esp: check command buffer length before write(CVE-2016-4439)

The 53C9X Fast SCSI Controller(FSC) comes with an internal 16-byte
FIFO buffer. It is used to handle command and data transfer. While
writing to this command buffer 's->cmdbuf[TI_BUFSZ=16]', a check
was missing to validate input length. Add check to avoid OOB write
access.

Fixes CVE-2016-4439.
Reported-by: NLi Qiang <liqiang6-s@360.cn>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPrasad J Pandit <pjp@fedoraproject.org>
Message-Id: <1463654371-11169-2-git-send-email-ppandit@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/esp.c

@@ -448,7 +448,11 @@ void esp_reg_write(ESPState *s, uint32_t saddr, uint64_t val)
         break;
     case ESP_FIFO:
         if (s->do_cmd) {
-            s->cmdbuf[s->cmdlen++] = val & 0xff;
+            if (s->cmdlen < TI_BUFSZ) {
+                s->cmdbuf[s->cmdlen++] = val & 0xff;
+            } else {
+                trace_esp_error_fifo_overrun();
+            }
         } else if (s->ti_size == TI_BUFSZ - 1) {
             trace_esp_error_fifo_overrun();
         } else {