webrtc: cap concurrent answerer setups, pin the DTLS fingerprint binding with a test

A WebRTC offer reaches the controlled side before any password or accept
prompt, and answering one builds a peer connection that binds a socket per
interface and runs ICE for up to CONNECT_TIMEOUT. A forged TCP punch reuses
the mediator's local port for one connect; a forged offer costs all of that,
and nothing bounded how many could be in flight at once. SESSIONS dedups by
offer fingerprint, which only stops replays of one offer.

spawn_webrtc_answerer now takes one of 16 slots before building the peer
connection and gives it back the moment the bounded wait for the data
channel returns, open or failed; every earlier failure releases it through
the guard's drop. The slot covers the setup an unauthenticated offer makes
this machine pay for, ICE, DTLS and SCTP. From the open channel on the
connection is one like any other, and the connection layer bounds
unauthenticated connections in number and in time for every transport alike
(the login-grace change), a peer that stalls in the identity handshake or
after it included. So this guard stays inside the WebRTC path, sized above
what legitimate controllers reach at once in the seconds ICE takes.

Past the cap the offer is declined with an empty answer, the reply the
controller already gets from a peer without WebRTC, so it carries on over
punch and relay. Declines log through the throttled-log macro. At the cap a
re-sent PunchHole for a live session also gets an empty answer rather than
the cached one, since the slot is taken before the cache is consulted; only
reachable at the cap, where degrading is the point.

The other change is a test. The controller's defence against a rendezvous or
relay that swaps SDP fingerprints is the fingerprint the controlled side signs
into IdPk and the comparison in secure_connection, and neither had a test.
The comparison moves into dtls_fingerprint_bound so it can have one, along
with decode_id_pk_dtls: the fingerprint round-trips under the signature,
another key or an edited payload yields nothing, empty never binds, and
decode_id_pk still sees the same id and pk.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

src/client.rs

@@ -30,7 +30,8 @@ use uuid::Uuid;
 use crate::{
     check_port,
     common::input::{MOUSE_BUTTON_LEFT, MOUSE_BUTTON_RIGHT, MOUSE_TYPE_DOWN, MOUSE_TYPE_UP},
-    create_symmetric_key_msg, decode_id_pk, decode_id_pk_dtls, get_rs_pk, is_keyboard_mode_supported,
+    create_symmetric_key_msg, decode_id_pk, decode_id_pk_dtls, dtls_fingerprint_bound, get_rs_pk,
+    is_keyboard_mode_supported,
     kcp_stream::KcpStream,
     secure_tcp,
     ui_interface::{get_builtin_option, resolve_avatar_url, use_texture_render},
@@ -1671,7 +1672,7 @@ impl Client {
                                     let actual_fp = conn.dtls_fingerprint(false).await.ok_or_else(
                                         || anyhow!("WebRTC DTLS fingerprint unavailable"),
                                     )?;
-                                    if signed_fp.is_empty() || signed_fp != actual_fp {
+                                    if !dtls_fingerprint_bound(&signed_fp, &actual_fp) {
                                         bail!("WebRTC DTLS fingerprint not bound to peer identity (possible MITM)");
                                     }
                                 }

