[pbs-devel] [PATCH v2 proxmox 3/3] fix #5105: rest-server: connection: overhaul TLS handshake check logic

[Max Carrara <m.carrara@proxmox.com>]

On rare occasions, the TLS "client hello" message [1] is delayed after
a connection with the server was established, which causes HTTPS
requests to fail before TLS was even negotiated. In these cases, the
server would incorrectly respond with "HTTP/1.1 400 Bad Request"
instead of closing the connection (or similar).

The reasons for the "client hello" being delayed seem to vary; one
user noticed that the issue went away completely after they turned off
UFW [2]. Another user noticed (during private correspondence) that the
issue only appeared when connecting to their PBS instance via WAN, but
not from within their VPN. In the WAN case a firewall was also
present. The same user kindly provided tcpdumps and strace logs on
request.

The issue was finally reproduced with the following Python script:

  import socket
  import time

  HOST: str = ...
  PORT: int = ...

  with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
      sock.connect((HOST, PORT))
      time.sleep(1.5) # simulate firewall / proxy / etc. delay
      sock.sendall(b"\x16\x03\x01\x02\x00")
      data = sock.recv(256)
      print(data)

The additional delay before sending the first 5 bytes of the "client
hello" message causes the handshake checking logic to incorrectly fall
back to plain HTTP.

All of this is fixed by the following:

  1. Increase the timeout duration to 10 seconds (from 1)
  2. Instead of falling back to plain HTTP, refuse to accept the
     connection if the TLS handshake wasn't initiated before the
     timeout limit is reached
  3. Only accept plain HTTP if the first 5 bytes do not correspond to
     a TLS handshake fragment [3]
  4. Do not take the last number of bytes that were in the buffer into
     account; instead, only perform the actual handshake check if
     5 bytes are in the peek buffer using some of tokio's low-level
     functionality

Regarding 1.: This should be generous enough for any client to be able
to initiate a TLS handshake, despite its surrounding circumstances.

Regarding 4.: While this is not 100% related to the issue, peeking into
the buffer in this manner should ensure that our implementation here
remains correct, even if the kernel's underlying behaviour regarding
edge-triggering is changed [4]. At the same time, there's no need for
busy-waiting and continuously yielding to the event loop anymore.

[1]: https://www.rfc-editor.org/rfc/rfc8446.html#section-4.1.2
[2]: https://forum.proxmox.com/threads/disable-default-http-redirects-on-8007.142312/post-675352
[3]: https://www.rfc-editor.org/rfc/rfc8446.html#section-5.1
[4]: https://lwn.net/Articles/864947/

Signed-off-by: Max Carrara <m.carrara@proxmox.com>
---
Changes v1 --> v2:
  * use `socket.async_io` instead of `socket.peek` and rework the future
    that performs the handshake check
  * adapt commit message
  * fix reference in commit message

 proxmox-rest-server/src/connection.rs | 110 ++++++++++++++------------
 1 file changed, 61 insertions(+), 49 deletions(-)

diff --git a/proxmox-rest-server/src/connection.rs b/proxmox-rest-server/src/connection.rs
index 470021d7..63fa8640 100644
--- a/proxmox-rest-server/src/connection.rs
+++ b/proxmox-rest-server/src/connection.rs
@@ -2,7 +2,9 @@
 //!
 //! Hyper building block.
 
+use std::io;
 use std::net::SocketAddr;
+use std::os::fd::FromRawFd;
 use std::os::unix::io::AsRawFd;
 use std::path::PathBuf;
 use std::pin::Pin;
@@ -418,70 +420,80 @@ impl AcceptBuilder {
         secure_sender: ClientSender,
         insecure_sender: InsecureClientSender,
     ) {
+        const CLIENT_HANDSHAKE_TIMEOUT: Duration = Duration::from_secs(10);
+
         let peer = state.peer;
 
-        let client_initiates_handshake = {
-            #[cfg(feature = "rate-limited-stream")]
-            let socket_ref = state.socket.inner();
+        #[cfg(feature = "rate-limited-stream")]
+        let socket_ref = state.socket.inner();
 
-            #[cfg(not(feature = "rate-limited-stream"))]
-            let socket_ref = &state.socket;
+        #[cfg(not(feature = "rate-limited-stream"))]
+        let socket_ref = &state.socket;
 
