remove now-obsolete note that engine's don't support ssh

docs/source/parallel/parallel_security.txt

@@ -1,254 +1,251 b''
 .. _parallelsecurity:
 
 ===========================
 Security details of IPython
 ===========================
 
 .. note::
 
     This section is not thorough, and IPython.zmq needs a thorough security
     audit.
 
 IPython's :mod:`IPython.zmq` package exposes the full power of the
 Python interpreter over a TCP/IP network for the purposes of parallel
 computing. This feature brings up the important question of IPython's security
 model. This document gives details about this model and how it is implemented
 in IPython's architecture.
 
 Process and network topology
 ============================
 
 To enable parallel computing, IPython has a number of different processes that
 run. These processes are discussed at length in the IPython documentation and
 are summarized here:
 
 * The IPython *engine*.  This process is a full blown Python
   interpreter in which user code is executed.  Multiple
   engines are started to make parallel computing possible.
 * The IPython *hub*.  This process monitors a set of 
   engines and schedulers, and keeps track of the state of the processes. It listens
   for registration connections from engines and clients, and monitor connections
   from schedulers.
 * The IPython *schedulers*. This is a set of processes that relay commands and results
   between clients and engines. They are typically on the same machine as the controller,
   and listen for connections from engines and clients, but connect to the Hub.
 * The IPython *client*.  This process is typically an
   interactive Python process that is used to coordinate the
   engines to get a parallel computation done.
 
 Collectively, these processes are called the IPython *cluster*, and the hub and schedulers
 together are referred to as the *controller*.
 
 
 These processes communicate over any transport supported by ZeroMQ (tcp,pgm,infiniband,ipc)
 with a well defined topology. The IPython hub and schedulers listen on sockets. Upon
 starting, an engine connects to a hub and registers itself, which then informs the engine
 of the connection information for the schedulers, and the engine then connects to the
 schedulers. These engine/hub and engine/scheduler connections persist for the
 lifetime of each engine.
 
 The IPython client also connects to the controller processes using a number of socket
 connections. As of writing, this is one socket per scheduler (4), and 3 connections to the
 hub for a total of 7. These connections persist for the lifetime of the client only.
 
 A given IPython controller and set of engines engines typically has a relatively
 short lifetime. Typically this lifetime corresponds to the duration of a single parallel
 simulation performed by a single user. Finally, the hub, schedulers, engines, and client
 processes typically execute with the permissions of that same user. More specifically, the
 controller and engines are *not* executed as root or with any other superuser permissions.
 
 Application logic
 =================
 
 When running the IPython kernel to perform a parallel computation, a user
 utilizes the IPython client to send Python commands and data through the
 IPython schedulers to the IPython engines, where those commands are executed
 and the data processed. The design of IPython ensures that the client is the
 only access point for the capabilities of the engines. That is, the only way
 of addressing the engines is through a client.
 
 A user can utilize the client to instruct the IPython engines to execute
 arbitrary Python commands. These Python commands can include calls to the
 system shell, access the filesystem, etc., as required by the user's
 application code. From this perspective, when a user runs an IPython engine on
 a host, that engine has the same capabilities and permissions as the user
 themselves (as if they were logged onto the engine's host with a terminal).
 
 Secure network connections
 ==========================
 
 Overview
 --------
 
 ZeroMQ provides exactly no security. For this reason, users of IPython must be very
 careful in managing connections, because an open TCP/IP socket presents access to
 arbitrary execution as the user on the engine machines. As a result, the default behavior
 of controller processes is to only listen for clients on the loopback interface, and the
 client must establish SSH tunnels to connect to the controller processes.
 
 .. warning:: 
 
     If the controller's loopback interface is untrusted, then IPython should be considered
     vulnerable, and this extends to the loopback of all connected clients, which have
     opened a loopback port that is redirected to the controller's loopback port.
 
 
 SSH
 ---
 
 Since ZeroMQ provides no security, SSH tunnels are the primary source of secure
 connections. A connector file, such as `ipcontroller-client.json`, will contain
 information for connecting to the controller, possibly including the address of an
 ssh-server through with the client is to tunnel. The Client object then creates tunnels
 using either [OpenSSH]_ or [Paramiko]_, depending on the platform. If users do not wish to
 use OpenSSH or Paramiko, or the tunneling utilities are insufficient, then they may
 construct the tunnels themselves, and simply connect clients and engines as if the
 controller were on loopback on the connecting machine.
 
-.. note::
-
-    There is not currently tunneling available for engines.
 
 Authentication
 --------------
 
 To protect users of shared machines, [HMAC]_ digests are used to sign messages, using a
 shared key.
 
 The Session object that handles the message protocol uses a unique key to verify valid
 messages. This can be any value specified by the user, but the default behavior is a
 pseudo-random 128-bit number, as generated by `uuid.uuid4()`. This key is used to
 initialize an HMAC object, which digests all messages, and includes that digest as a
 signature and part of the message. Every message that is unpacked (on Controller, Engine,
 and Client) will also be digested by the receiver, ensuring that the sender's key is the
 same as the receiver's. No messages that do not contain this key are acted upon in any
 way. The key itself is never sent over the network.
 
