Fix for a fuzzer-discovered integer underflow of the flow control win…

…dow size (#692)

Removed the SubAssign, etc. syntactic sugar functions and switched to return Result on over/underflow

Whenever possible, switched to returning a library GoAway protocol
error. Otherwise we check for over/underflow only with `debug_assert!`,
assuming that those code paths do not over/underflow.


Signed-off-by: Michael Rodler <mrodler@amazon.de>
Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>
Co-authored-by: Michael Rodler <mrodler@amazon.de>
Co-authored-by: Daniele Ahmed <ahmeddan@amazon.de>

src/proto/connection.rs

@@ -145,7 +145,9 @@ where
 
     /// connection flow control
     pub(crate) fn set_target_window_size(&mut self, size: WindowSize) {
-        self.inner.streams.set_target_connection_window_size(size);
+        let _res = self.inner.streams.set_target_connection_window_size(size);
+        // TODO: proper error handling
+        debug_assert!(_res.is_ok());
     }
 
     /// Send a new SETTINGS frame with an updated initial window size.

src/proto/mod.rs

@@ -30,7 +30,7 @@ pub type PingPayload = [u8; 8];
 pub type WindowSize = u32;
 
 // Constants
-pub const MAX_WINDOW_SIZE: WindowSize = (1 << 31) - 1;
+pub const MAX_WINDOW_SIZE: WindowSize = (1 << 31) - 1; // i32::MAX as u32
 pub const DEFAULT_REMOTE_RESET_STREAM_MAX: usize = 20;
 pub const DEFAULT_RESET_STREAM_MAX: usize = 10;
 pub const DEFAULT_RESET_STREAM_SECS: u64 = 30;

src/proto/streams/flow_control.rs

@@ -75,12 +75,12 @@ impl FlowControl {
         self.window_size > self.available
     }
 
-    pub fn claim_capacity(&mut self, capacity: WindowSize) {
-        self.available -= capacity;
+    pub fn claim_capacity(&mut self, capacity: WindowSize) -> Result<(), Reason> {
+        self.available.decrease_by(capacity)
     }
 
-    pub fn assign_capacity(&mut self, capacity: WindowSize) {
-        self.available += capacity;
+    pub fn assign_capacity(&mut self, capacity: WindowSize) -> Result<(), Reason> {
+        self.available.increase_by(capacity)
     }
 
     /// If a WINDOW_UPDATE frame should be sent, returns a positive number
@@ -136,36 +136,38 @@ impl FlowControl {
     ///
     /// This is called after receiving a SETTINGS frame with a lower
     /// INITIAL_WINDOW_SIZE value.
-    pub fn dec_send_window(&mut self, sz: WindowSize) {
+    pub fn dec_send_window(&mut self, sz: WindowSize) -> Result<(), Reason> {
         tracing::trace!(
             "dec_window; sz={}; window={}, available={}",
             sz,
             self.window_size,
             self.available
         );
-        // This should not be able to overflow `window_size` from the bottom.
-        self.window_size -= sz;
+        // ~~This should not be able to overflow `window_size` from the bottom.~~ wrong. it can.
+        self.window_size.decrease_by(sz)?;
+        Ok(())
     }
 
     /// Decrement the recv-side window size.
     ///
     /// This is called after receiving a SETTINGS ACK frame with a lower
     /// INITIAL_WINDOW_SIZE value.
-    pub fn dec_recv_window(&mut self, sz: WindowSize) {
+    pub fn dec_recv_window(&mut self, sz: WindowSize) -> Result<(), Reason> {
         tracing::trace!(
             "dec_recv_window; sz={}; window={}, available={}",
             sz,
             self.window_size,
             self.available
         );
         // This should not be able to overflow `window_size` from the bottom.
-        self.window_size -= sz;
-        self.available -= sz;
+        self.window_size.decrease_by(sz)?;
+        self.available.decrease_by(sz)?;
+        Ok(())
     }
 
     /// Decrements the window reflecting data has actually been sent. The caller
     /// must ensure that the window has capacity.
-    pub fn send_data(&mut self, sz: WindowSize) {
+    pub fn send_data(&mut self, sz: WindowSize) -> Result<(), Reason> {
         tracing::trace!(
             "send_data; sz={}; window={}; available={}",
             sz,
@@ -176,12 +178,13 @@ impl FlowControl {
         // If send size is zero it's meaningless to update flow control window
         if sz > 0 {
             // Ensure that the argument is correct
-            assert!(self.window_size >= sz as usize);
+            assert!(self.window_size.0 >= sz as i32);
 
             // Update values
-            self.window_size -= sz;
-            self.available -= sz;
+            self.window_size.decrease_by(sz)?;
+            self.available.decrease_by(sz)?;
         }
+        Ok(())
     }
 }
 
