified entry from the neighbor cache.
+//
+// Prerequisite: n.mu and entry.mu MUST be locked.
 func (n *neighborCache) removeEntryLocked(entry *neighborEntry) {
 	if entry.neigh.State != Static {
 		n.dynamic.lru.Remove(entry)

pkg/tcpip/stack/neighbor_cache_test.go

@@ -80,7 +80,7 @@ func entryDiffOptsWithSort() []cmp.Option {
 func newTestNeighborCache(nudDisp NUDDispatcher, config NUDConfigurations, clock tcpip.Clock) *neighborCache {
 	config.resetInvalidFields()
 	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
-	return &neighborCache{
+	neigh := &neighborCache{
 		nic: &NIC{
 			stack: &Stack{
 				clock:   clock,
@@ -92,6 +92,8 @@ func newTestNeighborCache(nudDisp NUDDispatcher, config NUDConfigurations, clock
 		state: NewNUDState(config, rng),
 		cache: make(map[tcpip.Address]*neighborEntry, neighborCacheSize),
 	}
+	neigh.nic.neigh = neigh
+	return neigh
 }
 
 // testEntryStore contains a set of IP to NeighborEntry mappings.

pkg/tcpip/stack/neighbor_entry.go

@@ -258,7 +258,7 @@ func (e *neighborEntry) setStateLocked(next NeighborState) {
 
 	case Failed:
 		e.notifyWakersLocked()
-		e.job = e.nic.stack.newJob(&e.mu, func() {
+		e.job = e.nic.stack.newJob(&doubleLock{first: &e.nic.neigh.mu, second: &e.mu}, func() {
 			e.nic.neigh.removeEntryLocked(e)
 		})
 		e.job.Schedule(config.UnreachableTime)
@@ -512,3 +512,23 @@ func (e *neighborEntry) handleUpperLevelConfirmationLocked() {
 		panic(fmt.Sprintf("Invalid cache entry state: %s", e.neigh.State))
 	}
 }
+
+// doubleLock combines two locks into one while maintaining lock ordering.
+//
+// TODO(gvisor.dev/issue/4796): Remove this once subsequent traffic to a Failed
+// neighbor is allowed.
+type doubleLock struct {
+	first, second sync.Locker
+}
+
+// Lock locks both locks in order: first then second.
+func (l *doubleLock) Lock() {
+	l.first.Lock()
+	l.second.Lock()
+}
+
+// Unlock unlocks both locks in reverse order: second then first.
+func (l *doubleLock) Unlock() {
+	l.second.Unlock()
+	l.first.Unlock()
+}