Merge branch 'main' into fix/sec-mcp-token-endpoint

The bridge picked the token by port alone and sent it to whatever the address resolved to, so a listener another account opened on ::1 with the same port received the desktop's credential. The token is now attached only when the connected peer is exactly 127.0.0.1:PORT, where the app listens. Reading the token refuses a symlink, a non-plain file and a file others can read, and the shared temporary directory is no longer a fallback location for it.

Cargo.lock

@@ -772,6 +772,7 @@ dependencies = [
  "cadcraft-render",
  "clap",
  "getrandom 0.4.3",
+ "libc",
  "log",
  "serde_json",
 ]

crates/engine/src/lisp.rs

@@ -123,12 +123,6 @@ pub struct LispState {
     acad_loaded: bool,
     /// Memory caps (lowered by tests).
     pub(crate) limits: Limits,
-    /// Interpreter steps all synchronous callbacks have run, nested ones included. A callback's
-    /// allowance counts what ran since it started, so the callbacks it calls spend its steps.
-    pub(crate) callback_used: u64,
-    /// The `callback_used` value the callback running now (and so every one it calls) must not
-    /// pass: the lowest of its own and its callers' allowances.
-    pub(crate) callback_ceiling: Option<u64>,
 }
 
 /// What U and REDO inside the running routine take back: the commands it ran, each as the drawing

crates/engine/src/lisp/builtins/strings.rs

@@ -1,5 +1,5 @@
 use super::args::{R, arity, int, list_arg, string};
-use super::{LispError, OsStr, Value, charge_bytes, check_string_len, read, to_text};
+use super::{LispError, OsStr, Value, check_string_len, read, to_text};
 
 // ---------------- strings ----------------
 
@@ -114,13 +114,7 @@ pub(super) fn read_fn(a: &[Value]) -> R {
     arity(a, 0, 1)?;
     match a.first() {
         None | Some(Value::Nil) => Ok(Value::Nil),
-        Some(v) => {
-            // The strings in the list it makes are new, each as long as its text at most: the
-            // text is charged, though a list result isn't inspected by `charge_result`.
-            let text = string(v)?;
-            charge_bytes(text.len())?;
-            read::read_one(&text)
-        }
+        Some(v) => read::read_one(&string(v)?),
     }
 }
 

crates/engine/src/lisp/drawing.rs

@@ -179,12 +179,7 @@ fn to_json_bounded(v: &Value, depth: usize, budget: &mut u64) -> Option<serde_js
         Value::Nil => serde_json::Value::Null,
         Value::Int(i) => serde_json::json!(i),
         Value::Real(r) => serde_json::json!(r),
-        Value::Str(s) => {
-            // A string is copied whole each time it occurs: it spends the allowance by its length
-            // (one unit per 16 bytes, so about 32 MB in all), shared or not.
-            *budget = budget.checked_sub(s.len() as u64 / 16)?;
-            serde_json::json!(&**s)
-        }
+        Value::Str(s) => serde_json::json!(&**s),
         Value::Cons(_) => {
             let mut arr = Vec::new();
             for x in v.iter() {
@@ -301,16 +296,12 @@ fn groups_to_list(tags: &[Tag]) -> Vec<Value> {
     out
 }
 
