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

[Philippe Mathieu-Daudé]

On 6/3/20 8:55 PM, P J P wrote:
> 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);

We usually log unexpected guest accesses with:

           } else {
               qemu_log_mask(LOG_GUEST_ERROR, ...

>          }
>          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);

Ditto:

           } else {
               qemu_log_mask(LOG_GUEST_ERROR, ...

>          }
>          break;
>