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

[Paolo Bonzini]

On 04/06/20 13:49, Michael S. Tsirkin wrote:
> 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?

In this case the path should be

  address_space_stl_le
    address_space_stl_internal
      memory_region_dispatch_write
        memory_region_access_valid

and then it does check valid.unaligned.  Is there a testcase?

Paolo

> 
>> ---
>>  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
>