Fix insufficient length validation in IPv4 packets.

Found via cargo-fuzz.

Author: whitequark

src/wire/ipv4.rs

@@ -119,19 +119,21 @@ impl<T: AsRef<[u8]>> Packet<T> {
     /// Ensure that no accessor method will panic if called.
     /// Returns `Err(Error::Truncated)` if the buffer is too short.
     ///
-    /// The result of this check is invalidated by calling [set_header_len].
+    /// The result of this check is invalidated by calling [set_header_len]
+    /// and [set_total_len].
     ///
     /// [set_header_len]: #method.set_header_len
+    /// [set_total_len]: #method.set_total_len
     pub fn check_len(&self) -> Result<()> {
         let len = self.buffer.as_ref().len();
         if len < field::DST_ADDR.end {
             Err(Error::Truncated)
+        } else if len < self.header_len() as usize {
+            Err(Error::Truncated)
+        } else if len < self.total_len() as usize {
+            Err(Error::Truncated)
         } else {
-            if len < self.header_len() as usize {
-                Err(Error::Truncated)
-            } else {
-                Ok(())
-            }
+            Ok(())
         }
     }
 
@@ -634,6 +636,16 @@ mod test {
                    PAYLOAD_BYTES.len());
     }
 
+    #[test]
+    fn test_total_len_overflow() {
+        let mut bytes = vec![];
+        bytes.extend(&PACKET_BYTES[..]);
+        Packet::new(&mut bytes).set_total_len(128);
+
+        assert_eq!(Packet::new_checked(&bytes).unwrap_err(),
+                   Error::Truncated);
+    }
+
     static REPR_PACKET_BYTES: [u8; 24] =
         [0x45, 0x00, 0x00, 0x18,
          0x00, 0x00, 0x40, 0x00,