bootstrap SFTP subsystem

Cargo.lock

@@ -422,6 +422,17 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "cluelessh-sftp"
+version = "0.1.0"
+
+[[package]]
+name = "cluelessh-sftp-proto"
+version = "0.1.0"
+dependencies = [
+ "cluelessh-transport",
+]
+
 [[package]]
 name = "cluelessh-tokio"
 version = "0.1.0"
@@ -489,6 +500,15 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "cluelesshd-sftp-server"
+version = "0.1.0"
+dependencies = [
+ "eyre",
+ "tracing",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "cobs"
 version = "0.2.3"

bin/cluelessh-faked/src/main.rs

@@ -230,6 +230,11 @@ async fn handle_session_channel(
                             channel.send(ChannelOperationKind::Eof).await?;
                             channel.send(ChannelOperationKind::Close).await?;
                         }
+                        ChannelRequest::Subsystem { want_reply, .. } => {
+                            if want_reply {
+                                channel.send(ChannelOperationKind::Failure).await?;
+                            }
+                        }
                         ChannelRequest::ExitStatus { .. } => {}
                         ChannelRequest::Env { .. } => {}
                     };

bin/cluelesshd-sftp-server/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "cluelesshd-sftp-server"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+eyre.workspace = true
+tracing.workspace = true
+tracing-subscriber = { version = "0.3.18", features = ["env-filter"] }
+
+[lints]
+workspace = true

bin/cluelesshd-sftp-server/README.md

@@ -0,0 +1,3 @@
+# cluelesshd-sftp-server
+
+SFTP server for cluelesshd.

bin/cluelesshd-sftp-server/src/main.rs

@@ -0,0 +1,17 @@
+use eyre::Result;
+use tracing::info;
+use tracing_subscriber::EnvFilter;
+
+fn main() -> Result<()> {
+    let env_filter =
+        EnvFilter::try_from_env("SFTP_LOG").unwrap_or_else(|_| EnvFilter::new("debug"));
+
+    tracing_subscriber::fmt()
+        .with_writer(std::io::stderr)
+        .with_env_filter(env_filter)
+        .init();
+
+    info!("mroooow!");
+
+    Ok(())
+}

bin/cluelesshd/cluelesshd.toml

@@ -1,5 +1,9 @@
 log_level = "info"
 
+
+[subsystem.sftp]
+path = "../../target/debug/cluelesshd-sftp-server"
+
 [net]
 ip = "0.0.0.0"
 port = 2223

bin/cluelesshd/src/config.rs

@@ -1,12 +1,15 @@
 use eyre::{Context, Result};
 use serde::{Deserialize, Serialize};
 use std::{
+    collections::HashMap,
     net::{IpAddr, Ipv4Addr},
     path::PathBuf,
 };
 
 use crate::Args;
 
+// TODO: validate config and user nicer structs to consume it
+
 #[derive(Clone, Serialize, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub struct Config {
@@ -15,6 +18,8 @@ pub struct Config {
     pub net: NetConfig,
     pub auth: AuthConfig,
     pub security: SecurityConfig,
+    #[serde(default)]
+    pub subsystem: HashMap<String, SubsystemConfig>,
 }
 
 #[derive(Clone, Serialize, Deserialize)]
@@ -52,8 +57,21 @@ pub struct SecurityConfig {
     pub experimental_seccomp: bool,
 }
 
+/// Add arbitrary subsystems.
+/// # Subsystem Protocol
+/// Every subsystem process gets spawned in the home directory of the user, as the user.
+/// Several FDs are guaranteed to be open.
+/// - stdin (0): data from the client channel
+/// - stdout (1): data to the client channel
+/// - stderr (2): data to the client channel extended stderr (used for debugging)
+#[derive(Clone, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct SubsystemConfig {
+    pub path: PathBuf,
+}
+
 impl Config {
-    pub fn find(args: &Args) -> Result<Self> {
+    pub fn load(args: &Args) -> Result<Self> {
         let path = std::env::var("CLUELESSHD_CONFIG")
             .map(PathBuf::from)
             .or(args.config.clone().ok_or(std::env::VarError::NotPresent))
@@ -63,8 +81,19 @@ impl Config {
             format!("failed to open config file '{}', refusing to start. you can change the config file path with the --config arg or the CLUELESSHD_CONFIG environment variable", path.display())
         })?;
 