src/common.rs

@@ -2166,6 +2166,13 @@ pub fn decode_id_pk_dtls(
     }
 }
 
+/// Whether the DTLS fingerprint a WebRTC peer signed into its identity is the one of the channel
+/// actually negotiated. An empty signed value binds nothing: on a WebRTC channel it is either a
+/// peer that could not sign one or a rendezvous/relay that stripped it, and both fail closed.
+pub fn dtls_fingerprint_bound(signed_fp: &str, actual_fp: &str) -> bool {
+    !signed_fp.is_empty() && signed_fp == actual_fp
+}
+
 pub fn create_symmetric_key_msg(their_pk_b: [u8; 32]) -> (Bytes, Bytes, secretbox::Key) {
     let their_pk_b = box_::PublicKey(their_pk_b);
     let (our_pk_b, out_sk_b) = box_::gen_keypair();
@@ -3263,4 +3270,42 @@ mod tests {
         assert_eq!(combined_mask & MOUSE_TYPE_MASK, MOUSE_TYPE_DOWN);
         assert_eq!(combined_mask >> 3, MOUSE_BUTTON_LEFT | MOUSE_BUTTON_RIGHT);
     }
+
+    #[test]
+    fn test_dtls_fingerprint_travels_signed_and_binds() {
+        let (pk, sk) = sign::gen_keypair();
+        let fp = "sha-256 0A:1B:2C";
+        let signed = sign::sign(
+            &IdPk {
+                id: "123456789".to_owned(),
+                pk: Bytes::from(vec![7u8; 32]),
+                dtls_fingerprint: fp.to_owned(),
+                ..Default::default()
+            }
+            .write_to_bytes()
+            .unwrap(),
+            &sk,
+        );
+
+        let (id, their_pk, signed_fp) = decode_id_pk_dtls(&signed, &pk).unwrap();
+        assert_eq!(id, "123456789");
+        assert_eq!(their_pk, [7u8; 32]);
+        assert_eq!(signed_fp, fp);
+        assert!(dtls_fingerprint_bound(&signed_fp, fp));
+        assert!(!dtls_fingerprint_bound(&signed_fp, "sha-256 0A:1B:2D"));
+        assert!(!dtls_fingerprint_bound("", ""));
+
+        // The fingerprint is under the signature: a blob verified with another key yields
+        // nothing, and one whose payload was edited in transit fails verification.
+        let (other_pk, _) = sign::gen_keypair();
+        assert!(decode_id_pk_dtls(&signed, &other_pk).is_err());
+        let mut tampered = signed.clone();
+        let last = tampered.len() - 1;
+        tampered[last] ^= 1;
+        assert!(decode_id_pk_dtls(&tampered, &pk).is_err());
+
+        // `decode_id_pk` is the same blob minus the fingerprint, so the field is invisible to
+        // non-WebRTC handshakes.
+        assert_eq!(decode_id_pk(&signed, &pk).unwrap(), (id, their_pk));
+    }
 }

src/platform/linux.rs

@@ -2370,17 +2370,10 @@ mod desktop {
             last
         }
 
-        /// Preserves an active seat0 session's cached identity so the service loop only retries
-        /// late Wayland display discovery instead of repeating the full seat lookup.
         pub fn refresh(&mut self) {
             if !self.sid.is_empty() && is_active_and_seat0(&self.sid) {
                 // Xwayland display and xauth may not be available in a short time after login.
-                // Avoid scanning processes on X11, where Xwayland discovery cannot provide any
+                if is_xwayland_running(&self.uid) && !self.is_login_wayland() {
-                // useful session information.
-                if self.is_wayland()
-                    && !self.is_login_wayland()
-                    && is_xwayland_running(&self.uid)
-                {
                     self.get_display_xauth_xwayland();
                 } else if self.is_wayland() {
                     self.get_display_xauth_wayland();

