[pbs-devel] [PATCH v1 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

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, altering
this condition simplifies the overall logic and removes a potential
source of unintended behaviour.

[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-4.1.2

Signed-off-by: Max Carrara <m.carrara@proxmox.com>
Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5105
---
 proxmox-rest-server/src/connection.rs | 69 ++++++++++++---------------
 1 file changed, 31 insertions(+), 38 deletions(-)

diff --git a/proxmox-rest-server/src/connection.rs b/proxmox-rest-server/src/connection.rs
index 470021d7..a2d54b18 100644
--- a/proxmox-rest-server/src/connection.rs
+++ b/proxmox-rest-server/src/connection.rs
@@ -418,70 +418,63 @@ impl AcceptBuilder {
         secure_sender: ClientSender,
         insecure_sender: InsecureClientSender,
     ) {
-        let peer = state.peer;
+        const CLIENT_HANDSHAKE_TIMEOUT: Duration = Duration::from_secs(10);
 
-        let client_initiates_handshake = {
-            #[cfg(feature = "rate-limited-stream")]
-            let socket_ref = state.socket.inner();
+        let peer = state.peer;
 
-            #[cfg(not(feature = "rate-limited-stream"))]
-            let socket_ref = &state.socket;
+        #[cfg(feature = "rate-limited-stream")]
+        let socket_ref = state.socket.inner();
 
-            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;
-                }
-            }
-        };
+        #[cfg(not(feature = "rate-limited-stream"))]
+        let socket_ref = &state.socket;
 
-        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;
+    async fn wait_for_client_tls_handshake(
+        incoming_stream: &TcpStream,
+        timeout: Duration,
+    ) -> Result<bool, Error> {
+        const HANDSHAKE_BYTES_LEN: usize = 5;
 
-        let mut buf = [0; BYTES_BUF_SIZE];
-        let mut last_peek_size = 0;
+        let mut buf = [0; HANDSHAKE_BYTES_LEN];
 
         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);
-                }
+                    .context("couldn't peek into incoming TCP stream")?;
 
-                // No more new data came in
-                if peek_size == last_peek_size {
-                    return Ok(false);
+                if peek_size == HANDSHAKE_BYTES_LEN {
+                    return Ok(contains_tls_handshake_fragment(&buf));
                 }
 
-                last_peek_size = peek_size;
-
                 // explicitly yield to event loop; this future otherwise blocks ad infinitum
                 tokio::task::yield_now().await;
             }
         };
 
-        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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