the used_wrap_counter and the vq->last_used_idx may get
out of sync if they are separate assignment，and interrupt
might use an incorrect value to check for the used index.

for example:OOB access
ksoftirqd may consume the packet and it will call:
virtnet_poll
	-->virtnet_receive
		-->virtqueue_get_buf_ctx
			-->virtqueue_get_buf_ctx_packed
and in virtqueue_get_buf_ctx_packed:

vq->last_used_idx += vq->packed.desc_state[id].num;
if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
         vq->last_used_idx -= vq->packed.vring.num;
         vq->packed.used_wrap_counter ^= 1;
}

if at the same time, there comes a vring interrupt，in vring_interrupt:
we will call:
vring_interrupt
	-->more_used
		-->more_used_packed
			-->is_used_desc_packed
in is_used_desc_packed, the last_used_idx maybe >= vq->packed.vring.num.
so this could case a memory out of bounds bug.

this patch is to keep the used_wrap_counter in vq->last_used_idx
so we can get the correct value to check for used index in interrupt.

v3->v4:
- use READ_ONCE/WRITE_ONCE to get/set vq->last_used_idx

v2->v3:
- add inline function to get used_wrap_counter and last_used
- when use vq->last_used_idx, only read once
  if vq->last_used_idx is read twice, the values can be inconsistent.
- use last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR))
  to get the all bits below VRING_PACKED_EVENT_F_WRAP_CTR

v1->v2:
- reuse the VRING_PACKED_EVENT_F_WRAP_CTR
- Remove parameter judgment in is_used_desc_packed,
because it can't be illegal
Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
Message-Id: <20220617020411.80367-1-huangjie.albert@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>