vmm: Limit the size of the SerialBuffer

We must limit how much the buffer can grow, otherwise this could lead
the process to consume all the memory on the machine. This could happen
if the output from the guest was very important and nothing would
connect to the PTY for a long time.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vmm/src/serial_buffer.rs

@@ -12,6 +12,8 @@ use std::{
     },
 };
 
+const MAX_BUFFER_SIZE: usize = 1 << 20;
+
 // Circular buffer implementation for serial output.
 // Read from head; push to tail
 pub(crate) struct SerialBuffer {
@@ -28,6 +30,22 @@ impl SerialBuffer {
             write_out,
         }
     }
+
+    fn fill_buffer(&mut self, buf: &[u8]) {
+        if buf.len() >= MAX_BUFFER_SIZE {
+            let offset = buf.len() - MAX_BUFFER_SIZE;
+            self.buffer = VecDeque::from(buf[offset..].to_vec());
+            return;
+        }
+
+        let num_allowed_bytes = MAX_BUFFER_SIZE - buf.len();
+        if self.buffer.len() > num_allowed_bytes {
+            let num_bytes_to_remove = self.buffer.len() - num_allowed_bytes;
+            self.buffer.drain(..num_bytes_to_remove);
+        }
+
+        self.buffer.extend(buf);
+    }
 }
 
 impl Write for SerialBuffer {
@@ -35,7 +53,7 @@ impl Write for SerialBuffer {
         // Simply fill the buffer if we're not allowed to write to the out
         // device.
         if !self.write_out.load(Ordering::Acquire) {
-            self.buffer.extend(buf);
+            self.fill_buffer(buf);
             return Ok(buf.len());
         }
 
@@ -47,7 +65,7 @@ impl Write for SerialBuffer {
         // only a subset of the bytes was written and we should fill the buffer
         // with what's coming from the serial.
         if !self.buffer.is_empty() {
-            self.buffer.extend(buf);
+            self.fill_buffer(buf);
             return Ok(buf.len());
         }
 
@@ -66,7 +84,7 @@ impl Write for SerialBuffer {
                     if !matches!(e.kind(), std::io::ErrorKind::WouldBlock) {
                         return Err(e);
                     }
-                    self.buffer.extend(&buf[offset..]);
+                    self.fill_buffer(&buf[offset..]);
                 }
             }
             break;