russh: peer-declared zero maximum packet size permanently stalls channel writes (silent AsyncWrite hang)

Summary

A peer can advertise maximum packet size = 0 in its channel-open message (CHANNEL_OPEN when attacking a server, CHANNEL_OPEN_CONFIRMATION when attacking a client). russh accepts the value without validation and copies it into the channel write state, where it caps the size of every outgoing data chunk. In the AsyncWrite code path (ChannelTx::poll_writable, used by Channel::data(), Channel::make_writer() and Channel::into_stream()), a cap of 0 makes every write future stay Poll::Pending forever: writable = max_packet_size.min(window_size).min(buf_len) is always 0, and no future event can ever raise it (a static maximum packet size of 0 can never be satisfied by any amount of peer-granted window). The write neither completes nor errors, so the application task hangs silently.

Every other write path in russh already treats max_packet_size == 0 as an error (Error::Inconsistent): Channel::reserve_writable_chunk (russh/src/channels/mod.rs:363-365) and the session packetizers (russh/src/session.rs:489-492 and 549-552). The AsyncWrite path is the only one that lacks the guard - it contradicts the crate's own invariant.

Root cause

(1) Peer value accepted without validation:

• russh/src/parsing.rs:44 (OpenChannelMessage::parse): let maxpacket = map_err!(u32::decode(r))?; stored as recipient_maximum_packet_size (:25-30); no range check. Same for ChannelOpenConfirmation::decode at russh/src/parsing.rs:194-207.
• The only packet-size validation anywhere is an upper bound on the local config that merely logs (russh/src/client/mod.rs:1189-1194, russh/src/server/mod.rs:912-917). Nothing validates the peer's value - 0 passes silently.

(2) The value flows into the channel write state verbatim:

• Server: russh/src/server/encrypted.rs:1877-1900 - ChannelParams { recipient_maximum_packet_size: msg.recipient_maximum_packet_size, .. }, then Channel::new(..., channel_params.recipient_maximum_packet_size, ...).
• Client: russh/src/client/encrypted.rs:471-488 - parameters.confirm(&msg) (russh/src/lib_inner.rs:519-523) and ChannelMsg::Open { max_packet_size: msg.maximum_packet_size, .. }, consumed by wait_channel_confirmation (russh/src/client/mod.rs:698-722), which builds ChannelWriteHalf { max_packet_size, .. }.
• The write half hands the value to the writer: make_writer_ext (russh/src/channels/mod.rs:433-441) and into_stream (russh/src/channels/mod.rs:661-670) pass self.max_packet_size to ChannelTx::new.

(3) The hang - russh/src/channels/io/tx.rs:82-108 (current v0.64.1, byte-identical in v0.60.0):

fn poll_writable(&mut self, cx: &mut Context<'_>, buf_len: usize) -> Poll<NonZeroUsize> {
    ...
    let writable = (self.max_packet_size).min(*window_size).min(buf_len as u32) as usize;
    match NonZeroUsize::try_from(writable) {
        Ok(w) => {
            *window_size -= writable as u32;
            if *window_size > 0 {
                self.notify.notify_one();
            }
            Poll::Ready(w)
        }
        Err(_) => {
            drop(window_size);
            ready!(self.window_size_notication.poll_unpin(cx));
            self.window_size_notication = WatchNotification::new(Arc::clone(&self.notify));
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }
}

With max_packet_size == 0, writable is always 0 regardless of window_size and buf_len; the task parks on the window notification, gets re-polled on every CHANNEL_WINDOW_ADJUST, recomputes 0, and parks again - forever.

(4) The guard exists everywhere else:

// russh/src/channels/mod.rs:363-365 (used by send_bytes -> data_bytes / extended_data_bytes)
if self.max_packet_size == 0 {
    return Err(Error::Inconsistent);
}

// russh/src/session.rs:490-492 and :550-552 (packetizer)
if max_packet_size == 0 {
    return Err(crate::Error::Inconsistent);
}

Note that Channel::data(reader) (russh/src/channels/mod.rs:321-323) goes through send_data -> tokio::io::copy(&mut data, &mut tx) (:350-360) into the same ChannelTx, so the primary documented data API hangs as well; only the *_bytes variants reach the guard.

