python-netfilterqueue/tests/conftest.py

import math
import os
import pytest
import socket
import subprocess
import sys
import trio
import unshare
import netfilterqueue
from functools import partial
from typing import AsyncIterator, Callable, Optional, Tuple
from async_generator import asynccontextmanager
from pytest_trio.enable_trio_mode import *


# We'll create three network namespaces, representing a router (which
# has interfaces on ROUTER_IP[1, 2]) and two hosts connected to it
# (PEER_IP[1, 2] respectively). The router (in the parent pytest
# process) will configure netfilterqueue iptables rules and use them
# to intercept and modify traffic between the two hosts (each of which
# is implemented in a subprocess).
#
# The 'peer' subprocesses communicate with each other over UDP, and
# with the router parent over a UNIX domain SOCK_SEQPACKET socketpair.
# Each packet sent from the parent to one peer over the UNIX domain
# socket will be forwarded to the other peer over UDP. Each packet
# received over UDP by either of the peers will be forwarded to its
# parent.

ROUTER_IP = {1: "172.16.101.1", 2: "172.16.102.1"}
PEER_IP = {1: "172.16.101.2", 2: "172.16.102.2"}


def enter_netns() -> None:
    # Create new namespaces of the other types we need
    unshare.unshare(unshare.CLONE_NEWNS | unshare.CLONE_NEWNET)

    # Mount /sys so network tools work
    subprocess.run("/bin/mount -t sysfs sys /sys".split(), check=True)

    # Bind-mount /run so iptables can get its lock
    subprocess.run("/bin/mount -t tmpfs tmpfs /run".split(), check=True)

    # Set up loopback interface
    subprocess.run("/sbin/ip link set lo up".split(), check=True)


@pytest.hookimpl(tryfirst=True)
def pytest_runtestloop():
    if os.getuid() != 0:
        # Create a new user namespace for the whole test session
        outer = {"uid": os.getuid(), "gid": os.getgid()}
        unshare.unshare(unshare.CLONE_NEWUSER)
        with open("/proc/self/setgroups", "wb") as fp:
            # This is required since we're unprivileged outside the namespace
            fp.write(b"deny")
        for idtype in ("uid", "gid"):
            with open(f"/proc/self/{idtype}_map", "wb") as fp:
                fp.write(b"0 %d 1" % (outer[idtype],))
        assert os.getuid() == os.getgid() == 0

    # Create a new network namespace for this pytest process
    enter_netns()
    with open("/proc/sys/net/ipv4/ip_forward", "wb") as fp:
        fp.write(b"1\n")


async def peer_main(idx: int, parent_fd: int) -> None:
    parent = trio.socket.fromfd(parent_fd, socket.AF_UNIX, socket.SOCK_SEQPACKET)

    # Tell parent we've set up our netns, wait for it to confirm it's
    # created our veth interface
    await parent.send(b"ok")
    assert b"ok" == await parent.recv(4096)

    my_ip = PEER_IP[idx]
    router_ip = ROUTER_IP[idx]
    peer_ip = PEER_IP[3 - idx]

    for cmd in (
        f"ip link set veth0 up",
        f"ip addr add {my_ip}/24 dev veth0",
        f"ip route add default via {router_ip} dev veth0",
    ):
        await trio.run_process(cmd.split(), capture_stdout=True, capture_stderr=True)

    peer = trio.socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    await peer.bind((my_ip, 0))

    # Tell the parent our port and get our peer's port
    await parent.send(b"%d" % peer.getsockname()[1])
    peer_port = int(await parent.recv(4096))
    await peer.connect((peer_ip, peer_port))

    # Enter the message-forwarding loop
    async def proxy_one_way(src, dest):
        while src.fileno() >= 0:
            try:
                msg = await src.recv(4096)
            except trio.ClosedResourceError:
                return
            if not msg:
                dest.close()
                return
            try:
                await dest.send(msg)
            except BrokenPipeError:
                return

    async with trio.open_nursery() as nursery:
        nursery.start_soon(proxy_one_way, parent, peer)
        nursery.start_soon(proxy_one_way, peer, parent)


def _default_capture_cb(
    target: "trio.MemorySendChannel[netfilterqueue.Packet]",
    packet: netfilterqueue.Packet,
) -> None:
    packet.retain()
    target.send_nowait(packet)


class Harness:
    def __init__(self):
        self._received = {}
        self._conn = {}
        self.dest_addr = {}
        self.failed = False

    async def _run_peer(self, idx: int, *, task_status):
        their_ip = PEER_IP[idx]
        my_ip = ROUTER_IP[idx]
        conn, child_conn = trio.socket.socketpair(socket.AF_UNIX, socket.SOCK_SEQPACKET)
        with conn:
            try:
                process = await trio.open_process(
                    [sys.executable, __file__, str(idx), str(child_conn.fileno())],
                    stdin=subprocess.DEVNULL,
                    pass_fds=[child_conn.fileno()],
                    preexec_fn=enter_netns,
                )
            finally:
                child_conn.close()
            assert b"ok" == await conn.recv(4096)
            for cmd in (
                f"ip link add veth{idx} type veth peer netns {process.pid} name veth0",
                f"ip link set veth{idx} up",
                f"ip addr add {my_ip}/24 dev veth{idx}",
            ):
                await trio.run_process(cmd.split())

