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

[P J P <ppandit@redhat.com>]

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>
---
 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