net: limit the size of the guest buffer in TX path

When we switched to using writev for copying a network packet from guest
memory to the tap device we dropped an (implicit) check for the size of
the TX frame. Reintroduce that check so that we ensure that we can catch
such errors early rather than rely on the underlying TAP device. This
also, controls the amount of memory we allocate in the Firecracker
process for copying frames that are destined for MMDS from guest memory
to Firecracker memory.

Signed-off-by: Babis Chalios <[email protected]>

Author: bchalios

src/vmm/src/devices/virtio/net/device.rs

@@ -607,6 +607,17 @@ impl Net {
                     continue;
                 }
             };
+
+            // We only handle frames that are up to MAX_BUFFER_SIZE
+            if buffer.len() > MAX_BUFFER_SIZE {
+                error!("net: received too big frame from driver");
+                self.metrics.tx_fails.inc();
+                tx_queue
+                    .add_used(mem, head_index, 0)
+                    .map_err(DeviceError::QueueError)?;
+                continue;
+            }
+
             if !Self::rate_limiter_consume_op(&mut self.tx_rate_limiter, buffer.len() as u64) {
                 tx_queue.undo_pop();
                 self.metrics.tx_rate_limiter_throttled.inc();