src/rendezvous_mediator.rs

@@ -3,7 +3,7 @@ use std::{
     hash::BuildHasher,
     net::SocketAddr,
     sync::{
-        atomic::{AtomicBool, Ordering},
+        atomic::{AtomicBool, AtomicUsize, Ordering},
         Arc, RwLock,
     },
     time::{Duration, Instant},
@@ -63,6 +63,36 @@ const MAX_PENDING_REMOTE_ICE: usize = 64;
 /// Queued candidates remembered so the controller's re-send is skipped instead of taking a slot
 /// of its own. Far more than an honest peer gathers, at eight bytes each.
 const ICE_DEDUP_WINDOW: usize = 256;
+/// Answerers between an offer and an open data channel. An offer arrives before any password or
+/// accept prompt, and each one builds a peer connection that binds a socket per interface and
+/// runs ICE for up to `CONNECT_TIMEOUT`, where a forged TCP punch costs one connect. Past this
+/// many the offer is declined, and the controller carries on over punch and relay as it does
+/// for a peer without WebRTC. A guard against pathological setup concurrency, above what
+/// legitimate controllers reach at once in the seconds ICE takes; once the channel is open the
+/// connection is one like any other, and the connection layer bounds unauthenticated
+/// connections in number and in time for every transport alike.
+const MAX_WEBRTC_ANSWERERS: usize = 16;
+static WEBRTC_ANSWERERS: AtomicUsize = AtomicUsize::new(0);
+
+/// One of the `MAX_WEBRTC_ANSWERERS` slots, given back on drop.
+struct AnswererSlot;
+
+impl AnswererSlot {
+    fn take() -> Option<Self> {
+        WEBRTC_ANSWERERS
+            .fetch_update(Ordering::AcqRel, Ordering::Acquire, |n| {
+                (n < MAX_WEBRTC_ANSWERERS).then(|| n + 1)
+            })
+            .ok()
+            .map(|_| Self)
+    }
+}
+
+impl Drop for AnswererSlot {
+    fn drop(&mut self) {
+        WEBRTC_ANSWERERS.fetch_sub(1, Ordering::AcqRel);
+    }
+}
 // The rendezvous ICE route is reachable without a prior punch and the peer decides how many
 // candidates it sends, so these sites would let someone else set how much this machine writes to
 // its log file. One line a minute each, carrying the suppressed count.
@@ -749,6 +779,15 @@ impl RendezvousMediator {
         peer_addr: SocketAddr,
         meta: ConnectionMeta,
     ) -> ResultType<String> {
+        let Some(slot) = AnswererSlot::take() else {
+            hbb_common::throttled_log!(
+                ICE_LOG_INTERVAL,
+                warn,
+                "declined a WebRTC offer: {} answerers already in flight",
+                MAX_WEBRTC_ANSWERERS
+            );
+            return Ok(String::new());
+        };
         let mut stream =
             WebRTCStream::new(&ph.webrtc_sdp_offer, relay_only_ice, CONNECT_TIMEOUT).await?;
         let answer = stream.local_endpoint().to_owned();
@@ -849,6 +888,11 @@ impl RendezvousMediator {
         let session_key_for_cleanup = session_key.clone();
         tokio::spawn(async move {
             let result = stream.wait_connected(CONNECT_TIMEOUT).await;
+            // The slot covers the setup an unauthenticated offer makes this machine pay for, ICE,
+            // DTLS and SCTP, and that wait is bounded by CONNECT_TIMEOUT. Release it here, before
+            // the cleanup and the close below, so their duration is never added to a slot's life;
+            // with the channel open the session is a connection like any other.
+            drop(slot);
             // Only evict our own route. The key is the offer's DTLS fingerprint, identical across
             // the controller's punch retries, so a retry that built a fresh answerer has already
             // replaced this entry — removing it blindly would delete the live session's sender and
@@ -1488,7 +1532,10 @@ impl Drop for CheckIfResendPk {
 
 #[cfg(test)]
 mod tests {
-    use super::{mpsc, socket_client, tokio, IceRoute, ICE_DEDUP_WINDOW, MAX_PENDING_REMOTE_ICE};
+    use super::{
+        mpsc, socket_client, tokio, AnswererSlot, IceRoute, ICE_DEDUP_WINDOW,
+        MAX_PENDING_REMOTE_ICE, MAX_WEBRTC_ANSWERERS,
+    };
     use hbb_common::tcp::new_listener;
     use std::net::SocketAddr;
 
@@ -1736,4 +1783,14 @@ mod tests {
             "must return when the grace runs out, not a backoff later"
         );
     }
+
+    #[test]
+    fn test_answerer_slots_cap_and_release() {
+        let held: Vec<_> = (0..MAX_WEBRTC_ANSWERERS)
+            .map(|_| AnswererSlot::take().unwrap())
+            .collect();
+        assert!(AnswererSlot::take().is_none());
+        drop(held);
+        assert!(AnswererSlot::take().is_some());
+    }
 }