-/// The most string bytes an entity list may hold (counted per occurrence, shared or not).
-const MAX_GROUP_TEXT: usize = 32 << 20;
-
 /// An association list back to DXF groups (points split into x, y, z groups). The type
 /// (`0`), the entity name (`-1`), the handle (`5`), the owner (`330`) and the layout (`410`) are
 /// left out: they aren't the entity's to set.
 fn list_to_groups(list: &Value) -> Result<(String, Vec<Tag>), LispError> {
     let mut kind = None;
     let mut tags = Vec::new();
-    let mut text_left = MAX_GROUP_TEXT;
     for item in list.iter() {
         let Value::Cons(c) = &item else { return Err(LispError::bad_arg("listp", &item)) };
         let Value::Int(code) = c.car else { return Err(LispError::bad_arg("fixnump", &c.car)) };
@@ -332,11 +323,7 @@ fn list_to_groups(list: &Value) -> Result<(String, Vec<Tag>), LispError> {
             _ => tags.push(match &v {
                 Value::Int(i) => Tag::i(code, i64::from(*i)),
                 Value::Real(r) => Tag::f(code, *r),
-                Value::Str(s) => {
-                    // Each occurrence of a shared string becomes its own copy in the tag.
-                    text_left = text_left.checked_sub(s.len()).ok_or_else(|| LispError::new("out of memory: the entity list holds too much text"))?;
-                    Tag::s(code, &**s)
-                }
+                Value::Str(s) => Tag::s(code, &**s),
                 Value::Ename(h) => Tag::s(code, h.hex()),
                 other => return Err(LispError::bad_arg("DXF group value", other)),
             }),
@@ -1138,27 +1125,3 @@ pub fn load_source(s: &Session, name: &str) -> Option<String> {
         s.lisp_support_dir().map(|d| candidates.iter().map(|c| d.join(c).to_string_lossy().into_owned()).collect::<Vec<_>>()).unwrap_or_default();
     candidates.iter().chain(&support).find_map(super::builtins::read_text_file)
 }
-
-#[cfg(test)]
-mod budget_tests {
-    use super::*;
-
-    /// One 64 KB string occurring many times is charged by its length each time.
-    #[test]
-    fn shared_strings_spend_the_conversion_budgets() {
-        let big = Value::str(&"x".repeat(64 << 10));
-        // 600 x 64 KB is over the 32 MiB an entity list may hold; 100 x 64 KB is not.
-        let list = |n: usize| {
-            let mut items = vec![Value::cons(Value::Int(0), Value::str("TEXT"))];
-            items.extend(std::iter::repeat_n(Value::cons(Value::Int(1), big.clone()), n));
-            Value::list(items)
-        };
-        assert!(list_to_groups(&list(600)).is_err());
-        assert!(list_to_groups(&list(100)).is_ok());
-        // 1,000 x 64 KB is 4,096,000 units of the 2,000,000 SETVAR allows; 100 x 64 KB is not.
-        let mut budget = TO_JSON_BUDGET;
-        assert!(to_json_bounded(&Value::list(vec![big.clone(); 1_000]), 0, &mut budget).is_none());
-        let mut budget = TO_JSON_BUDGET;
-        assert!(to_json_bounded(&Value::list(vec![big; 100]), 0, &mut budget).is_some());
-    }
-}

crates/engine/src/lisp/eval.rs

@@ -175,19 +175,17 @@ pub struct Machine {
     stack: Vec<Frame>,
     state: State,
     need: Option<Need>,
-    /// Steps the last [`Machine::run`] took.
-    last_steps: u64,
 }
 
 impl Machine {
     /// A machine evaluating `forms` in turn.
     pub fn forms(forms: Vec<Value>) -> Machine {
-        Machine { stack: vec![Frame::Forms { rest: forms.into_iter().rev().collect() }], state: State::Ret(Value::Nil), need: None, last_steps: 0 }
+        Machine { stack: vec![Frame::Forms { rest: forms.into_iter().rev().collect() }], state: State::Ret(Value::Nil), need: None }
     }
 
     /// A machine calling `f` with `args`.
     pub fn calling(f: Value, args: Vec<Value>) -> Machine {
-        Machine { stack: Vec::new(), state: State::Call(f, args), need: None, last_steps: 0 }
+        Machine { stack: Vec::new(), state: State::Call(f, args), need: None }
     }
 