Attack scenarios

• Server (authenticated remote DoS): with the default configuration, an authenticated client opens a session channel advertising maximum packet size = 0. Whenever the server application writes on that channel through AsyncWrite (echo, shell prompts, SFTP - russh's own examples echo via into_stream()), the write future never completes. The handler task is stranded and no error surfaces, so application cleanup paths do not trigger. Repeat per channel and per connection; keep-alive traffic resets the server inactivity_timeout (default 600 s), so stranded resources persist as long as the attacker wants.
• Client (malicious server): a malicious or buggy server confirms an opened channel with maximum packet size = 0; the client application's writes on that channel stall forever. The client inactivity_timeout defaults to None, so nothing ever tears the session down.
• This is distinct from a window_size == 0 stall: a zero window is dynamic flow control that a well-behaved peer releases as soon as its buffers drain, while maximum packet size is a static peer declaration where 0 can never be satisfied by any future packet. RFC 4254 section 5.1 recommends a minimum of 32768 - legitimate peers do not send 0.

Impact

Permanent, silent denial of service for the affected writes, with stranded tasks/channels/connections that accumulate for the attacker - resource exhaustion of a server or of a client application. Because the failure mode is Poll::Pending instead of Err, ordinary application mitigations (error handling, timeouts on writes, connection teardown) do not engage on their own.

Reproducer

Two stock russh instances are enough (no raw SSH crafting required):

1. Server: default config plus a host key; channel_open_session accepts and echoes using tokio::io::copy into channel.into_stream() (the pattern from russh's own examples).
2. Client: default config except config.maximum_packet_size = 0 (accepted silently; see above), then authenticate_publickey, channel_open_session().await, channel.data(&b"hello"[..]).await.

The client's CHANNEL_OPEN goes out with maximum packet size = 0 (russh/src/client/session.rs:21-44 encodes self.common.config.maximum_packet_size verbatim). All three scenarios below were verified end-to-end against a stock russh v0.64.1 build (ring backend), default settings otherwise:

• control (both sides default, 32768): the server echoes and the client receives hello;
• client maximum_packet_size = 0: the server prints the received bytes and then the echo write never completes - no data, no error, the client times out;
• server maximum_packet_size = 0: the client's channel.data(...) future never completes - no error, it times out.

Observed output (abridged):

=== SCENARIO A (control): both sides default (maxpacket 32768) ===
[server] read 5 bytes; echoing via AsyncWrite ...
[server] echo write completed
RESULT A: echo received: "hello"

=== SCENARIO B: client advertises maximum_packet_size = 0 ===
[server] read 5 bytes; echoing via AsyncWrite ...
RESULT B: TIMEOUT after 6s - server-side echo write is permanently stalled (no error, no data)

=== SCENARIO C: server advertises maximum_packet_size = 0 ===
RESULT C: TIMEOUT after 6s - client-side channel.data() is permanently stalled

The server task stays parked inside ChannelTx::poll_writable; nothing is logged and nothing is returned to the handler. The harness source (server: into_stream() echo; client: one config toggle) can be attached on request.

Unit-level sketch of the core defect:

let mut tx = ChannelTx::<Msg>::new(
    sender, ChannelId(0),
    Arc::new(Mutex::new(4096)),   // window_size
    Arc::new(Notify::new()),      // notify
    0,                            // max_packet_size
    None,
);
// poll_write(&mut tx, cx, b"x") stays Poll::Pending forever,
// even after CHANNEL_WINDOW_ADJUST raises `window_size`.

Suggested fix

• Validate at parse time (preferred): reject maximum_packet_size == 0 with Error::Inconsistent in OpenChannelMessage::parse and ChannelOpenConfirmation::decode (russh/src/parsing.rs), consistent with the existing guards; optionally clamp any value below the RFC 4254 recommended minimum (32768) instead.
• Defense in depth: in ChannelTx::poll_writable (russh/src/channels/io/tx.rs:82), when self.max_packet_size == 0, surface an io::Error from poll_write instead of returning Poll::Pending.

Both match the treatment PR #695 already applied to the other write paths in v0.61.0 (the reserve_writable_chunk and session packetizer guards).

Affected versions

Verified by inspecting the release artifacts: 0.60.0 through 0.64.1 (current) are affected - poll_writable is byte-identical in v0.60.0 and the peer-value plumbing is present in all of them. The ChannelTx write logic and the peer-value flow predate 0.60 (they go back to the channels::io refactor), so we recommend treating all releases that ship channels::io::ChannelTx as affected. The zero-guards in the other write paths were only added in 0.61.0, so on 0.60.x those paths do not even fail with an error.

Reporter

Tshellauth