@@ -208,6 +211,29 @@ impl Window {
         assert!(self.0 >= 0, "negative Window");
         self.0 as WindowSize
     }
+
+    pub fn decrease_by(&mut self, other: WindowSize) -> Result<(), Reason> {
+        if let Some(v) = self.0.checked_sub(other as i32) {
+            self.0 = v;
+            Ok(())
+        } else {
+            Err(Reason::FLOW_CONTROL_ERROR)
+        }
+    }
+
+    pub fn increase_by(&mut self, other: WindowSize) -> Result<(), Reason> {
+        let other = self.add(other)?;
+        self.0 = other.0;
+        Ok(())
+    }
+
+    pub fn add(&self, other: WindowSize) -> Result<Self, Reason> {
+        if let Some(v) = self.0.checked_add(other as i32) {
+            Ok(Self(v))
+        } else {
+            Err(Reason::FLOW_CONTROL_ERROR)
+        }
+    }
 }
 
 impl PartialEq<usize> for Window {
@@ -230,25 +256,6 @@ impl PartialOrd<usize> for Window {
     }
 }
 
-impl ::std::ops::SubAssign<WindowSize> for Window {
-    fn sub_assign(&mut self, other: WindowSize) {
-        self.0 -= other as i32;
-    }
-}
-
-impl ::std::ops::Add<WindowSize> for Window {
-    type Output = Self;
-    fn add(self, other: WindowSize) -> Self::Output {
-        Window(self.0 + other as i32)
-    }
-}
-
-impl ::std::ops::AddAssign<WindowSize> for Window {
-    fn add_assign(&mut self, other: WindowSize) {
-        self.0 += other as i32;
-    }
-}
-
 impl fmt::Display for Window {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)

src/proto/streams/prioritize.rs

@@ -87,7 +87,9 @@ impl Prioritize {
         flow.inc_window(config.remote_init_window_sz)
             .expect("invalid initial window size");
 
-        flow.assign_capacity(config.remote_init_window_sz);
+        // TODO: proper error handling
+        let _res = flow.assign_capacity(config.remote_init_window_sz);
+        debug_assert!(_res.is_ok());
 
         tracing::trace!("Prioritize::new; flow={:?}", flow);
 
@@ -253,7 +255,9 @@ impl Prioritize {
                 if available as usize > capacity {
                     let diff = available - capacity as WindowSize;
 
-                    stream.send_flow.claim_capacity(diff);
+                    // TODO: proper error handling
+                    let _res = stream.send_flow.claim_capacity(diff);
+                    debug_assert!(_res.is_ok());
 
                     self.assign_connection_capacity(diff, stream, counts);
                 }
@@ -324,7 +328,9 @@ impl Prioritize {
     pub fn reclaim_all_capacity(&mut self, stream: &mut store::Ptr, counts: &mut Counts) {
         let available = stream.send_flow.available().as_size();
         if available > 0 {
-            stream.send_flow.claim_capacity(available);
+            // TODO: proper error handling
+            let _res = stream.send_flow.claim_capacity(available);
+            debug_assert!(_res.is_ok());
             // Re-assign all capacity to the connection
             self.assign_connection_capacity(available, stream, counts);
         }
@@ -337,7 +343,9 @@ impl Prioritize {
         if stream.requested_send_capacity as usize > stream.buffered_send_data {
             let reserved = stream.requested_send_capacity - stream.buffered_send_data as WindowSize;
 
-            stream.send_flow.claim_capacity(reserved);
+            // TODO: proper error handling
+            let _res = stream.send_flow.claim_capacity(reserved);
+            debug_assert!(_res.is_ok());
             self.assign_connection_capacity(reserved, stream, counts);
         }
     }
@@ -363,7 +371,9 @@ impl Prioritize {
         let span = tracing::trace_span!("assign_connection_capacity", inc);
         let _e = span.enter();
 
-        self.flow.assign_capacity(inc);
+        // TODO: proper error handling
+        let _res = self.flow.assign_capacity(inc);
+        debug_assert!(_res.is_ok());
 
         // Assign newly acquired capacity to streams pending capacity.
         while self.flow.available() > 0 {
@@ -443,7 +453,9 @@ impl Prioritize {
             stream.assign_capacity(assign, self.max_buffer_size);
 
             // Claim the capacity from the connection
-            self.flow.claim_capacity(assign);
+            // TODO: proper error handling
+            let _res = self.flow.claim_capacity(assign);
+            debug_assert!(_res.is_ok());
         }
 
         tracing::trace!(
@@ -763,12 +775,16 @@ impl Prioritize {
                                 // Assign the capacity back to the connection that
                                 // was just consumed from the stream in the previous
                                 // line.
-                                self.flow.assign_capacity(len);
+                                // TODO: proper error handling
+                                let _res = self.flow.assign_capacity(len);
+                                debug_assert!(_res.is_ok());
                             });
 