-        toml::from_str(&content)
+        let mut config: Config = toml::from_str(&content)
-            .wrap_err_with(|| format!("invalid config file '{}'", path.display()))
+            .wrap_err_with(|| format!("invalid config file '{}'", path.display()))?;
+
+        for sub in config.subsystem.values_mut() {
+            sub.path = sub.path.canonicalize().wrap_err_with(|| {
+                format!(
+                    "error canonicalizing subsystem path: {}",
+                    sub.path.display()
+                )
+            })?;
+        }
+
+        Ok(config)
     }
 }
 

bin/cluelesshd/src/connection.rs

@@ -292,7 +292,7 @@ impl SessionState {
                             }
                         }
                     }
-                    ChannelRequest::Shell { want_reply } => match self.shell(None).await {
+                    ChannelRequest::Shell { want_reply } => match self.shell(None, None).await {
                         Ok(()) => {
                             if want_reply {
                                 self.channel.send(ChannelOperationKind::Success).await?;
@@ -309,7 +309,7 @@ impl SessionState {
                         want_reply,
                         command,
                     } => match String::from_utf8(command) {
-                        Ok(command) => match self.shell(Some(&command)).await {
+                        Ok(command) => match self.shell(Some(command), None).await {
                             Ok(()) => {
                                 if want_reply {
                                     self.channel.send(ChannelOperationKind::Success).await?;
@@ -330,6 +330,21 @@ impl SessionState {
                             }
                         }
                     },
+                    ChannelRequest::Subsystem { want_reply, name } => {
+                        match self.shell(None, Some(name)).await {
+                            Ok(()) => {
+                                if want_reply {
+                                    self.channel.send(ChannelOperationKind::Success).await?;
+                                }
+                            }
+                            Err(err) => {
+                                debug!(%err, "Failed to spawn subsystem");
+                                if want_reply {
+                                    self.channel.send(ChannelOperationKind::Failure).await?;
+                                }
+                            }
+                        }
+                    }
                     ChannelRequest::Env {
                         name,
                         value,
@@ -394,11 +409,12 @@ impl SessionState {
         Ok(())
     }
 
-    async fn shell(&mut self, shell_command: Option<&str>) -> Result<()> {
+    async fn shell(&mut self, shell_command: Option<String>, subsystem: Option<String>) -> Result<()> {
         let mut fds = self
             .rpc_client
             .shell(
-                shell_command.map(ToOwned::to_owned),
+                shell_command,
+                subsystem,
                 self.pty_term.clone(),
                 self.envs.clone(),
             )

bin/cluelesshd/src/main.rs

@@ -25,7 +25,7 @@ use eyre::{bail, eyre, Context, Result};
 use rustix::fs::MemfdFlags;
 use serde::{Deserialize, Serialize};
 use tokio::net::{TcpListener, TcpStream};
-use tracing::{error, info, warn};
+use tracing::{ error, info, warn};
 
 use tracing_subscriber::EnvFilter;
 
@@ -51,10 +51,14 @@ fn main() -> eyre::Result<()> {
             // Initial setup
             let args = Args::parse();
 
-            let config = config::Config::find(&args)?;
+            let config = config::Config::load(&args)?;
 
             setup_tracing(&config);
 
+            for (name, system) in &config.subsystem {
+                info!(%name, path = %system.path.display(), "Loaded subsystem");
+            }
+
             if !rustix::process::getuid().is_root() {
                 warn!("Daemon not started as root. This disables several security mitigations and permits logging in as any other user");
             }
@@ -197,7 +201,8 @@ async fn spawn_connection_child(
 ) -> Result<()> {
     let stream_fd = stream.as_raw_fd();
 
-    let mut rpc_server = rpc::Server::new(host_keys).wrap_err("creating RPC server")?;
+    let mut rpc_server =
+        rpc::Server::new(config.clone(), host_keys).wrap_err("creating RPC server")?;
 
     let rpc_client_fd = rpc_server.client_fd().as_raw_fd();
 

bin/cluelesshd/src/rpc.rs

@@ -41,6 +41,8 @@ use users::os::unix::UserExt;
 use users::User;
 use zeroize::Zeroizing;
 
+use crate::config::Config;
+
 #[derive(Debug, Serialize, Deserialize)]
 enum Request {
     /// Performs the key exchange by generating a private key, deriving the shared secret,
@@ -136,6 +138,7 @@ struct ShellRequest {
     /// Whether a PTY is used and if yes, the TERM env var.
     pty_term: Option<String>,
     command: Option<String>,
+    subsystem: Option<String>,
     env: Vec<(String, String)>,
 }
 
@@ -162,17 +165,20 @@ pub struct Server {
     host_keys: Vec<PlaintextPrivateKey>,
     authenticated_user: Option<users::User>,
 
