fix(remote): make Ctrl-C interrupt blocking getaddrinfo() (#2540)

[ChrisJr404]

Closes #2540.

What

pwnlib.tubes.remote.remote(host, port) calls socket.getaddrinfo() to resolve host. On glibc, getaddrinfo holds the GIL during the libc DNS round-trip and swallows SIGINT, so a hung resolver leaves the script unkillable from Ctrl-C until the kernel-level resolver timeout (often ~30s).

This PR follows the workaround @Arusekk suggested in the issue thread: swap the SIGINT handler to SIG_DFL for the duration of the resolution call, then restore the original handler.

@contextlib.contextmanager
def _interruptible_block():
    if threading.current_thread() is not threading.main_thread():
        yield
        return
    try:
        old = signal.signal(signal.SIGINT, signal.SIG_DFL)
    except ValueError:
        yield
        return
    try:
        yield
    finally:
        signal.signal(signal.SIGINT, old)

remote._connect now wraps the socket.getaddrinfo(...) call in this context.

Why

Quoted from the issue's main reporter (@tesuji):

I would like to use ctrl-C to quit immediately in these cases.

And from @Arusekk:

Maybe we can set SIGINT handler to SIG_DFL as a workaround?

That's exactly what this does, scoped narrowly to the resolver call so it doesn't perturb anything else.

Tests

• Existing remote() round-trip against a local listen() socket still passes (resolution to a literal IP is instantaneous).
• _interruptible_block swap+restore verified directly: signal.getsignal(SIGINT) is SIG_DFL inside, equal to the original handler outside.
• On a non-main thread the context is a no-op (signal handlers can only be changed from the main thread, so we don't try).

Notes

• Pure-additive: no existing call signature changes. Anyone who wants to keep their custom SIGINT handler around their own getaddrinfo can call it directly without going through remote().
• The fix only covers pwnlib.tubes.remote.remote._connect; if a similar hang ever shows up in another tube's resolver call it can reuse _interruptible_block (or be lifted into a shared module).

[peace-maker]

Thanks for this! This is a cleaner solution than #2541 if it works as well. @k4lizen can you try this too with your flakey domains please?

[k4lizen]

i can confirm it fixes the issue for me

[peace-maker]

This global signal handler doesn't cause trouble in asyncio since we don't await I guess. Thank you for testing @k4lizen and thank you for your resources @ChrisJr404

How could we test this? Configure some DNS server that doesn't exist and abort a query?

[Arusekk]

I can confirm that this works for me, too.

The simplest way to test it is echo nameserver x.y.z.w > /etc/resolv.conf for any x.y.z.w that ignores udp53 packets. You can even try a random routable address, good chance it will hit some CGNAT and get dropped.

Is there any official account of this problem? Any upstream bug report that we can link?

[peace-maker]

Is there any official account of this problem? Any upstream bug report that we can link?

https://bugs.python.org/issue22889

[peace-maker]

This solution kills the python process without triggering the cleanup routines.

from pwn import *
try:
  io = remote('notexisting.lol', 2451)
  io.interactive()
finally:
  print('DONE!!')

Replacing the target nameserver in resolv.conf with some random IP not hosting a dns server stalls the DNS resolution. Pressing Ctrl+C once immediately stops the python process but doesn't print DONE!! nor any other interrupt trace.

The other solution in #2541 using asyncio in a new thread allows to abort the dns resolution somewhat while running the cleanup code. It requires three Ctrl+C presses before the process stops though, so it's not perfect.

But I think allowing graceful shutdown should be desirable, so this solution feels too disruptive. @Arusekk What do you think?

[-] Opening connection to notexisting.lol on port 2451: Failed
DONE!!
Traceback (most recent call last):
  File "/home/peace/tools/pwntools/pwnlib/tubes/remote.py", line 131, in sync_getaddrinfo
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/concurrent/futures/_base.py", line 451, in result
    self._condition.wait(timeout)
  File "/usr/lib/python3.12/threading.py", line 355, in wait
    waiter.acquire()
KeyboardInterrupt

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/peace/tools/pwntools/testdns.py", line 4, in <module>
    io = remote('notexisting.lol', 2451)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/peace/tools/pwntools/pwnlib/tubes/remote.py", line 84, in __init__
    self.sock   = self._connect(fam, typ)
                  ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/peace/tools/pwntools/pwnlib/tubes/remote.py", line 138, in _connect
    hostnames = sync_getaddrinfo(self.rhost, self.rport, fam, typ, 0, socket.AI_PASSIVE)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/peace/tools/pwntools/pwnlib/tubes/remote.py", line 128, in sync_getaddrinfo
    with concurrent.futures.ThreadPoolExecutor() as executor:
  File "/usr/lib/python3.12/concurrent/futures/_base.py", line 647, in __exit__
    self.shutdown(wait=True)
  File "/usr/lib/python3.12/concurrent/futures/thread.py", line 238, in shutdown
    t.join()
  File "/usr/lib/python3.12/threading.py", line 1147, in join
    self._wait_for_tstate_lock()
  File "/usr/lib/python3.12/threading.py", line 1167, in _wait_for_tstate_lock
    if lock.acquire(block, timeout):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyboardInterrupt
^CException ignored in: <module 'threading' from '/usr/lib/python3.12/threading.py'>
Traceback (most recent call last):
  File "/usr/lib/python3.12/threading.py", line 1592, in _shutdown
    atexit_call()
  File "/usr/lib/python3.12/concurrent/futures/thread.py", line 31, in _python_exit
    t.join()
  File "/usr/lib/python3.12/threading.py", line 1147, in join
    self._wait_for_tstate_lock()
  File "/usr/lib/python3.12/threading.py", line 1167, in _wait_for_tstate_lock
    if lock.acquire(block, timeout):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyboardInterrupt: