hw/acpi/erst.c: Fix memory handling issues

- Fix memset argument order: The second argument is
  the value, the length goes last.
- Fix an integer overflow reported by Alexander Bulekov.

Both issues allow the guest to overrun the host buffer
allocated for the ERST memory device.

Cc: Eric DeVolder <eric.devolder@oracle.com
Cc: Alexander Bulekov <alxndr@bu.edu>
Cc: qemu-stable@nongnu.org
Fixes: f7e26ffa ("ACPI ERST: support for ACPI ERST feature")
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
Message-Id: <20221024154233.1043347-1-lk@c--e.de>
Fixes: #1268


Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/acpi/erst.c

@@ -635,7 +635,7 @@ static unsigned read_erst_record(ERSTDeviceState *s)
         if (record_length < UEFI_CPER_RECORD_MIN_SIZE) {
             rc = STATUS_FAILED;
         }
-        if ((s->record_offset + record_length) > exchange_length) {
+        if (record_length > exchange_length - s->record_offset) {
             rc = STATUS_FAILED;
         }
         /* If all is ok, copy the record to the exchange buffer */
@@ -684,7 +684,7 @@ static unsigned write_erst_record(ERSTDeviceState *s)
     if (record_length < UEFI_CPER_RECORD_MIN_SIZE) {
         return STATUS_FAILED;
     }
-    if ((s->record_offset + record_length) > exchange_length) {
+    if (record_length > exchange_length - s->record_offset) {
         return STATUS_FAILED;
     }
 
@@ -716,7 +716,7 @@ static unsigned write_erst_record(ERSTDeviceState *s)
     if (nvram) {
         /* Write the record into the slot */
         memcpy(nvram, exchange, record_length);
-        memset(nvram + record_length, exchange_length - record_length, 0xFF);
+        memset(nvram + record_length, 0xFF, exchange_length - record_length);
         /* If a new record, increment the record_count */
         if (!record_found) {
             uint32_t record_count;