     /// Give the value a [`Need`] waited for, and carry on.
@@ -198,15 +196,9 @@ impl Machine {
     /// Run until done, waiting, or `budget` steps.
     pub fn run(&mut self, ctx: &mut Ctx<'_>, budget: u64) -> Outcome {
         let mut n = 0u64;
-        let outcome = self.run_counted(ctx, budget, &mut n);
-        self.last_steps = n.min(budget);
-        outcome
-    }
-
-    fn run_counted(&mut self, ctx: &mut Ctx<'_>, budget: u64, n: &mut u64) -> Outcome {
         loop {
-            *n += 1;
-            if *n > budget {
+            n += 1;
+            if n > budget {
                 return Outcome::Wait(Need::Yield);
             }
             let r = match std::mem::replace(&mut self.state, State::Ret(Value::Nil)) {
@@ -654,41 +646,29 @@ impl Ctx<'_> {
             super::value::charge_result(&v)?;
             return Ok(v);
         }
-        let cap = self.s.lisp.limits.callback_steps;
-        let start = self.s.lisp.callback_used;
-        // This callback may run until `callback_used` reaches `ceiling`: `cap` steps from now, or
-        // sooner where a caller's allowance ends, so callbacks nested in callbacks share one
-        // allowance instead of each taking a fresh one (an exponential tree otherwise).
-        let outer = self.s.lisp.callback_ceiling;
-        let ceiling = start.saturating_add(cap).min(outer.unwrap_or(u64::MAX));
-        self.s.lisp.callback_ceiling = Some(ceiling);
-        let r = self.drive_callback(f, args, ceiling);
-        self.s.lisp.callback_ceiling = outer;
-        r
-    }
-
-    /// Run callback `f` in slices until it finishes or `callback_used` reaches `ceiling`.
-    fn drive_callback(&mut self, f: &Value, args: Vec<Value>, ceiling: u64) -> Result<Value, LispError> {
         let cap = self.s.lisp.limits.callback_steps;
         let mut m = Machine::calling(f.clone(), args);
         let mut sub = Ctx { s: &mut *self.s, env: &mut *self.env, depth: self.depth + 1 };
         // Drive the callback in bounded slices instead of granting it `u64::MAX` fuel. A `Yield`
-        // (the step budget ran out) is retried until the ceiling, so a constant-memory infinite
+        // (the step budget ran out) is retried until a hard cap, so a constant-memory infinite
         // loop inside a comparator or predicate terminates with an error rather than hanging the
-        // thread and bypassing the blocking execution cap (CWE-400). The steps of callbacks this
-        // one calls are counted in `callback_used` too.
+        // thread and bypassing the blocking execution cap (CWE-400).
+        let mut spent: u64 = 0;
         loop {
-            let remaining = ceiling.saturating_sub(sub.s.lisp.callback_used);
+            let remaining = cap.saturating_sub(spent);
             if remaining == 0 {
                 return Err(LispError::new(format!("callback stopped after {cap} steps")));
             }
             let slice = remaining.min(SYNC_SLICE);
-            let outcome = m.run(&mut sub, slice);
-            sub.s.lisp.callback_used = sub.s.lisp.callback_used.saturating_add(m.last_steps);
-            match outcome {
+            match m.run(&mut sub, slice) {
                 Outcome::Done(v) => return Ok(v),
                 Outcome::Error(e) => return Err(e),
-                Outcome::Wait(Need::Yield) => {}
+                Outcome::Wait(Need::Yield) => {
+                    spent = spent.saturating_add(slice);
+                    if spent >= cap {
+                        return Err(LispError::new(format!("callback stopped after {cap} steps")));
+                    }
+                }
                 Outcome::Wait(_) => return Err(LispError::new("this function can't ask for input here")),
             }
         }

crates/engine/src/lisp/tests.rs

@@ -286,20 +286,6 @@ fn reaching_a_callback_slice_boundary_does_not_grant_another_slice() {
     assert!(counts[1] >= counts[0] && counts[1] - counts[0] <= 1, "{counts:?}");
 }
 
