crypto/tls: fix deadlock when Read and Close called concurrently

The existing implementation of TLS connection has a deadlock. It occurs
when client connects to TLS server and doesn't send data for
handshake, so server calls Close on this connection. This is because
server reads data under locked mutex, while Close method tries to
lock the same mutex.

Fixes #23518

Change-Id: I4fb0a2a770f3d911036bfd9a7da7cc41c1b27e19
Reviewed-on: https://go-review.googlesource.com/90155
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

conn.go

@@ -27,15 +27,16 @@ type Conn struct {
 	conn     net.Conn
 	isClient bool
 
+	// handshakeStatus is 1 if the connection is currently transferring
+	// application data (i.e. is not currently processing a handshake).
+	// This field is only to be accessed with sync/atomic.
+	handshakeStatus uint32
 	// constant after handshake; protected by handshakeMutex
 	handshakeMutex sync.Mutex
 	handshakeErr   error   // error resulting from handshake
 	vers           uint16  // TLS version
 	haveVers       bool    // version has been negotiated
 	config         *Config // configuration passed to constructor
-	// handshakeComplete is true if the connection is currently transferring
-	// application data (i.e. is not currently processing a handshake).
-	handshakeComplete bool
 	// handshakes counts the number of handshakes performed on the
 	// connection so far. If renegotiation is disabled then this is either
 	// zero or one.
@@ -571,12 +572,12 @@ func (c *Conn) readRecord(want recordType) error {
 		c.sendAlert(alertInternalError)
 		return c.in.setErrorLocked(errors.New("tls: unknown record type requested"))
 	case recordTypeHandshake, recordTypeChangeCipherSpec:
-		if c.handshakeComplete {
+		if c.handshakeComplete() {
 			c.sendAlert(alertInternalError)
 			return c.in.setErrorLocked(errors.New("tls: handshake or ChangeCipherSpec requested while not in handshake"))
 		}
 	case recordTypeApplicationData:
-		if !c.handshakeComplete {
+		if !c.handshakeComplete() {
 			c.sendAlert(alertInternalError)
 			return c.in.setErrorLocked(errors.New("tls: application data record requested while in handshake"))
 		}
@@ -1048,7 +1049,7 @@ func (c *Conn) Write(b []byte) (int, error) {
 		return 0, err
 	}
 
-	if !c.handshakeComplete {
+	if !c.handshakeComplete() {
 		return 0, alertInternalError
 	}
 
@@ -1114,7 +1115,7 @@ func (c *Conn) handleRenegotiation() error {
 	c.handshakeMutex.Lock()
 	defer c.handshakeMutex.Unlock()
 
-	c.handshakeComplete = false
+	atomic.StoreUint32(&c.handshakeStatus, 0)
 	if c.handshakeErr = c.clientHandshake(); c.handshakeErr == nil {
 		c.handshakes++
 	}
@@ -1215,11 +1216,9 @@ func (c *Conn) Close() error {
 
 	var alertErr error
 
-	c.handshakeMutex.Lock()
-	if c.handshakeComplete {
+	if c.handshakeComplete() {
 		alertErr = c.closeNotify()
 	}
-	c.handshakeMutex.Unlock()
 
 	if err := c.conn.Close(); err != nil {
 		return err
@@ -1233,9 +1232,7 @@ var errEarlyCloseWrite = errors.New("tls: CloseWrite called before handshake com
 // called once the handshake has completed and does not call CloseWrite on the
 // underlying connection. Most callers should just use Close.
 func (c *Conn) CloseWrite() error {
-	c.handshakeMutex.Lock()
-	defer c.handshakeMutex.Unlock()
-	if !c.handshakeComplete {
+	if !c.handshakeComplete() {
 		return errEarlyCloseWrite
 	}
 
@@ -1264,7 +1261,7 @@ func (c *Conn) Handshake() error {
 	if err := c.handshakeErr; err != nil {
 		return err
 	}
-	if c.handshakeComplete {
+	if c.handshakeComplete() {
 		return nil
 	}
 
@@ -1284,7 +1281,7 @@ func (c *Conn) Handshake() error {
 		c.flush()
 	}
 
-	if c.handshakeErr == nil && !c.handshakeComplete {
+	if c.handshakeErr == nil && !c.handshakeComplete() {
 		panic("handshake should have had a result.")
 	}
 
@@ -1297,10 +1294,10 @@ func (c *Conn) ConnectionState() ConnectionState {
 	defer c.handshakeMutex.Unlock()
 
 	var state ConnectionState
-	state.HandshakeComplete = c.handshakeComplete
+	state.HandshakeComplete = c.handshakeComplete()
 	state.ServerName = c.serverName
 
-	if c.handshakeComplete {
+	if state.HandshakeComplete {
 		state.Version = c.vers
 		state.NegotiatedProtocol = c.clientProtocol
 		state.DidResume = c.didResume
@@ -1345,7 +1342,7 @@ func (c *Conn) VerifyHostname(host string) error {
 	if !c.isClient {
 		return errors.New("tls: VerifyHostname called on TLS server connection")
 	}
-	if !c.handshakeComplete {
+	if !c.handshakeComplete() {
 		return errors.New("tls: handshake has not yet been performed")
 	}
 	if len(c.verifiedChains) == 0 {
@@ -1353,3 +1350,7 @@ func (c *Conn) VerifyHostname(host string) error {
 	}
 	return c.peerCertificates[0].VerifyHostname(host)
 }
+
+func (c *Conn) handshakeComplete() bool {
+	return atomic.LoadUint32(&c.handshakeStatus) == 1
+}