char: cadence: check baud rate generator and divider values

The Cadence UART device emulator calculates speed by dividing the
baud rate by a 'baud rate generator' & 'baud rate divider' value.
The device specification defines these register values to be
non-zero and within certain limits. Add checks for these limits
to avoid errors like divide by zero.

Reported-by: Huawei PSIRT <psirt@huawei.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1477596278-1470-1-git-send-email-ppandit@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/hw/char/cadence_uart.c b/hw/char/cadence_uart.c
index def34cd..0215d65 100644
--- a/hw/char/cadence_uart.c
+++ b/hw/char/cadence_uart.c
@@ -1,6 +1,11 @@
 /*
  * Device model for Cadence UART
  *
+ * Reference: Xilinx Zynq 7000 reference manual
+ *   - http://www.xilinx.com/support/documentation/user_guides/ug585-Zynq-7000-TRM.pdf
+ *   - Chapter 19 UART Controller
+ *   - Appendix B for Register details
+ *
  * Copyright (c) 2010 Xilinx Inc.
  * Copyright (c) 2012 Peter A.G. Crosthwaite (peter.crosthwaite@petalogix.com)
  * Copyright (c) 2012 PetaLogix Pty Ltd.
@@ -402,6 +407,16 @@ static void uart_write(void *opaque, hwaddr offset,
             break;
         }
         break;
+    case R_BRGR: /* Baud rate generator */
+        if (value >= 0x01) {
+            s->r[offset] = value & 0xFFFF;
+        }
+        break;
+    case R_BDIV:    /* Baud rate divider */
+        if (value >= 0x04) {
+            s->r[offset] = value & 0xFF;
+        }
+        break;
     default:
         s->r[offset] = value;
     }