[PATCH for-8.0] hw/char/cadence_uart: Fix guards on invalid BRGR/BDIV settings

[Peter Maydell]

The cadence UART attempts to avoid allowing the guset to set invalid
baud rate register values in the uart_write() function.  However it
does the "mask to the size of the register field" and "check for
invalid values" in the wrong order, which means that a malicious
guest can get a bogus value into the register by setting also some
high bits in the value, and cause QEMU to crash by division-by-zero.

Do the mask before the bounds check instead of afterwards.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1493
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/char/cadence_uart.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/char/cadence_uart.c b/hw/char/cadence_uart.c
index c069a30842e..807e3985419 100644
--- a/hw/char/cadence_uart.c
+++ b/hw/char/cadence_uart.c
@@ -450,13 +450,15 @@ static MemTxResult uart_write(void *opaque, hwaddr offset,
         }
         break;
     case R_BRGR: /* Baud rate generator */
+        value &= 0xffff;
         if (value >= 0x01) {
-            s->r[offset] = value & 0xFFFF;
+            s->r[offset] = value;
         }
         break;
     case R_BDIV:    /* Baud rate divider */
+        value &= 0xff;
         if (value >= 0x04) {
-            s->r[offset] = value & 0xFF;
+            s->r[offset] = value;
         }
         break;
     default:
-- 
2.34.1