+    config: Config,
+
     pty_user: Option<OwnedFd>,
     shell_process: Option<Child>,
 }
 
 impl Server {
-    pub fn new(host_keys: Vec<PlaintextPrivateKey>) -> Result<Self> {
+    pub fn new(config: Config, host_keys: Vec<PlaintextPrivateKey>) -> Result<Self> {
         let (server, client) = UnixDatagram::pair().wrap_err("creating socketpair")?;
 
         Ok(Self {
             server,
             client,
+            config,
             host_keys,
             authenticated_user: None,
             pty_user: None,
@@ -366,13 +372,28 @@ impl Server {
     }
 
     async fn shell(&mut self, user: &User, req: ShellRequest) -> Result<Vec<OwnedFd>> {
+        let subsystem = match req.subsystem.as_deref() {
+            Some(subsystem) => match self.config.subsystem.get(subsystem) {
+                Some(system) => Some(system.path.clone()),
+                None => bail!("unsupported subsystem: {subsystem}"),
+            },
+            None => None,
+        };
+
         let shell = user.shell();
 
-        let mut cmd = Command::new(shell);
+        let cmd_arg0 = subsystem.as_deref().unwrap_or(shell);
+
+        // TODO: the SSH RFC mentions subsystems going through shell... should we?
+        let mut cmd = Command::new(cmd_arg0);
+
+        if subsystem.is_none() {
             if let Some(shell_command) = req.command {
                 cmd.arg("-c");
                 cmd.arg(shell_command);
             }
+        };
+
         cmd.env_clear();
 
         let has_pty = req.pty_term.is_some();
@@ -404,7 +425,7 @@ impl Server {
             cmd.env(k, v);
         }
 
-        debug!(cmd = %shell.display(), uid = %user.uid(), gid = %user.primary_group_id(), "Executing process");
+        debug!(cmd = %cmd_arg0.display(), uid = %user.uid(), gid = %user.primary_group_id(), "Executing process");
 
         let mut shell = cmd.spawn()?;
 
@@ -533,12 +554,14 @@ impl Client {
     pub async fn shell(
         &self,
         command: Option<String>,
+        subsystem: Option<String>,
         pty_term: Option<String>,
         env: Vec<(String, String)>,
     ) -> Result<Vec<OwnedFd>> {
         self.send_request(&Request::Shell(ShellRequest {
             pty_term,
             command,
+            subsystem,
             env,
         }))
         .await?;

lib/cluelessh-connection/src/lib.rs

@@ -105,6 +105,11 @@ pub enum ChannelRequest {
 
         command: Vec<u8>,
     },
+    Subsystem {
+        want_reply: bool,
+
+        name: String,
+    },
     Env {
         want_reply: bool,
 
@@ -471,6 +476,19 @@ impl ChannelsState {
                             command: command.to_owned(),
                         }
                     }
+                    "subsystem" => {
+                        if !self.is_server {
+                            return Err(peer_error!("server tried to set environment var"));
+                        }
+
+                        let name = p.utf8_string()?;
+
+                        info!(channel = %our_channel, %name, "Starting subsystem");
+                        ChannelRequest::Subsystem {
+                            want_reply,
+                            name: name.to_owned(),
+                        }
+                    }
                     "env" => {
                         if !self.is_server {
                             return Err(peer_error!("server tried to set environment var"));
@@ -632,6 +650,7 @@ impl ChannelsState {
                         Packet::new_msg_channel_request_shell(peer, b"shell", want_reply)
                     }
                     ChannelRequest::Exec { .. } => todo!("exec"),
+                    ChannelRequest::Subsystem { .. } => todo!("subsystem"),
                     ChannelRequest::Env { .. } => todo!("env"),
                     ChannelRequest::ExitStatus { status } => {
                         Packet::new_msg_channel_request_exit_status(
@@ -787,6 +806,7 @@ impl ChannelOperation {
                 ChannelRequest::PtyReq { .. } => "pty-req",
                 ChannelRequest::Shell { .. } => "shell",
                 ChannelRequest::Exec { .. } => "exec",
+                ChannelRequest::Subsystem { .. } => "subsystem",
                 ChannelRequest::Env { .. } => "env",
                 ChannelRequest::ExitStatus { .. } => "exit-status",
             },

lib/cluelessh-sftp-proto/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "cluelessh-sftp-proto"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+cluelessh-transport = { path = "../cluelessh-transport" }
+
+[lints]
+workspace = true

lib/cluelessh-sftp-proto/src/lib.rs

@@ -0,0 +1 @@
+

lib/cluelessh-sftp/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "cluelessh-sftp"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[lints]
+workspace = true

lib/cluelessh-sftp/src/lib.rs

@@ -0,0 +1,14 @@
+pub fn add(left: u64, right: u64) -> u64 {
+    left + right
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+}