ipn, ipnserver: only require sudo on Linux for mutable CLI actions

This partially reverts d6e9fb1df0, which modified the permissions
on the tailscaled Unix socket and thus required "sudo tailscale" even
for "tailscale status".

Instead, open the permissions back up (on Linux only) but have the
server look at the peer creds and only permit read-only actions unless
you're root.

In the future we'll also have a group that can do mutable actions.

On OpenBSD and FreeBSD, the permissions on the socket remain locked
down to 0600 from d6e9fb1df0.

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

safesocket/unixsocket.go

@@ -64,10 +64,33 @@ func listen(path string, port uint16) (ln net.Listener, _ uint16, err error) {
 	if err != nil {
 		return nil, 0, err
 	}
-	os.Chmod(path, 0600)
+	os.Chmod(path, socketPermissionsForOS())
 	return pipe, 0, err
 }
 
+// socketPermissionsForOS returns the permissions to use for the
+// tailscaled.sock.
+func socketPermissionsForOS() os.FileMode {
+	if runtime.GOOS == "linux" {
+		// On Linux, the ipn/ipnserver package looks at the Unix peer creds
+		// and only permits read-only actions from non-root users, so we want
+		// this opened up wider.
+		//
+		// TODO(bradfitz): unify this all one in place probably, moving some
+		// of ipnserver (which does much of the "safe" bits) here. Maybe
+		// instead of net.Listener, we should return a type that returns
+		// an identity in addition to a net.Conn? (returning a wrapped net.Conn
+		// would surprise downstream callers probably)
+		//
+		// TODO(bradfitz): if OpenBSD and FreeBSD do the equivalent peercreds
+		// stuff that's in ipn/ipnserver/conn_ucred.go, they should also
+		// return 0666 here.
+		return 0666
+	}
+	// Otherwise, root only.
+	return 0600
+}
+
 // connectMacOSAppSandbox connects to the Tailscale Network Extension,
 // which is necessarily running within the macOS App Sandbox.  Our
 // little dance to connect a regular user binary to the sandboxed