[PATCH v2] ati-vga: check mm_index before recursive call

[P J P]

From: Prasad J Pandit <pjp@fedoraproject.org>

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 it.

Reported-by: Ren Ding <rding@gatech.edu>
Reported-by: Hanqing Zhao <hanqing@gatech.edu>
Reported-by: Yi Ren <c4tren@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 hw/display/ati.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Update v2: add check before recursive call
  -> https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg00781.html

diff --git a/hw/display/ati.c b/hw/display/ati.c
index 065f197678..bda4a2d816 100644
--- a/hw/display/ati.c
+++ b/hw/display/ati.c
@@ -285,7 +285,7 @@ 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);
         }
         break;
@@ -520,7 +520,7 @@ 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);
         }
         break;
-- 
2.26.2