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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 04, 2020 at 01:56:45PM +0200, Philippe Mathieu-Daudé wrote:
> On 6/4/20 1:49 PM, 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?
> 
> Related:
> https://www.mail-archive.com/qemu-devel@nongnu.org/msg695362.html
> earlier comment:
> https://www.mail-archive.com/qemu-devel@nongnu.org/msg694805.html

Yea looks like more devices following documentation and memory core
doing something else instead.

> > 
> >> ---
> >>  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
> >