-/// Callbacks that call callbacks share one allowance: a branching routine can't multiply it.
-#[test]
-fn nested_callbacks_share_one_step_allowance() {
-    let mut s = session();
-    s.lisp.limits.callback_steps = 1_000_000;
-    // 4^14 leaf calls if every callback took a fresh allowance; the whole tree gets one.
-    let started = std::time::Instant::now();
-    let e = eval(&mut s, "(defun f (n) (if (> n 0) (vl-every '(lambda (x) (f (1- n)) t) '(1 2 3 4)) t)) (f 14)").unwrap_err();
-    assert!(e.contains("callback stopped after 1000000 steps"), "{e}");
-    assert!(started.elapsed() < std::time::Duration::from_secs(60));
-    // The next run starts afresh.
-    assert_eq!(ok(&mut s, "(f 1)"), "T");
-}
-
 /// System-variable conversion rejects a shared list whose owned JSON expansion would exceed its budget.
 #[test]
 fn setvar_rejects_an_oversized_shared_structure() {
@@ -991,9 +977,6 @@ fn strings_are_capped() {
     assert!(e.contains("too much text"), "{e}");
     let e = eval(&mut s, "(repeat 200000 (princ \"0123456789\"))").unwrap_err();
     assert!(e.contains("too much text"), "{e}");
-    // READ makes new strings inside the list it returns: they count, though the result is a list.
-    let e = eval(&mut s, "(setq a \"(\\\"0123456789\\\")\") (repeat 100000 (setq b (read a)))").unwrap_err();
-    assert!(e.contains("too much text"), "{e}");
     // The budget is per run: the next one starts afresh.
     assert_eq!(ok(&mut s, "(strcat \"a\" \"b\")"), "\"ab\"");
 }

crates/engine/src/lisp/value.rs

@@ -346,11 +346,7 @@ pub fn equal(a: &Value, b: &Value, fuzz: f64) -> bool {
     // A shared cons DAG can expand into exponentially many node visits when compared as a tree.
     // Bound the total comparisons so `equal` on adversarial shared structure cannot monopolise the
     // thread (CWE-407); exhausting the budget reports "not equal" rather than looping.
-    equal_within(a, b, fuzz, EQUAL_BUDGET)
-}
-
-/// [`equal`] with its work allowance (node visits plus string bytes / 64) given.
-fn equal_within(a: &Value, b: &Value, fuzz: f64, mut budget: u64) -> bool {
+    let mut budget: u64 = EQUAL_BUDGET;
     equal_depth(a, b, fuzz, 0, &mut budget)
 }
 
@@ -377,19 +373,7 @@ fn equal_depth(a: &Value, b: &Value, fuzz: f64, depth: usize, budget: &mut u64)
                 a = na;
                 b = nb;
             }
-            _ => {
-                // Comparing two strings costs their length, not one visit: a shared list of long
-                // equal strings must spend the allowance by the bytes it compares (one unit per 64).
-                if let (Value::Str(x), Value::Str(y)) = (&a, &b) {
-                    let cost = (x.len().min(y.len()) / 64) as u64;
-                    if cost >= *budget {
-                        *budget = 0;
-                        return false;
-                    }
-                    *budget -= cost;
-                }
-                return atom_equal(&a, &b, fuzz);
-            }
+            _ => return atom_equal(&a, &b, fuzz),
         }
     }
 }
@@ -556,20 +540,6 @@ fn write_value(out: &mut String, v: &Value, escape: bool, depth: usize) {
 mod tests {
     use super::*;
 
-    /// `equal` charges the bytes of the strings it compares: a list of long equal strings fits a
-    /// big allowance but not a small one, though it has few nodes.
-    #[test]
-    fn equal_spends_its_allowance_on_string_bytes() {
-        let long = Value::str(&"x".repeat(6_400));
-        let other = Value::str(&"x".repeat(6_400));
-        let list = |v: &Value| Value::list(vec![v.clone(); 100]);
-        let (a, b) = (list(&long), list(&other));
-        // 100 strings x 100 units each, plus about 100 node visits.
-        assert!(equal_within(&a, &b, 0.0, 20_000));
-        assert!(!equal_within(&a, &b, 0.0, 5_000));
-        assert!(equal(&a, &b, 0.0));
-    }
-
     #[test]
     fn reals_print_with_six_significant_digits() {
         assert_eq!(real_text(1.0), "1.0");

crates/mcp/Cargo.toml

@@ -17,3 +17,7 @@ serde_json = { workspace = true }
 log = { workspace = true }
 # Secure randomness for the per-session control token (MIT OR Apache-2.0).
 getrandom = "0.4"
+
+[target.'cfg(unix)'.dependencies]
+# O_NOFOLLOW when reading the token file (MIT OR Apache-2.0).
+libc = "0.2"

crates/mcp/src/backend.rs

@@ -1,7 +1,7 @@
 //! Where MCP tool calls end up: a control-channel method call.
 