                             let (eos, len) = tracing::trace_span!("updating connection flow")
                                 .in_scope(|| {
-                                    self.flow.send_data(len);
+                                    // TODO: proper error handling
+                                    let _res = self.flow.send_data(len);
+                                    debug_assert!(_res.is_ok());
 
                                     // Wrap the frame's data payload to ensure that the
                                     // correct amount of data gets written.

src/proto/streams/recv.rs

@@ -90,7 +90,7 @@ impl Recv {
         // settings
         flow.inc_window(DEFAULT_INITIAL_WINDOW_SIZE)
             .expect("invalid initial remote window size");
-        flow.assign_capacity(DEFAULT_INITIAL_WINDOW_SIZE);
+        flow.assign_capacity(DEFAULT_INITIAL_WINDOW_SIZE).unwrap();
 
         Recv {
             init_window_sz: config.local_init_window_sz,
@@ -363,7 +363,9 @@ impl Recv {
         self.in_flight_data -= capacity;
 
         // Assign capacity to connection
-        self.flow.assign_capacity(capacity);
+        // TODO: proper error handling
+        let _res = self.flow.assign_capacity(capacity);
+        debug_assert!(_res.is_ok());
 
         if self.flow.unclaimed_capacity().is_some() {
             if let Some(task) = task.take() {
@@ -391,7 +393,9 @@ impl Recv {
         stream.in_flight_recv_data -= capacity;
 
         // Assign capacity to stream
-        stream.recv_flow.assign_capacity(capacity);
+        // TODO: proper error handling
+        let _res = stream.recv_flow.assign_capacity(capacity);
+        debug_assert!(_res.is_ok());
 
         if stream.recv_flow.unclaimed_capacity().is_some() {
             // Queue the stream for sending the WINDOW_UPDATE frame.
@@ -437,7 +441,11 @@ impl Recv {
     ///
     /// The `task` is an optional parked task for the `Connection` that might
     /// be blocked on needing more window capacity.
-    pub fn set_target_connection_window(&mut self, target: WindowSize, task: &mut Option<Waker>) {
+    pub fn set_target_connection_window(
+        &mut self,
+        target: WindowSize,
+        task: &mut Option<Waker>,
+    ) -> Result<(), Reason> {
         tracing::trace!(
             "set_target_connection_window; target={}; available={}, reserved={}",
             target,
@@ -450,11 +458,15 @@ impl Recv {
         //
         // Update the flow controller with the difference between the new
         // target and the current target.
-        let current = (self.flow.available() + self.in_flight_data).checked_size();
+        let current = self
+            .flow
+            .available()
+            .add(self.in_flight_data)?
+            .checked_size();
         if target > current {
-            self.flow.assign_capacity(target - current);
+            self.flow.assign_capacity(target - current)?;
         } else {
-            self.flow.claim_capacity(current - target);
+            self.flow.claim_capacity(current - target)?;
         }
 
         // If changing the target capacity means we gained a bunch of capacity,
@@ -465,6 +477,7 @@ impl Recv {
                 task.wake();
             }
         }
+        Ok(())
     }
 
     pub(crate) fn apply_local_settings(
@@ -504,9 +517,13 @@ impl Recv {
                     let dec = old_sz - target;
                     tracing::trace!("decrementing all windows; dec={}", dec);
 
-                    store.for_each(|mut stream| {
-                        stream.recv_flow.dec_recv_window(dec);
-                    })
+                    store.try_for_each(|mut stream| {
+                        stream
+                            .recv_flow
+                            .dec_recv_window(dec)
+                            .map_err(proto::Error::library_go_away)?;
+                        Ok::<_, proto::Error>(())
+                    })?;
                 }
                 Ordering::Greater => {
                     // We must increase the (local) window on every open stream.
@@ -519,7 +536,10 @@ impl Recv {
                             .recv_flow
                             .inc_window(inc)
                             .map_err(proto::Error::library_go_away)?;
-                        stream.recv_flow.assign_capacity(inc);
+                        stream
+                            .recv_flow
+                            .assign_capacity(inc)
+                            .map_err(proto::Error::library_go_away)?;
                         Ok::<_, proto::Error>(())
                     })?;
                 }
@@ -626,7 +646,10 @@ impl Recv {
         }
 
         // Update stream level flow control
-        stream.recv_flow.send_data(sz);
+        stream
+            .recv_flow
+            .send_data(sz)
+            .map_err(proto::Error::library_go_away)?;
 
         // Track the data as in-flight
         stream.in_flight_recv_data += sz;
@@ -667,7 +690,7 @@ impl Recv {
         }
 
         // Update connection level flow control
-        self.flow.send_data(sz);
+        self.flow.send_data(sz).map_err(Error::library_go_away)?;
 
         // Track the data as in-flight
         self.in_flight_data += sz;