            try:
                await conn.send(b"ok")
                self._conn[idx] = conn
                task_status.started()
                retval = await process.wait()
            except BaseException:
                process.kill()
                with trio.CancelScope(shield=True):
                    await process.wait()
                raise
            else:
                if retval != 0:
                    raise RuntimeError(
                        "peer subprocess exited with code {}".format(retval)
                    )
            finally:
                # On some kernels the veth device is removed when the subprocess exits
                # and its netns goes away. check=False to suppress that error.
                await trio.run_process(f"ip link delete veth{idx}".split(), check=False)

    async def _manage_peer(self, idx: int, *, task_status):
        async with trio.open_nursery() as nursery:
            await nursery.start(self._run_peer, idx)
            packets_w, packets_r = trio.open_memory_channel(math.inf)
            self._received[idx] = packets_r
            task_status.started()
            async with packets_w:
                while True:
                    msg = await self._conn[idx].recv(4096)
                    if not msg:
                        break
                    await packets_w.send(msg)

    @asynccontextmanager
    async def run(self):
        async with trio.open_nursery() as nursery:
            async with trio.open_nursery() as start_nursery:
                start_nursery.start_soon(nursery.start, self._manage_peer, 1)
                start_nursery.start_soon(nursery.start, self._manage_peer, 2)
            # Tell each peer about the other one's port
            for idx in (1, 2):
                self.dest_addr[idx] = (
                    PEER_IP[idx],
                    int(await self._received[idx].receive()),
                )
                await self._conn[3 - idx].send(b"%d" % self.dest_addr[idx][1])
            yield
            self._conn[1].shutdown(socket.SHUT_WR)
            self._conn[2].shutdown(socket.SHUT_WR)

        if not self.failed:
            for idx in (1, 2):
                async for remainder in self._received[idx]:
                    raise AssertionError(
                        f"Peer {idx} received unexepcted packet {remainder!r}"
                    )

    def bind_queue(
        self,
        cb: Callable[[netfilterqueue.Packet], None],
        *,
        queue_num: int = -1,
        **options: int,
    ) -> Tuple[int, netfilterqueue.NetfilterQueue]:
        nfq = netfilterqueue.NetfilterQueue()
        # Use a smaller socket buffer to avoid a warning in CI
        options.setdefault("sock_len", 131072)
        if queue_num >= 0:
            nfq.bind(queue_num, cb, **options)
        else:
            for queue_num in range(16):
                try:
                    nfq.bind(queue_num, cb, **options)
                    break
                except Exception as ex:
                    last_error = ex
            else:
                raise RuntimeError(
                    "Couldn't bind any netfilter queue number between 0-15"
                ) from last_error
        return queue_num, nfq

    @asynccontextmanager
    async def enqueue_packets_to(
        self, idx: int, queue_num: int, *, forwarded: bool = True
    ) -> AsyncIterator[None]:
        if forwarded:
            chain = "FORWARD"
        else:
            chain = "OUTPUT"

        rule = f"{chain} -d {PEER_IP[idx]} -j NFQUEUE --queue-num {queue_num}"
        await trio.run_process(f"/sbin/iptables -A {rule}".split())
        try:
            yield
        finally:
            await trio.run_process(f"/sbin/iptables -D {rule}".split())

    @asynccontextmanager
    async def capture_packets_to(
        self,
        idx: int,
        cb: Callable[
            ["trio.MemorySendChannel[netfilterqueue.Packet]", netfilterqueue.Packet],
            None,
        ] = _default_capture_cb,
        **options: int,
    ) -> AsyncIterator["trio.MemoryReceiveChannel[netfilterqueue.Packet]"]:

        packets_w, packets_r = trio.open_memory_channel(math.inf)
        queue_num, nfq = self.bind_queue(partial(cb, packets_w), **options)
        try:
            async with self.enqueue_packets_to(idx, queue_num):
                async with packets_w, trio.open_nursery() as nursery:

                    @nursery.start_soon
                    async def listen_for_packets():
                        while True:
                            await trio.lowlevel.wait_readable(nfq.get_fd())
                            nfq.run(block=False)

                    yield packets_r
                    nursery.cancel_scope.cancel()
        finally:
            nfq.unbind()

    async def expect(self, idx: int, *packets: bytes):
        for expected in packets:
            with trio.move_on_after(5) as scope:
                received = await self._received[idx].receive()
            if scope.cancelled_caught:
                self.failed = True
                raise AssertionError(
                    f"Timeout waiting for peer {idx} to receive {expected!r}"
                )
            if received != expected:
                self.failed = True
                raise AssertionError(
                    f"Expected peer {idx} to receive {expected!r} but it "
                    f"received {received!r}"
                )

    async def send(self, idx: int, *packets: bytes):
        for packet in packets:
            await self._conn[3 - idx].send(packet)


@pytest.fixture
async def harness() -> Harness:
    h = Harness()
    async with h.run():
        yield h


if __name__ == "__main__":
    trio.run(peer_main, int(sys.argv[1]), int(sys.argv[2]))