hw/char/pl011: Avoid division-by-zero in pl011_get_baudrate()

In pl011_get_baudrate(), when we calculate the baudrate we can
accidentally divide by zero. This happens because although (as the
specification requires) we treat UARTIBRD = 0 as invalid, we aren't
correctly limiting UARTIBRD and UARTFBRD values to the 16-bit and 6-bit
ranges the hardware allows, and so some non-zero values of UARTIBRD can
result in a zero divisor.

Enforce the correct register field widths on guest writes and on inbound
migration to avoid the division by zero.

ASAN log:
==2973125==ERROR: AddressSanitizer: FPE on unknown address 0x55f72629b348
(pc 0x55f72629b348 bp 0x7fffa24d0e00 sp 0x7fffa24d0d60 T0)
     #0 0x55f72629b348 in pl011_get_baudrate hw/char/pl011.c:255:17
     #1 0x55f726298d94 in pl011_trace_baudrate_change hw/char/pl011.c:260:33
     #2 0x55f726296fc8 in pl011_write hw/char/pl011.c:378:9

Reproducer:
cat << EOF | qemu-system-aarch64 -display \
none -machine accel=qtest, -m 512M -machine realview-pb-a8 -qtest stdio
writeq 0x1000b024 0xf8000000
EOF

Suggested-by: Philippe Mathieu-Daudé <[email protected]>
Suggested-by: Peter Maydell <[email protected]>
Signed-off-by: Zheyu Ma <[email protected]>
Message-Id: <[email protected]>

hw/char/pl011.c

@@ -87,6 +87,12 @@ DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev *chr)
 #define CR_DTR      (1 << 10)
 #define CR_LBE      (1 << 7)
 
+/* Integer Baud Rate Divider, UARTIBRD */
+#define IBRD_MASK 0x3f
+
+/* Fractional Baud Rate Divider, UARTFBRD */
+#define FBRD_MASK 0xffff
+
 static const unsigned char pl011_id_arm[8] =
   { 0x11, 0x10, 0x14, 0x00, 0x0d, 0xf0, 0x05, 0xb1 };
 static const unsigned char pl011_id_luminary[8] =
@@ -374,11 +380,11 @@ static void pl011_write(void *opaque, hwaddr offset,
         s->ilpr = value;
         break;
     case 9: /* UARTIBRD */
-        s->ibrd = value;
+        s->ibrd = value & IBRD_MASK;
         pl011_trace_baudrate_change(s);
         break;
     case 10: /* UARTFBRD */
-        s->fbrd = value;
+        s->fbrd = value & FBRD_MASK;
         pl011_trace_baudrate_change(s);
         break;
     case 11: /* UARTLCR_H */
@@ -531,6 +537,9 @@ static int pl011_post_load(void *opaque, int version_id)
         s->read_pos = 0;
     }
 
+    s->ibrd &= IBRD_MASK;
+    s->fbrd &= FBRD_MASK;
+
     return 0;
 }