From: Prasad J Pandit <address@hidden>

virtio back end uses set of buffers to facilitate I/O operations.
If its size is too large, 'cpu_physical_memory_map' could return
a null address. This would result in a null dereference
while un-mapping descriptors. Add check to avoid it.

Reported-by: Qinghao Tang <address@hidden>
Signed-off-by: Prasad J Pandit <address@hidden>
---
 hw/virtio/virtio.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 15ee3a7..0a4c5b6 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -472,12 +472,14 @@ static void virtqueue_map_desc(unsigned int *p_num_sg, hwaddr *addr, struct iove
         }
 
         iov[num_sg].iov_base = cpu_physical_memory_map(pa, &len, is_write);
-        iov[num_sg].iov_len = len;
-        addr[num_sg] = pa;
+        if (iov[num_sg].iov_base) {
+            iov[num_sg].iov_len = len;
+            addr[num_sg] = pa;
 
+            pa += len;
+            num_sg++;
+        }
         sz -= len;
-        pa += len;
-        num_sg++;
     }
     *p_num_sg = num_sg;
 }
-- 
2.5.5