tokio-xmpp: Add support for kernel TLS

This drastically improve the debuggability of the network parts, by
moving the task of encrypting/decrypting TLS packets from userland to
the kernel.  This makes them appear in clear in strace as sendto() and
recvfrom().

I introduced a new tls-rust-ktls feature which depends on both rustls
and ktls, but isn’t enabled by default since it probably isn’t available
on every computer.  It requires the tls kernel module to be loaded,
which then can offload encryption/decryption to dedicated hardware if
available.

I have tested this change on Linux 6.11 on a rk3588.

tokio-xmpp/Cargo.toml

@@ -35,6 +35,7 @@ idna = { version = "1.0", optional = true}
 native-tls = { version = "0.2", optional = true }
 tokio-native-tls = { version = "0.3", optional = true }
 tokio-rustls = { version = "0.26", optional = true }
+ktls = { version = "6", optional = true }
 
 [dev-dependencies]
 env_logger = { version = "0.11", default-features = false, features = ["auto-color", "humantime"] }
@@ -46,6 +47,7 @@ tokio-xmpp = { path = ".", features = ["insecure-tcp"]}
 default = ["starttls-rust", "rustls-native-certs"]
 starttls = ["dns"]
 tls-rust = ["tokio-rustls"]
+tls-rust-ktls = ["tls-rust", "ktls"]
 tls-rust-native-certs = ["tls-rust", "rustls-native-certs"]
 tls-rust-webpki-roots = ["tls-rust", "webpki-roots"]
 tls-native = ["tokio-native-tls", "native-tls"]

tokio-xmpp/src/connect/starttls.rs

@@ -6,6 +6,7 @@ use std::borrow::Cow;
 use std::error::Error as StdError;
 use std::fmt;
 use std::io;
+use std::os::fd::AsRawFd;
 #[cfg(all(feature = "tls-rust", not(feature = "tls-native")))]
 use tokio_rustls::rustls::pki_types::InvalidDnsNameError;
 #[cfg(all(feature = "tls-rust", not(feature = "tls-native")))]
@@ -17,13 +18,22 @@ use futures::{sink::SinkExt, stream::StreamExt};
 use {
     std::sync::Arc,
     tokio_rustls::{
-        client::TlsStream,
         rustls::pki_types::ServerName,
         rustls::{ClientConfig, RootCertStore},
         TlsConnector,
     },
 };
 
+#[cfg(all(
+    feature = "tls-rust",
+    not(feature = "tls-native"),
+    not(feature = "tls-rust-ktls")
+))]
+use tokio_rustls::client::TlsStream;
+
+#[cfg(all(feature = "tls-rust-ktls", not(feature = "tls-native")))]
+type TlsStream<S> = ktls::KtlsStream<S>;
+
 #[cfg(feature = "tls-native")]
 use {
     native_tls::TlsConnector as NativeTlsConnector,
@@ -116,7 +126,16 @@ impl ServerConnector for StartTlsServerConnector {
             log::warn!("tls-native doesn’t support channel binding, please use tls-rust if you want this feature!");
             Ok(ChannelBinding::None)
         }
-        #[cfg(all(feature = "tls-rust", not(feature = "tls-native")))]
+        #[cfg(all(feature = "tls-rust-ktls", not(feature = "tls-native")))]
+        {
+            log::warn!("Kernel TLS doesn’t support channel binding yet, we would have to extract the secrets in the rustls TlsStream before converting it into a KtlsStream.");
+            Ok(ChannelBinding::None)
+        }
+        #[cfg(all(
+            feature = "tls-rust",
+            not(feature = "tls-native"),
+            not(feature = "tls-rust-ktls")
+        ))]
         {
             let (_, connection) = stream.get_ref().get_ref();
             Ok(match connection.protocol_version() {
@@ -149,7 +168,7 @@ async fn get_tls_stream<S: AsyncRead + AsyncWrite + Unpin>(
 }
 
 #[cfg(all(feature = "tls-rust", not(feature = "tls-native")))]
-async fn get_tls_stream<S: AsyncRead + AsyncWrite + Unpin>(
+async fn get_tls_stream<S: AsyncRead + AsyncWrite + Unpin + AsRawFd>(
     xmpp_stream: XmppStream<BufStream<S>>,
     domain: &str,
 ) -> Result<TlsStream<S>, Error> {
@@ -164,19 +183,29 @@ async fn get_tls_stream<S: AsyncRead + AsyncWrite + Unpin>(
     {
         root_store.add_parsable_certificates(rustls_native_certs::load_native_certs()?);
     }
-    let config = ClientConfig::builder()
+    #[allow(unused_mut, reason = "This config is mutable when using ktls")]
+    let mut config = ClientConfig::builder()
         .with_root_certificates(root_store)
         .with_no_client_auth();
+    #[cfg(feature = "tls-rust-ktls")]
+    let stream = {
+        config.enable_secret_extraction = true;
+        ktls::CorkStream::new(stream)
+    };
     let tls_stream = TlsConnector::from(Arc::new(config))
         .connect(domain, stream)
         .await
         .map_err(|e| Error::from(crate::Error::Io(e)))?;
+    #[cfg(feature = "tls-rust-ktls")]
+    let tls_stream = ktls::config_ktls_client(tls_stream)
+        .await
+        .map_err(StartTlsError::KtlsError)?;
     Ok(tls_stream)
 }
 
 /// Performs `<starttls/>` on an XmppStream and returns a binary
 /// TlsStream.
-pub async fn starttls<S: AsyncRead + AsyncWrite + Unpin>(
+pub async fn starttls<S: AsyncRead + AsyncWrite + Unpin + AsRawFd>(
     mut stream: XmppStream<BufStream<S>>,
     domain: &str,
 ) -> Result<TlsStream<S>, Error> {
@@ -214,6 +243,9 @@ pub enum StartTlsError {
     #[cfg(all(feature = "tls-rust", not(feature = "tls-native")))]
     /// DNS name parsing error
     DnsNameError(InvalidDnsNameError),
+    #[cfg(feature = "tls-rust-ktls")]
+    /// Error while setting up kernel TLS
+    KtlsError(ktls::Error),
 }
 
 impl ServerConnectorError for StartTlsError {}
@@ -224,6 +256,8 @@ impl fmt::Display for StartTlsError {
             Self::Tls(e) => write!(fmt, "TLS error: {}", e),
             #[cfg(all(feature = "tls-rust", not(feature = "tls-native")))]
             Self::DnsNameError(e) => write!(fmt, "DNS name error: {}", e),
+            #[cfg(feature = "tls-rust-ktls")]
+            Self::KtlsError(e) => write!(fmt, "Kernel TLS error: {}", e),
         }
     }
 }