Subject: Re: [PATCH net V5] virtio-net: fix the race between refill work and close

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 4 Jul 2022 04:52:55 -0400

On Mon, Jul 04, 2022 at 03:48:59PM +0800, Jason Wang wrote:
> We try using cancel_delayed_work_sync() to prevent the work from
> enabling NAPI. This is insufficient since we don't disable the source
> of the refill work scheduling. This means an NAPI poll callback after
> cancel_delayed_work_sync() can schedule the refill work then can
> re-enable the NAPI that leads to use-after-free [1].
> 
> Since the work can enable NAPI, we can't simply disable NAPI before
> calling cancel_delayed_work_sync(). So fix this by introducing a
> dedicated boolean to control whether or not the work could be
> scheduled from NAPI.
> 
> [1]
> ==================================================================
> BUG: KASAN: use-after-free in refill_work+0x43/0xd4
> Read of size 2 at addr ffff88810562c92e by task kworker/2:1/42
> 
> CPU: 2 PID: 42 Comm: kworker/2:1 Not tainted 5.19.0-rc1+ #480
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> Workqueue: events refill_work
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x34/0x44
>  print_report.cold+0xbb/0x6ac
>  ? _printk+0xad/0xde
>  ? refill_work+0x43/0xd4
>  kasan_report+0xa8/0x130
>  ? refill_work+0x43/0xd4
>  refill_work+0x43/0xd4
>  process_one_work+0x43d/0x780
>  worker_thread+0x2a0/0x6f0
>  ? process_one_work+0x780/0x780
>  kthread+0x167/0x1a0
>  ? kthread_exit+0x50/0x50
>  ret_from_fork+0x22/0x30
>  </TASK>
> ...
> 
> Fixes: b2baed69e605c ("virtio_net: set/cancel work on ndo_open/ndo_stop")
> Signed-off-by: Jason Wang <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

> ---
> Changes since V4:
> - Tweak the variable name (using delayed_refill)
> Changes since V3:
> - rebase to -net
> Changes since V2:
> - use spin_unlock()/lock_bh() in open/stop to synchronize with bh
> Changes since V1:
> - Tweak the changelog
> ---
>  drivers/net/virtio_net.c | 35 +++++++++++++++++++++++++++++++++--
>  1 file changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 356cf8dd4164..b9ac4431becb 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -245,6 +245,12 @@ struct virtnet_info {
>  	/* Work struct for refilling if we run low on memory. */

let's update this comment to "for delayed refilling" for consistency

>  	struct delayed_work refill;
>  
> +	/* Is delayed refill enabled? */
> +	bool delayed_refill_enabled;

I would keep the name refill_enabled, refill refers to the field "refill"
above.

> +
> +	/* The lock to synchronize the access to delayed_refill_enabled */

add:

... and to refill

> +	spinlock_t refill_lock;
> +
>  	/* Work struct for config space updates */
>  	struct work_struct config_work;
>  
> @@ -348,6 +354,20 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
>  	return p;
>  }
>  
> +static void enable_delayed_refill(struct virtnet_info *vi)
> +{
> +	spin_lock_bh(&vi->refill_lock);
> +	vi->delayed_refill_enabled = true;
> +	spin_unlock_bh(&vi->refill_lock);
> +}
> +
> +static void disable_delayed_refill(struct virtnet_info *vi)
> +{
> +	spin_lock_bh(&vi->refill_lock);
> +	vi->delayed_refill_enabled = false;
> +	spin_unlock_bh(&vi->refill_lock);
> +}
> +
>  static void virtqueue_napi_schedule(struct napi_struct *napi,
>  				    struct virtqueue *vq)
>  {
> @@ -1527,8 +1547,12 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  	}
>  
>  	if (rq->vq->num_free > min((unsigned int)budget, virtqueue_get_vring_size(rq->vq)) / 2) {
> -		if (!try_fill_recv(vi, rq, GFP_ATOMIC))
> -			schedule_delayed_work(&vi->refill, 0);
> +		if (!try_fill_recv(vi, rq, GFP_ATOMIC)) {
> +			spin_lock(&vi->refill_lock);
> +			if (vi->delayed_refill_enabled)
> +				schedule_delayed_work(&vi->refill, 0);
> +			spin_unlock(&vi->refill_lock);
> +		}
>  	}
>  
>  	u64_stats_update_begin(&rq->stats.syncp);
> @@ -1651,6 +1675,8 @@ static int virtnet_open(struct net_device *dev)
>  	struct virtnet_info *vi = netdev_priv(dev);
>  	int i, err;
>  
> +	enable_delayed_refill(vi);
> +
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
>  		if (i < vi->curr_queue_pairs)
>  			/* Make sure we have some buffers: if oom use wq. */
> @@ -2033,6 +2059,8 @@ static int virtnet_close(struct net_device *dev)
>  	struct virtnet_info *vi = netdev_priv(dev);
>  	int i;
>  
> +	/* Make sure NAPI doesn't schedule refill work */
> +	disable_delayed_refill(vi);
>  	/* Make sure refill_work doesn't re-enable napi! */
>  	cancel_delayed_work_sync(&vi->refill);
>  
> @@ -2792,6 +2820,8 @@ static int virtnet_restore_up(struct virtio_device *vdev)
>  
>  	virtio_device_ready(vdev);
>  
> +	enable_delayed_refill(vi);
> +
>  	if (netif_running(vi->dev)) {
>  		err = virtnet_open(vi->dev);
>  		if (err)
> @@ -3535,6 +3565,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	vdev->priv = vi;
>  
>  	INIT_WORK(&vi->config_work, virtnet_config_changed_work);
> +	spin_lock_init(&vi->refill_lock);
>  
>  	/* If we can receive ANY GSO packets, we must allocate large ones. */
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
> -- 
> 2.25.1