-            match Self::wait_for_client_tls_handshake(socket_ref).await {
-                Ok(initiates_handshake) => initiates_handshake,
-                Err(err) => {
-                    log::error!("[{peer}] error checking for TLS handshake: {err}");
-                    return;
-                }
-            }
-        };
-
-        if !client_initiates_handshake {
-            let insecure_stream = Box::pin(state.socket);
+        let handshake_res =
+            Self::wait_for_client_tls_handshake(socket_ref, CLIENT_HANDSHAKE_TIMEOUT).await;
 
-            if insecure_sender.send(Ok(insecure_stream)).await.is_err() && flags.is_debug {
-                log::error!("[{peer}] detected closed connection channel")
+        match handshake_res {
+            Ok(true) => {
+                Self::do_accept_tls(state, flags, secure_sender).await;
             }
+            Ok(false) => {
+                let insecure_stream = Box::pin(state.socket);
 
-            return;
+                if let Err(send_err) = insecure_sender.send(Ok(insecure_stream)).await {
+                    log::error!("[{peer}] failed to accept connection - connection channel closed: {send_err}");
+                }
+            }
+            Err(err) => {
+                log::error!("[{peer}] failed to check for TLS handshake: {err}");
+            }
         }
-
-        Self::do_accept_tls(state, flags, secure_sender).await
     }
 
-    async fn wait_for_client_tls_handshake(incoming_stream: &TcpStream) -> Result<bool, Error> {
-        const MS_TIMEOUT: u64 = 1000;
-        const BYTES_BUF_SIZE: usize = 128;
-
-        let mut buf = [0; BYTES_BUF_SIZE];
-        let mut last_peek_size = 0;
+    async fn wait_for_client_tls_handshake(
+        incoming_stream: &TcpStream,
+        timeout: Duration,
+    ) -> Result<bool, Error> {
+        const HANDSHAKE_BYTES_LEN: usize = 5;
 
         let future = async {
-            loop {
-                let peek_size = incoming_stream
-                    .peek(&mut buf)
-                    .await
-                    .context("couldn't peek into incoming tcp stream")?;
-
-                if contains_tls_handshake_fragment(&buf) {
-                    return Ok(true);
-                }
-
-                // No more new data came in
-                if peek_size == last_peek_size {
-                    return Ok(false);
-                }
-
-                last_peek_size = peek_size;
-
-                // explicitly yield to event loop; this future otherwise blocks ad infinitum
-                tokio::task::yield_now().await;
-            }
+            incoming_stream
+                .async_io(tokio::io::Interest::READABLE, || {
+                    let mut buf = [0; HANDSHAKE_BYTES_LEN];
+
+                    // Convert to standard lib TcpStream so we can peek without interfering
+                    // with tokio's internals
+                    let raw_fd = incoming_stream.as_raw_fd();
+                    let std_stream = unsafe { std::net::TcpStream::from_raw_fd(raw_fd) };
+
+                    let peek_res = std_stream.peek(&mut buf);
+
+                    // Prevent destructor from being called, closing the connection
+                    // and messing up invariants
+                    std::mem::forget(std_stream);
+
+                    match peek_res {
+                        // If we didn't get enough bytes, raise an EAGAIN / EWOULDBLOCK which tells
+                        // tokio to await the readiness of the socket again. This should normally
+                        // only be used if the socket isn't actually ready, but is fine to do here
+                        // in our case.
+                        //
+                        // This means we will peek into the stream's queue until we got
+                        // HANDSHAKE_BYTE_LEN bytes or an error.
+                        Ok(peek_len) if peek_len < HANDSHAKE_BYTES_LEN => {
+                            Err(io::ErrorKind::WouldBlock.into())
+                        }
+                        // Either we got Ok(HANDSHAKE_BYTES_LEN) or some error.
+                        res => res.map(|_| contains_tls_handshake_fragment(&buf)),
+                    }
+                })
+                .await
+                .context("couldn't peek into incoming TCP stream")
         };
 
-        tokio::time::timeout(Duration::from_millis(MS_TIMEOUT), future)
+        tokio::time::timeout(timeout, future)
             .await
-            .unwrap_or(Ok(false))
+            .context("timed out while waiting for client to initiate TLS handshake")?
     }
 }
 
-- 
2.39.2



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