cmd/derper: fix mesh auth for DERP servers (#16061)

To authenticate mesh keys, the DERP servers used a simple == comparison,
which is susceptible to a side channel timing attack.

By extracting the mesh key for a DERP server, an attacker could DoS it
by forcing disconnects using derp.Client.ClosePeer. They could also
enumerate the public Wireguard keys, IP addresses and ports for nodes
connected to that DERP server.

DERP servers configured without mesh keys deny all such requests.

This patch also extracts the mesh key logic into key.DERPMesh, to
prevent this from happening again.

Security bulletin: https://tailscale.com/security-bulletins#ts-2025-003

Fixes tailscale/corp#28720

Signed-off-by: Simon Law <sfllaw@tailscale.com>

derp/derp_server.go

@@ -134,7 +134,7 @@ type Server struct {
 	publicKey   key.NodePublic
 	logf        logger.Logf
 	memSys0     uint64 // runtime.MemStats.Sys at start (or early-ish)
-	meshKey     string
+	meshKey     key.DERPMesh
 	limitedLogf logger.Logf
 	metaCert    []byte // the encoded x509 cert to send after LetsEncrypt cert+intermediate
 	dupPolicy   dupPolicy
@@ -464,8 +464,13 @@ func genDroppedCounters() {
 // amongst themselves.
 //
 // It must be called before serving begins.
-func (s *Server) SetMeshKey(v string) {
-	s.meshKey = v
+func (s *Server) SetMeshKey(v string) error {
+	k, err := key.ParseDERPMesh(v)
+	if err != nil {
+		return err
+	}
+	s.meshKey = k
+	return nil
 }
 
 // SetVerifyClients sets whether this DERP server verifies clients through tailscaled.
@@ -506,10 +511,10 @@ func (s *Server) SetTCPWriteTimeout(d time.Duration) {
 }
 
 // HasMeshKey reports whether the server is configured with a mesh key.
-func (s *Server) HasMeshKey() bool { return s.meshKey != "" }
+func (s *Server) HasMeshKey() bool { return !s.meshKey.IsZero() }
 
 // MeshKey returns the configured mesh key, if any.
-func (s *Server) MeshKey() string { return s.meshKey }
+func (s *Server) MeshKey() key.DERPMesh { return s.meshKey }
 
 // PrivateKey returns the server's private key.
 func (s *Server) PrivateKey() key.NodePrivate { return s.privateKey }
@@ -1355,7 +1360,18 @@ func (c *sclient) requestMeshUpdate() {
 // isMeshPeer reports whether the client is a trusted mesh peer
 // node in the DERP region.
 func (s *Server) isMeshPeer(info *clientInfo) bool {
-	return info != nil && info.MeshKey != "" && info.MeshKey == s.meshKey
+	// Compare mesh keys in constant time to prevent timing attacks.
+	// Since mesh keys are a fixed length, we don’t need to be concerned
+	// about timing attacks on client mesh keys that are the wrong length.
+	// See https://github.com/tailscale/corp/issues/28720
+	if info == nil || info.MeshKey == "" {
+		return false
+	}
+	k, err := key.ParseDERPMesh(info.MeshKey)
+	if err != nil {
+		return false
+	}
+	return s.meshKey.Equal(k)
 }
 
 // verifyClient checks whether the client is allowed to connect to the derper,