Use atomic to avoid stale SRTP protection profile

`state` is acccessed without lock in the FSM.
In some cases, that leads to stale values.
For example, `srtpProtectionProfile` is set in flight
handlers (differnt flight handlers in client and server).
But, when it is accessed via the API
`SelectedSRTPProtectionProfile`,
it gets a stale value as it appears that the two goroutines
are out-of-sync on that piece of shared memory.

This is a larger concern for use of `state`.
Ideally, either
- `state` should have a lock internally and all fields are accessed
  through methods.
- carefully split fields of `state` to ensure process access/sync.

Doing the smaller change here to address one field that has
seen stale value.

conn.go

@@ -374,14 +374,12 @@ func (c *Conn) ConnectionState() State {
 
 // SelectedSRTPProtectionProfile returns the selected SRTPProtectionProfile
 func (c *Conn) SelectedSRTPProtectionProfile() (SRTPProtectionProfile, bool) {
-	c.lock.RLock()
-	defer c.lock.RUnlock()
-
-	if c.state.srtpProtectionProfile == 0 {
+	profile := c.state.getSRTPProtectionProfile()
+	if profile == 0 {
 		return 0, false
 	}
 
-	return c.state.srtpProtectionProfile, true
+	return profile, true
 }
 
 func (c *Conn) writePackets(ctx context.Context, pkts []*packet) error {

flight0handler.go

@@ -60,7 +60,7 @@ func flight0Parse(_ context.Context, _ flightConn, state *State, cache *handshak
 			if !ok {
 				return 0, &alert.Alert{Level: alert.Fatal, Description: alert.InsufficientSecurity}, errServerNoMatchingSRTPProfile
 			}
-			state.srtpProtectionProfile = profile
+			state.setSRTPProtectionProfile(profile)
 		case *extension.UseExtendedMasterSecret:
 			if cfg.extendedMasterSecret != DisableExtendedMasterSecret {
 				state.extendedMasterSecret = true

flight3handler.go

@@ -56,7 +56,7 @@ func flight3Parse(ctx context.Context, c flightConn, state *State, cache *handsh
 				if !found {
 					return 0, &alert.Alert{Level: alert.Fatal, Description: alert.IllegalParameter}, errClientNoMatchingSRTPProfile
 				}
-				state.srtpProtectionProfile = profile
+				state.setSRTPProtectionProfile(profile)
 			case *extension.UseExtendedMasterSecret:
 				if cfg.extendedMasterSecret != DisableExtendedMasterSecret {
 					state.extendedMasterSecret = true
@@ -71,7 +71,7 @@ func flight3Parse(ctx context.Context, c flightConn, state *State, cache *handsh
 		if cfg.extendedMasterSecret == RequireExtendedMasterSecret && !state.extendedMasterSecret {
 			return 0, &alert.Alert{Level: alert.Fatal, Description: alert.InsufficientSecurity}, errClientRequiredButNoServerEMS
 		}
-		if len(cfg.localSRTPProtectionProfiles) > 0 && state.srtpProtectionProfile == 0 {
+		if len(cfg.localSRTPProtectionProfiles) > 0 && state.getSRTPProtectionProfile() == 0 {
 			return 0, &alert.Alert{Level: alert.Fatal, Description: alert.InsufficientSecurity}, errRequestedButNoSRTPExtension
 		}
 

flight4bhandler.go

@@ -59,9 +59,9 @@ func flight4bGenerate(_ flightConn, state *State, cache *handshakeCache, cfg *ha
 			Supported: true,
 		})
 	}
-	if state.srtpProtectionProfile != 0 {
+	if state.getSRTPProtectionProfile() != 0 {
 		extensions = append(extensions, &extension.UseSRTP{
-			ProtectionProfiles: []SRTPProtectionProfile{state.srtpProtectionProfile},
+			ProtectionProfiles: []SRTPProtectionProfile{state.getSRTPProtectionProfile()},
 		})
 	}
 

flight4handler.go

@@ -228,9 +228,9 @@ func flight4Generate(_ flightConn, state *State, _ *handshakeCache, cfg *handsha
 			Supported: true,
 		})
 	}
-	if state.srtpProtectionProfile != 0 {
+	if state.getSRTPProtectionProfile() != 0 {
 		extensions = append(extensions, &extension.UseSRTP{
-			ProtectionProfiles: []SRTPProtectionProfile{state.srtpProtectionProfile},
+			ProtectionProfiles: []SRTPProtectionProfile{state.getSRTPProtectionProfile()},
 		})
 	}
 	if state.cipherSuite.AuthenticationType() == CipherSuiteAuthenticationTypeCertificate {

state.go

@@ -22,7 +22,7 @@ type State struct {
 	masterSecret              []byte
 	cipherSuite               CipherSuite // nil if a cipherSuite hasn't been chosen
 
-	srtpProtectionProfile SRTPProtectionProfile // Negotiated SRTPProtectionProfile
+	srtpProtectionProfile atomic.Value // Negotiated SRTPProtectionProfile
 	PeerCertificates      [][]byte
 	IdentityHint          []byte
 	SessionID             []byte
@@ -87,7 +87,7 @@ func (s *State) serialize() *serializedState {
 		SequenceNumber:        atomic.LoadUint64(&s.localSequenceNumber[epoch]),
 		LocalRandom:           localRnd,
 		RemoteRandom:          remoteRnd,
-		SRTPProtectionProfile: uint16(s.srtpProtectionProfile),
+		SRTPProtectionProfile: uint16(s.getSRTPProtectionProfile()),
 		PeerCertificates:      s.PeerCertificates,
 		IdentityHint:          s.IdentityHint,
 		SessionID:             s.SessionID,
@@ -123,7 +123,7 @@ func (s *State) deserialize(serialized serializedState) {
 	s.cipherSuite = cipherSuiteForID(CipherSuiteID(serialized.CipherSuiteID), nil)
 
 	atomic.StoreUint64(&s.localSequenceNumber[epoch], serialized.SequenceNumber)
-	s.srtpProtectionProfile = SRTPProtectionProfile(serialized.SRTPProtectionProfile)
+	s.setSRTPProtectionProfile(SRTPProtectionProfile(serialized.SRTPProtectionProfile))
 
 	// Set remote certificate
 	s.PeerCertificates = serialized.PeerCertificates
@@ -214,3 +214,15 @@ func (s *State) getLocalEpoch() uint16 {
 	}
 	return 0
 }
+
+func (s *State) setSRTPProtectionProfile(profile SRTPProtectionProfile) {
+	s.srtpProtectionProfile.Store(profile)
+}
+
+func (s *State) getSRTPProtectionProfile() SRTPProtectionProfile {
+	if val, ok := s.srtpProtectionProfile.Load().(SRTPProtectionProfile); ok {
+		return val
+	}
+
+	return 0
+}