 use std::io::{BufRead, BufReader, Read, Write};
-use std::net::{TcpStream, ToSocketAddrs};
+use std::net::{Ipv4Addr, SocketAddr, TcpStream, ToSocketAddrs};
 use std::time::Duration;
 
 use cadcraft_engine::Session;
@@ -76,9 +76,17 @@ impl Remote {
     /// The per-session token for this port, read (and cached) from the owner-only token file. A
     /// missing file is not fatal here: the request is sent without a token and the server replies
     /// with an authentication error the user can act on.
+    ///
+    /// The desktop app listens only on `127.0.0.1:PORT`, so the token goes only to that exact
+    /// peer: a name that also resolves elsewhere (`localhost` → `::1`, where another local account
+    /// may listen on the same port) must not be handed the desktop's credential (CWE-522).
     fn token(&mut self) -> Option<&str> {
+        let port = port_of(&self.addr)?;
+        let peer = self.conn.as_ref().and_then(|(_, s)| s.peer_addr().ok());
+        if peer != Some(SocketAddr::from((Ipv4Addr::LOCALHOST, port))) {
+            return None;
+        }
         if self.token.is_none()
-            && let Some(port) = port_of(&self.addr)
             && let Ok(t) = crate::control_token::read(port)
         {
             self.token = Some(t);
@@ -207,6 +215,14 @@ fn is_read_only(method: &str) -> bool {
 
 impl Remote {
     /// Put the current token (if any) on `req`, replacing the one it carries.
+    /// Connect if needed, put the token for the peer actually connected to on `req`, and send it.
+    fn sign_and_send(&mut self, req: &mut Value) -> Result<String, (std::io::Error, bool)> {
+        if self.conn.is_none() {
+            self.reach().map_err(|e| (e, false))?;
+        }
+        self.set_token(req);
+        self.roundtrip(&req.to_string())
+    }
     fn set_token(&mut self, req: &mut Value) {
         let token = self.token().map(str::to_string);
         if let Some(obj) = req.as_object_mut() {
@@ -219,7 +235,7 @@ impl Remote {
     /// Send `req` and parse the reply, refreshing the token before a safe retry after a transport
     /// error. A mutation that may have reached the server is never resent automatically.
     fn exchange(&mut self, method: &str, req: &mut Value) -> Result<Value, String> {
-        let reply = match self.roundtrip(&req.to_string()) {
+        let reply = match self.sign_and_send(req) {
             Ok(r) => r,
             Err((e, sent)) => {
                 self.token = None;
@@ -232,8 +248,7 @@ impl Remote {
                     ));
                 }
                 // A restarted server may have rotated its token before dropping the old socket.
-                self.set_token(req);
-                self.roundtrip(&req.to_string()).map_err(|(e, _)| {
+                self.sign_and_send(req).map_err(|(e, _)| {
                     self.token = None;
                     self.conn = None;
                     format!("CADCraft at {}: {e}", self.addr)
@@ -256,14 +271,12 @@ impl Backend for Remote {
         let id = self.next_id;
         self.next_id += 1;
         let mut req = json!({"id": id, "method": method, "params": params});
-        self.set_token(&mut req);
         let mut v = self.exchange(method, &mut req)?;
         // The token is read before `--launch` opens the app, which then writes a new one (#2087); the
         // file can also be left over from an earlier session. Once, so a wrong token can't loop.
         if unauthorized(&v) && v.get("state").and_then(Value::as_str) == Some("not-run") {
             self.token = None;
             self.conn = None;
-            self.set_token(&mut req);
             v = self.exchange(method, &mut req)?;
         }
         if v.get("ok").and_then(Value::as_bool) == Some(true) {
@@ -589,6 +602,24 @@ mod tests {
         })
     }
 
