Re: [PATCH v3] ati-vga: check address before reading configuration bytes (CVE-2020-13791)

[Michael S. Tsirkin]

On Thu, Jun 04, 2020 at 04:25:24PM +0530, 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>
> Suggested-by: BALATON Zoltan <balaton@eik.bme.hu>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>

BTW, this only happens on unaligned accesses.
And the IO memory region in question does not set valid.unaligned
or .impl.unaligned.

And the documentation says:

- .valid.unaligned specifies that the *device being modelled* supports
  unaligned accesses; if false, unaligned accesses will invoke the
  appropriate bus or CPU specific behaviour.

and

- .impl.unaligned specifies that the *implementation* supports unaligned
  accesses; if false, unaligned accesses will be emulated by two aligned
  accesses.

Is this then another case of a memory core bug which should have either
failed the access or split it?

> ---
>  hw/display/ati.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Update v3: avoid modifying 'addr' variable
>   -> https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg00834.html
> 
> diff --git a/hw/display/ati.c b/hw/display/ati.c
> index 67604e68de..b4d0fd88b7 100644
> --- a/hw/display/ati.c
> +++ b/hw/display/ati.c
> @@ -387,7 +387,9 @@ 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);
> +        if ((addr - 0xf00) + size <= pci_config_size(&s->dev)) {
> +            val = pci_default_read_config(&s->dev, addr - 0xf00, size);
> +        }
>          break;
>      case CUR_OFFSET:
>          val = s->regs.cur_offset;
> -- 
> 2.26.2