ati-vga: check mm_index before recursive call (CVE-2020-13800)

While accessing VGA registers via ati_mm_read/write routines,
a guest may set 's->regs.mm_index' such that it leads to infinite
recursion. Check mm_index value to avoid such recursion. Log an
error message for wrong values.

Reported-by: Ren Ding <rding@gatech.edu>
Reported-by: Hanqing Zhao <hanqing@gatech.edu>
Reported-by: Yi Ren <c4tren@gmail.com>
Message-id: 20200604090830.33885-1-ppandit@redhat.com
Suggested-by: BALATON Zoltan <balaton@eik.bme.hu>
Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/display/ati.c

@@ -285,8 +285,11 @@ static uint64_t ati_mm_read(void *opaque, hwaddr addr, unsigned int size)
             if (idx <= s->vga.vram_size - size) {
                 val = ldn_le_p(s->vga.vram_ptr + idx, size);
             }
-        } else {
+        } else if (s->regs.mm_index > MM_DATA + 3) {
             val = ati_mm_read(s, s->regs.mm_index + addr - MM_DATA, size);
+        } else {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                "ati_mm_read: mm_index too small: %u\n", s->regs.mm_index);
         }
         break;
     case BIOS_0_SCRATCH ... BUS_CNTL - 1:
@@ -520,8 +523,11 @@ static void ati_mm_write(void *opaque, hwaddr addr,
             if (idx <= s->vga.vram_size - size) {
                 stn_le_p(s->vga.vram_ptr + idx, size, data);
             }
-        } else {
+        } else if (s->regs.mm_index > MM_DATA + 3) {
             ati_mm_write(s, s->regs.mm_index + addr - MM_DATA, data, size);
+        } else {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                "ati_mm_write: mm_index too small: %u\n", s->regs.mm_index);
         }
         break;
     case BIOS_0_SCRATCH ... BUS_CNTL - 1: