Re: [PATCH net-next v2] net: WireGuard secure network tunnel

[Dmitry Vyukov]

On Mon, Dec 9, 2019 at 12:28 AM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
>
> WireGuard is a layer 3 secure networking tunnel made specifically for
> the kernel, that aims to be much simpler and easier to audit than IPsec.
> Extensive documentation and description of the protocol and
> considerations, along with formal proofs of the cryptography, are
> available at:
>
>   * https://www.wireguard.com/
>   * https://www.wireguard.com/papers/wireguard.pdf
>
> This commit implements WireGuard as a simple network device driver,
> accessible in the usual RTNL way used by virtual network drivers. It
> makes use of the udp_tunnel APIs, GRO, GSO, NAPI, and the usual set of
> networking subsystem APIs. It has a somewhat novel multicore queueing
> system designed for maximum throughput and minimal latency of encryption
> operations, but it is implemented modestly using workqueues and NAPI.
> Configuration is done via generic Netlink, and following a review from
> the Netlink maintainer a year ago, several high profile userspace tools
> have already implemented the API.
>
> This commit also comes with several different tests, both in-kernel
> tests and out-of-kernel tests based on network namespaces, taking profit
> of the fact that sockets used by WireGuard intentionally stay in the
> namespace the WireGuard interface was originally created, exactly like
> the semantics of userspace tun devices. See wireguard.com/netns/ for
> pictures and examples.
>
> The source code is fairly short, but rather than combining everything
> into a single file, WireGuard is developed as cleanly separable files,
> making auditing and comprehension easier. Things are laid out as
> follows:
>
>   * noise.[ch], cookie.[ch], messages.h: These implement the bulk of the
>     cryptographic aspects of the protocol, and are mostly data-only in
>     nature, taking in buffers of bytes and spitting out buffers of
>     bytes. They also handle reference counting for their various shared
>     pieces of data, like keys and key lists.
>
>   * ratelimiter.[ch]: Used as an integral part of cookie.[ch] for
>     ratelimiting certain types of cryptographic operations in accordance
>     with particular WireGuard semantics.
>
>   * allowedips.[ch], peerlookup.[ch]: The main lookup structures of
>     WireGuard, the former being trie-like with particular semantics, an
>     integral part of the design of the protocol, and the latter just
>     being nice helper functions around the various hashtables we use.
>
>   * device.[ch]: Implementation of functions for the netdevice and for
>     rtnl, responsible for maintaining the life of a given interface and
>     wiring it up to the rest of WireGuard.
>
>   * peer.[ch]: Each interface has a list of peers, with helper functions
>     available here for creation, destruction, and reference counting.
>
>   * socket.[ch]: Implementation of functions related to udp_socket and
>     the general set of kernel socket APIs, for sending and receiving
>     ciphertext UDP packets, and taking care of WireGuard-specific sticky
>     socket routing semantics for the automatic roaming.
>
>   * netlink.[ch]: Userspace API entry point for configuring WireGuard
>     peers and devices. The API has been implemented by several userspace
>     tools and network management utility, and the WireGuard project
>     distributes the basic wg(8) tool.
>
>   * queueing.[ch]: Shared function on the rx and tx path for handling
>     the various queues used in the multicore algorithms.
>
>   * send.c: Handles encrypting outgoing packets in parallel on
>     multiple cores, before sending them in order on a single core, via
>     workqueues and ring buffers. Also handles sending handshake and cookie
>     messages as part of the protocol, in parallel.
>
>   * receive.c: Handles decrypting incoming packets in parallel on
>     multiple cores, before passing them off in order to be ingested via
>     the rest of the networking subsystem with GRO via the typical NAPI
>     poll function. Also handles receiving handshake and cookie messages
>     as part of the protocol, in parallel.
>
>   * timers.[ch]: Uses the timer wheel to implement protocol particular
>     event timeouts, and gives a set of very simple event-driven entry
>     point functions for callers.
>
>   * main.c, version.h: Initialization and deinitialization of the module.
>
>   * selftest/*.h: Runtime unit tests for some of the most security
>     sensitive functions.
>
>   * tools/testing/selftests/wireguard/netns.sh: Aforementioned testing
>     script using network namespaces.
>
> This commit aims to be as self-contained as possible, implementing
> WireGuard as a standalone module not needing much special handling or
> coordination from the network subsystem. I expect for future
> optimizations to the network stack to positively improve WireGuard, and
> vice-versa, but for the time being, this exists as intentionally
> standalone.
>
> We introduce a menu option for CONFIG_WIREGUARD, as well as providing a
> verbose debug log and self-tests via CONFIG_WIREGUARD_DEBUG.

Hi Jason, Dave,

Some late feedback on CONFIG_WIREGUARD_DEBUG.

Does it really do "verbose debug log"? I only see it is used for
self-tests and debug checks:

linux$ grep DEBUG drivers/net/wireguard/*.c
drivers/net/wireguard/allowedips.c: WARN_ON(IS_ENABLED(DEBUG) && *len >= 128);
drivers/net/wireguard/allowedips.c: WARN_ON(IS_ENABLED(DEBUG) && len
>= 128);                      \
drivers/net/wireguard/main.c:#ifdef DEBUG
drivers/net/wireguard/noise.c: WARN_ON(IS_ENABLED(DEBUG) &&

There are 3 different things:
 - boot self-tests
 - additional debug checks
 - verbose logging

In different contexts one may enable different sets of these.
In particular in fuzzing context one absolutely wants additional debug
checks, but not self tests and definitely no verbose logging. CI and
various manual scenarios will require different sets as well.
If this does verbose logging, we won't get debug checks as well during
fuzzing, which is unfortunate.
Can make sense splitting CONFIG_WIREGUARD_DEBUG into 2 or 3 separate
configs (that's what I see frequently). Unfortunately there is no
standard conventions for anything of this, so CIs will never find your
boot tests and fuzzing won't find the additional checks...