crosvm: fix serial device panic on write to read-only registers

There were a few cases of a write into the serial device that would
panic because the registers were read-only. This change simply ignores
those writes instead of raising a panic.

This is also the first bug that was found by cargo-fuzz! Wohoo!

TEST=None
BUG=None

Change-Id: I8b6805617ac3dcfddd0555c1bb394a7bc1a7cf6d
Reviewed-on: https://chromium-review.googlesource.com/518445
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

1 files changed, 2 insertions, 7 deletions

diff --git a/src/hw/serial.rs b/src/hw/serial.rs
index f93631d..68bc63e 100644
--- a/src/hw/serial.rs
+++ b/src/hw/serial.rs
@@ -176,11 +176,10 @@ impl Serial {
                 }
             }
             IER => self.interrupt_enable = v & IER_FIFO_BITS,
-            FCR => {}
             LCR => self.line_control = v,
             MCR => self.modem_control = v,
             SCR => self.scratch = v,
-            o => panic!("bad write offset on serial device: {}", o),
+            _ => {}
         }
         Ok(())
     }
@@ -192,10 +191,6 @@ impl BusDevice for Serial {
             return;
         }
 
-        if offset >= 8 {
-            return;
-        }
-
         if let Err(e) = self.handle_write(offset as u8, data[0]) {
             println!("serial failed write: {:?}", e);
         }
@@ -227,7 +222,7 @@ impl BusDevice for Serial {
             LSR => self.line_status,
             MSR => self.modem_status,
             SCR => self.scratch,
-            o => panic!("bad read offset on serial device: {}", o),
+            _ => 0,
         };
     }
 }