wgengine/magicsock: fix panic when rebinding fails

We would replace the existing real implementation of nettype.PacketConn
with a blockForeverConn, but that violates the contract of atomic.Value
(where the type cannot change). Fix by switching to a pointer value
(atomic.Pointer[nettype.PacketConn]).

A longstanding issue, but became more prevalent when we started binding
connections to interfaces on macOS and iOS (#6566), which could lead to
the bind call failing if the interface was no longer available.

Fixes #6641

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -3008,13 +3008,14 @@ func (c *Conn) ParseEndpoint(nodeKeyStr string) (conn.Endpoint, error) {
 // RebindingUDPConn is a UDP socket that can be re-bound.
 // Unix has no notion of re-binding a socket, so we swap it out for a new one.
 type RebindingUDPConn struct {
-	// pconnAtomic is the same as pconn, but doesn't require acquiring mu. It's
-	// used for reads/writes and only upon failure do the reads/writes then
-	// check pconn (after acquiring mu) to see if there's been a rebind
-	// meanwhile.
+	// pconnAtomic is a pointer to the value stored in pconn, but doesn't
+	// require acquiring mu. It's used for reads/writes and only upon failure
+	// do the reads/writes then check pconn (after acquiring mu) to see if
+	// there's been a rebind meanwhile.
 	// pconn isn't really needed, but makes some of the code simpler
-	// to keep it in a type safe form.
-	pconnAtomic syncs.AtomicValue[nettype.PacketConn]
+	// to keep it distinct.
+	// Neither is expected to be nil, sockets are bound on creation.
+	pconnAtomic atomic.Pointer[nettype.PacketConn]
 
 	mu    sync.Mutex // held while changing pconn (and pconnAtomic)
 	pconn nettype.PacketConn
@@ -3023,7 +3024,7 @@ type RebindingUDPConn struct {
 
 func (c *RebindingUDPConn) setConnLocked(p nettype.PacketConn) {
 	c.pconn = p
-	c.pconnAtomic.Store(p)
+	c.pconnAtomic.Store(&p)
 	c.port = uint16(c.localAddrLocked().Port)
 }
 
@@ -3038,7 +3039,7 @@ func (c *RebindingUDPConn) currentConn() nettype.PacketConn {
 // It returns the number of bytes copied and the source address.
 func (c *RebindingUDPConn) ReadFrom(b []byte) (int, net.Addr, error) {
 	for {
-		pconn := c.pconnAtomic.Load()
+		pconn := *c.pconnAtomic.Load()
 		n, addr, err := pconn.ReadFrom(b)
 		if err != nil && pconn != c.currentConn() {
 			continue
@@ -3056,7 +3057,7 @@ func (c *RebindingUDPConn) ReadFrom(b []byte) (int, net.Addr, error) {
 // when c's underlying connection is a net.UDPConn.
 func (c *RebindingUDPConn) ReadFromNetaddr(b []byte) (n int, ipp netip.AddrPort, err error) {
 	for {
-		pconn := c.pconnAtomic.Load()
+		pconn := *c.pconnAtomic.Load()
 
 		// Optimization: Treat *net.UDPConn specially.
 		// This lets us avoid allocations by calling ReadFromUDPAddrPort.
@@ -3122,13 +3123,10 @@ func (c *RebindingUDPConn) closeLocked() error {
 
 func (c *RebindingUDPConn) WriteTo(b []byte, addr net.Addr) (int, error) {
 	for {
-		pconn := c.pconnAtomic.Load()
-
+		pconn := *c.pconnAtomic.Load()
 		n, err := pconn.WriteTo(b, addr)
-		if err != nil {
-			if pconn != c.currentConn() {
-				continue
-			}
+		if err != nil && pconn != c.currentConn() {
+			continue
 		}
 		return n, err
 	}
@@ -3136,13 +3134,10 @@ func (c *RebindingUDPConn) WriteTo(b []byte, addr net.Addr) (int, error) {
 
 func (c *RebindingUDPConn) WriteToUDPAddrPort(b []byte, addr netip.AddrPort) (int, error) {
 	for {
-		pconn := c.pconnAtomic.Load()
-
+		pconn := *c.pconnAtomic.Load()
 		n, err := pconn.WriteToUDPAddrPort(b, addr)
-		if err != nil {
-			if pconn != c.currentConn() {
-				continue
-			}
+		if err != nil && pconn != c.currentConn() {
+			continue
 		}
 		return n, err
 	}

wgengine/magicsock/magicsock_test.go

@@ -1803,3 +1803,16 @@ func TestDiscoMagicMatches(t *testing.T) {
 		t.Errorf("last 2 bytes of disco magic don't match, got %v want %v", discoMagic2, m2)
 	}
 }
+
+func TestRebindingUDPConn(t *testing.T) {
+	// Test that RebindingUDPConn can be re-bound to different connection
+	// types.
+	c := RebindingUDPConn{}
+	realConn, err := net.ListenPacket("udp4", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer realConn.Close()
+	c.setConnLocked(realConn.(nettype.PacketConn))
+	c.setConnLocked(newBlockForeverConn())
+}