device: make unmarshall length checks exact

This is already enforced in receive.go, but if these unmarshallers are
to have error return values anyway, make them as explicit as possible.

cherry picked from commit WireGuard/wireguard-go@842888a

Updates tailscale/corp#28879

Signed-off-by: Jason A. Donenfeld <[email protected]>

Author: zx2c4

device/noise-protocol.go

@@ -116,11 +116,11 @@ type MessageCookieReply struct {
 	Cookie   [blake2s.Size128 + poly1305.TagSize]byte
 }
 
-var errMessageTooShort = errors.New("message too short")
+var errMessageLengthMismatch = errors.New("message length mismatch")
 
 func (msg *MessageInitiation) unmarshal(b []byte) error {
-	if len(b) < MessageInitiationSize {
-		return errMessageTooShort
+	if len(b) != MessageInitiationSize {
+		return errMessageLengthMismatch
 	}
 
 	msg.Type = binary.LittleEndian.Uint32(b)
@@ -135,8 +135,8 @@ func (msg *MessageInitiation) unmarshal(b []byte) error {
 }
 
 func (msg *MessageResponse) unmarshal(b []byte) error {
-	if len(b) < MessageResponseSize {
-		return errMessageTooShort
+	if len(b) != MessageResponseSize {
+		return errMessageLengthMismatch
 	}
 
 	msg.Type = binary.LittleEndian.Uint32(b)
@@ -151,8 +151,8 @@ func (msg *MessageResponse) unmarshal(b []byte) error {
 }
 
 func (msg *MessageCookieReply) unmarshal(b []byte) error {
-	if len(b) < MessageCookieReplySize {
-		return errMessageTooShort
+	if len(b) != MessageCookieReplySize {
+		return errMessageLengthMismatch
 	}
 
 	msg.Type = binary.LittleEndian.Uint32(b)