 There is exactly one shared key per cluster - it must be the same everywhere. Typically,
 the controller creates this key, and stores it in the private connection files
 `ipython-{engine|client}.json`. These files are typically stored in the
 `~/.ipython/profile_<name>/security` directory, and are maintained as readable only by the
 owner, just as is common practice with a user's keys in their `.ssh` directory.
 
 .. warning::
 
     It is important to note that the signatures protect against unauthorized messages,
     but, as there is no encryption, provide exactly no protection of data privacy.  It is
     possible, however, to use a custom serialization scheme (via Session.packer/unpacker
     traits) that does incorporate your own encryption scheme.
 
 
 
 Specific security vulnerabilities
 =================================
 
 There are a number of potential security vulnerabilities present in IPython's
 architecture. In this section we discuss those vulnerabilities and detail how
 the security architecture described above prevents them from being exploited.
 
 Unauthorized clients
 --------------------
 
 The IPython client can instruct the IPython engines to execute arbitrary
 Python code with the permissions of the user who started the engines. If an
 attacker were able to connect their own hostile IPython client to the IPython
 controller, they could instruct the engines to execute code.
 
 
 On the first level, this attack is prevented by requiring access to the controller's
 ports, which are recommended to only be open on loopback if the controller is on an
 untrusted local network. If the attacker does have access to the Controller's ports, then
 the attack is prevented by the capabilities based client authentication of the execution
 key. The relevant authentication information is encoded into the JSON file that clients
 must present to gain access to the IPython controller. By limiting the distribution of
 those keys, a user can grant access to only authorized persons, just as with SSH keys.
 
 It is highly unlikely that an execution key could be guessed by an attacker
 in a brute force guessing attack. A given instance of the IPython controller
 only runs for a relatively short amount of time (on the order of hours). Thus
 an attacker would have only a limited amount of time to test a search space of
 size 2**128.  For added security, users can have arbitrarily long keys.
 
 .. warning::
 
     If the attacker has gained enough access to intercept loopback connections on *either* the
     controller or client, then a duplicate message can be sent. To protect against this,
     recipients only allow each signature once, and consider duplicates invalid.  However,
     the duplicate message could be sent to *another* recipient using the same key,
     and it would be considered valid.
 
 
 Unauthorized engines
 --------------------
 
 If an attacker were able to connect a hostile engine to a user's controller,
 the user might unknowingly send sensitive code or data to the hostile engine.
 This attacker's engine would then have full access to that code and data.
 
 This type of attack is prevented in the same way as the unauthorized client
 attack, through the usage of the capabilities based authentication scheme.
 
 Unauthorized controllers
 ------------------------
 
 It is also possible that an attacker could try to convince a user's IPython
 client or engine to connect to a hostile IPython controller. That controller
 would then have full access to the code and data sent between the IPython
 client and the IPython engines.
 
 Again, this attack is prevented through the capabilities in a connection file, which
 ensure that a client or engine connects to the correct controller. It is also important to
 note that the connection files also encode the IP address and port that the controller is
 listening on, so there is little chance of mistakenly connecting to a controller running
 on a different IP address and port.
 
 When starting an engine or client, a user must specify the key to use
 for that connection. Thus, in order to introduce a hostile controller, the
 attacker must convince the user to use the key associated with the
 hostile controller. As long as a user is diligent in only using keys from
 trusted sources, this attack is not possible.
 
 .. note::
 
     I may be wrong, the unauthorized controller may be easier to fake than this.
 
 Other security measures
 =======================
 
 A number of other measures are taken to further limit the security risks
 involved in running the IPython kernel.
 
 First, by default, the IPython controller listens on random port numbers.
 While this can be overridden by the user, in the default configuration, an
 attacker would have to do a port scan to even find a controller to attack.
 When coupled with the relatively short running time of a typical controller
 (on the order of hours), an attacker would have to work extremely hard and
 extremely *fast* to even find a running controller to attack.
 
 Second, much of the time, especially when run on supercomputers or clusters,
 the controller is running behind a firewall. Thus, for engines or client to
 connect to the controller:
 
 * The different processes have to all be behind the firewall.
 
 or:
 
 * The user has to use SSH port forwarding to tunnel the
   connections through the firewall.
    
 In either case, an attacker is presented with additional barriers that prevent
 attacking or even probing the system.
 
 Summary
 =======
 
 IPython's architecture has been carefully designed with security in mind. The
 capabilities based authentication model, in conjunction with SSH tunneled
 TCP/IP channels, address the core potential vulnerabilities in the system,
 while still enabling user's to use the system in open networks.
 
 .. [RFC5246] <http://tools.ietf.org/html/rfc5246>
 
 .. [OpenSSH] <http://www.openssh.com/>
 .. [Paramiko] <http://www.lag.net/paramiko/>
 .. [HMAC] <http://tools.ietf.org/html/rfc2104.html>