Re: [PATCH v2 1/2] ait-vga: check address before reading configuration bytes

[BALATON Zoltan]

On Thu, 4 Jun 2020, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> While reading PCI configuration bytes, a guest may send an
> address towards the end of the configuration space. It may lead
> to an OOB access issue. Add check to ensure 'address + size' is
> within PCI configuration space.
>
> 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 | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> Update v2: add check to avoid OOB PCI configuration space access
>  -> https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg00711.html
>
> diff --git a/hw/display/ati.c b/hw/display/ati.c
> index bda4a2d816..6671959e5d 100644
> --- a/hw/display/ati.c
> +++ b/hw/display/ati.c
> @@ -384,7 +384,10 @@ static uint64_t ati_mm_read(void *opaque, hwaddr addr, 
> unsigned int size)
>         val = s->regs.crtc_pitch;
>         break;
>     case 0xf00 ... 0xfff:
> -        val = pci_default_read_config(&s->dev, addr - 0xf00, size);
> +        addr = addr - 0xf00;

I'd write this as addr -= 0xf00 but modifying addr here breaks the trace 
print out at end of function so better drop this line and do

if (addr + size <= 0xfff) {

instead. Or is that really (addr + size - 1 <= 0xfff) considering that 
reading the last byte with addr = 0xfff size = 1 should probably be valid.

Regards,
BALATON Zoltan

> +        if (addr + size <= 0xff) {
> +            val = pci_default_read_config(&s->dev, addr, size);
> +        }
>         break;
>     case CUR_OFFSET:
>         val = s->regs.cur_offset;