virtio-blk: fix out-of-bounds access to bitmap in notify_guest_bh

When the number of a virtio-blk device's virtqueues is larger than
BITS_PER_LONG, the out-of-bounds access to bitmap[ ] will occur.

Fixes: e21737ab ("virtio-blk: multiqueue batch notify")
Cc: qemu-stable@nongnu.org
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLi Hangjing <lihangjing@baidu.com>
Reviewed-by: NXie Yongji <xieyongji@baidu.com>
Reviewed-by: NChai Wen <chaiwen@baidu.com>
Message-id: 20191216023050.48620-1-lihangjing@baidu.com
Message-Id: <20191216023050.48620-1-lihangjing@baidu.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -67,7 +67,7 @@ static void notify_guest_bh(void *opaque)
     memset(s->batch_notify_vqs, 0, sizeof(bitmap));
 
     for (j = 0; j < nvqs; j += BITS_PER_LONG) {
-        unsigned long bits = bitmap[j];
+        unsigned long bits = bitmap[j / BITS_PER_LONG];
 
         while (bits != 0) {
             unsigned i = j + ctzl(bits);