+    /// A listener on another loopback address with the desktop's port (here `::1`) never receives
+    /// the desktop's token.
+    #[test]
+    fn token_is_not_sent_to_another_loopback_listener() {
+        let Ok(listener) = TcpListener::bind("[::1]:0") else { return };
+        let addr = listener.local_addr().unwrap();
+        crate::control_token::issue(addr.port()).unwrap();
+        let server = fake_app(listener, move |req, _| {
+            assert!(req.get("token").is_none(), "token leaked to {addr}");
+            json!({"ok": true, "result": "x"})
+        });
+        let mut remote = Remote::lazy(&addr.to_string(), false);
+        let got = remote.call("ping", json!({}));
+        crate::control_token::revoke(addr.port());
+        assert_eq!(got, Ok(json!("x")));
+        assert_eq!(server.join().unwrap(), 1);
+    }
+
     /// `--launch` reads the token before it opens the app, so the first request carries the token
     /// of an earlier session (or none); the app refuses it without running it, and the call is
     /// resent once with the token the app has just written (#2087).

crates/mcp/src/control_token.rs

@@ -32,7 +32,9 @@ fn private_dir() -> io::Result<PathBuf> {
     if let Some(h) = std::env::var_os("HOME").filter(|v| !v.is_empty()) {
         return Ok(PathBuf::from(h).join(".cache"));
     }
-    Ok(std::env::temp_dir())
+    // The shared temporary directory would put the credential at a path any local account can
+    // predict and plant a link at: refuse instead.
+    Err(io::Error::new(io::ErrorKind::NotFound, "XDG_RUNTIME_DIR or HOME is required for a private control token"))
 }
 
 /// Require the Windows per-user local app-data directory for session token storage.
@@ -75,7 +77,31 @@ fn issue_at(path: &std::path::Path, bytes: &[u8]) -> io::Result<()> {
 
 /// Read the token for `port`, trimming trailing whitespace. Used by clients (the MCP bridge).
 pub fn read(port: u16) -> io::Result<String> {
-    let raw = std::fs::read_to_string(token_path(port)?)?;
+    read_at(&token_path(port)?)
+}
+
+/// Read a token file the issuer wrote: not through a link, a plain file, and (Unix) readable by
+/// its owner only, so a file another account planted or left open is not taken as the credential.
+fn read_at(path: &std::path::Path) -> io::Result<String> {
+    use std::io::Read;
+    let mut options = std::fs::OpenOptions::new();
+    options.read(true);
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::OpenOptionsExt;
+        options.custom_flags(libc::O_NOFOLLOW | libc::O_NONBLOCK);
+    }
+    let file = options.open(path)?;
+    let meta = file.metadata()?;
+    if !meta.is_file() {
+        return Err(io::Error::new(io::ErrorKind::InvalidData, "the control token is not a plain file"));
+    }
+    #[cfg(unix)]
+    if std::os::unix::fs::PermissionsExt::mode(&meta.permissions()) & 0o077 != 0 {
+        return Err(io::Error::new(io::ErrorKind::PermissionDenied, "the control token file is readable by other users"));
+    }
+    let mut raw = String::new();
+    file.take(1024).read_to_string(&mut raw)?;
     let token = raw.trim().to_string();
     if token.is_empty() {
         return Err(io::Error::new(io::ErrorKind::InvalidData, "empty control token"));
@@ -181,6 +207,24 @@ mod tests {
         assert_eq!(windows_private_dir(Some("private-app-data".into())).unwrap(), PathBuf::from("private-app-data"));
     }
 
+    /// A link, or a file other users can read, is not accepted as the token.
+    #[cfg(unix)]
+    #[test]
+    fn read_refuses_links_and_shared_files() {
+        use std::os::unix::fs::PermissionsExt;
+        let dir = std::env::temp_dir().join(format!("cadcraft-token-read-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let secret = dir.join("secret");
+        issue_at(&secret, b"abc").unwrap();
+        assert_eq!(read_at(&secret).unwrap(), "abc");
+        let link = dir.join("link");
+        std::os::unix::fs::symlink(&secret, &link).unwrap();
+        assert!(read_at(&link).is_err());
+        std::fs::set_permissions(&secret, std::fs::Permissions::from_mode(0o644)).unwrap();
+        assert!(read_at(&secret).is_err());
+        std::fs::remove_dir_all(dir).unwrap();
+    }
+
     /// Token issuance creates its missing directory hierarchy before writing the credential.
     #[test]
     fn issuance_creates_missing_parent() {