wgengine/magicsock: filter disco packets and packets when stopped from wireguard

Fixes #1167
Fixes tailscale/corp#219

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -304,6 +304,9 @@ type Conn struct {
 	// with IPv4 or IPv6). It's used to suppress log spam and prevent
 	// new connection that'll fail.
 	networkUp syncs.AtomicBool
+
+	// havePrivateKey is whether privateKey is non-zero.
+	havePrivateKey syncs.AtomicBool
 }
 
 // derpRoute is a route entry for a public key, saying that a certain
@@ -1588,6 +1591,9 @@ func (c *Conn) ReceiveIPv4(b []byte) (n int, ep conn.Endpoint, err error) {
 }
 
 // receiveIP is the shared bits of ReceiveIPv4 and ReceiveIPv6.
+//
+// ok is whether this read should be reported up to wireguard-go (our
+// caller).
 func (c *Conn) receiveIP(b []byte, ua *net.UDPAddr, cache *ippEndpointCache) (ep conn.Endpoint, ok bool) {
 	ipp, ok := netaddr.FromStdAddr(ua.IP, ua.Port, ua.Zone)
 	if !ok {
@@ -1600,6 +1606,13 @@ func (c *Conn) receiveIP(b []byte, ua *net.UDPAddr, cache *ippEndpointCache) (ep
 	if c.handleDiscoMessage(b, ipp) {
 		return nil, false
 	}
+	if !c.havePrivateKey.Get() {
+		// If we have no private key, we're logged out or
+		// stopped.  Don't try to pass these wireguard packets
+		// up to wireguard-go; it'll just complain (Issue
+		// 1167).
+		return nil, false
+	}
 	if cache.ipp == ipp && cache.de != nil && cache.gen == cache.de.numStopAndReset() {
 		ep = cache.de
 	} else {
@@ -1750,8 +1763,8 @@ func (c *Conn) sendDiscoMessage(dst netaddr.IPPort, dstKey tailcfg.NodeKey, dstD
 	return sent, err
 }
 
-// handleDiscoMessage reports whether msg was a Tailscale inter-node discovery message
+// handleDiscoMessage handles a discovery message and reports whether
-// that was handled.
+// msg was a Tailscale inter-node discovery message.
 //
 // A discovery message has the form:
 //
@@ -1762,11 +1775,18 @@ func (c *Conn) sendDiscoMessage(dst netaddr.IPPort, dstKey tailcfg.NodeKey, dstD
 //
 // For messages received over DERP, the addr will be derpMagicIP (with
 // port being the region)
-func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
+func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) (isDiscoMsg bool) {
 	const headerLen = len(disco.Magic) + len(tailcfg.DiscoKey{}) + disco.NonceLen
 	if len(msg) < headerLen || string(msg[:len(disco.Magic)]) != disco.Magic {
 		return false
 	}
+
+	// If the first four parts are the prefix of disco.Magic
+	// (0x5453f09f) then it's definitely not a valid Wireguard
+	// packet (which starts with little-endian uint32 1, 2, 3, 4).
+	// Use naked returns for all following paths.
+	isDiscoMsg = true
+
 	var sender tailcfg.DiscoKey
 	copy(sender[:], msg[len(disco.Magic):])
 
@@ -1774,20 +1794,21 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 	defer c.mu.Unlock()
 
 	if c.closed {
-		return true
+		return
 	}
 	if debugDisco {
 		c.logf("magicsock: disco: got disco-looking frame from %v", sender.ShortString())
 	}
 	if c.privateKey.IsZero() {
 		// Ignore disco messages when we're stopped.
-		return false
+		// Still return true, to not pass it down to wireguard.
+		return
 	}
 	if c.discoPrivate.IsZero() {
 		if debugDisco {
 			c.logf("magicsock: disco: ignoring disco-looking frame, no local key")
 		}
-		return false
+		return
 	}
 
 	peerNode, ok := c.nodeOfDisco[sender]
@@ -1795,9 +1816,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 		if debugDisco {
 			c.logf("magicsock: disco: ignoring disco-looking frame, don't know node for %v", sender.ShortString())
 		}
-		// Returning false keeps passing it down, to WireGuard.
+		return
-		// WireGuard will almost surely reject it, but give it a chance.
-		return false
 	}
 
 	needsRecvActivityCall := false
@@ -1810,7 +1829,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 		c.logf("magicsock: got disco message from idle peer, starting lazy conf for %v, %v", peerNode.Key.ShortString(), sender.ShortString())
 		if c.noteRecvActivity == nil {
 			c.logf("magicsock: [unexpected] have node without endpoint, without c.noteRecvActivity hook")
-			return false
+			return
 		}
 		needsRecvActivityCall = true
 	} else {
@@ -1829,7 +1848,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 		// Now, recheck invariants that might've changed while we'd
 		// released the lock, which isn't much:
 		if c.closed || c.privateKey.IsZero() {
-			return true
+			return
 		}
 		de, ok = c.endpointOfDisco[sender]
 		if !ok {
@@ -1838,7 +1857,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 				return false
 			}
 			c.logf("magicsock: [unexpected] lazy endpoint not created for %v, %v", peerNode.Key.ShortString(), sender.ShortString())
-			return false
+			return
 		}
 		if !endpointFound0 {
 			c.logf("magicsock: lazy endpoint created via disco message for %v, %v", peerNode.Key.ShortString(), sender.ShortString())
@@ -1865,7 +1884,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 			c.logf("magicsock: disco: failed to open naclbox from %v (wrong rcpt?)", sender)
 		}
 		// TODO(bradfitz): add some counter for this that logs rarely
-		return false
+		return
 	}
 
 	dm, err := disco.Parse(payload)
@@ -1879,7 +1898,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 		// understand. Not even worth logging about, lest it
 		// be too spammy for old clients.
 		// TODO(bradfitz): add some counter for this that logs rarely
-		return true
+		return
 	}
 
 	switch dm := dm.(type) {
@@ -1887,14 +1906,14 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 		c.handlePingLocked(dm, de, src, sender, peerNode)
 	case *disco.Pong:
 		if de == nil {
-			return true
+			return
 		}
 		de.handlePongConnLocked(dm, src)
 	case *disco.CallMeMaybe:
 		if src.IP != derpMagicIPAddr {
 			// CallMeMaybe messages should only come via DERP.
 			c.logf("[unexpected] CallMeMaybe packets should only come via DERP")
-			return true
+			return
 		}
 		if de != nil {
 			c.logf("magicsock: disco: %v<-%v (%v, %v)  got call-me-maybe, %d endpoints",
@@ -1904,8 +1923,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 			go de.handleCallMeMaybe(dm)
 		}
 	}
-
+	return
-	return true
 }
 
 func (c *Conn) handlePingLocked(dm *disco.Ping, de *discoEndpoint, src netaddr.IPPort, sender tailcfg.DiscoKey, peerNode *tailcfg.Node) {
@@ -2082,6 +2100,7 @@ func (c *Conn) SetPrivateKey(privateKey wgkey.Private) error {
 		return nil
 	}
 	c.privateKey = newKey
+	c.havePrivateKey.Set(!newKey.IsZero())
 
 	if oldKey.IsZero() {
 		c.everHadKey = true