# Re: Kernel panic with vhost-vdpa

- From: Jason Wang \<hidden\>
- Date: 2021-02-18 08:48:46 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2021/02/15817813
- Thread: https://ratatoskr.run/virtualization/2021/02/15817813/t.md

```text
Hi Gautam:

On 2021/2/15 9:01 下午, Gautam Dawar wrote:
>
> Hi Jason/Michael,
>
> I observed a kernel panic while testing vhost-vdpa with Xilinx adapters. Here are the details for your review:
>
> Problem statement:
>
> When qemu with vhost-vdpa netdevice is run for the first time, it works well. But after the VM is powered off, next qemu run causes kernel panic due to a NULL pointer dereference in irq_bypass_register_producer().
>
> Root cause analysis:
>
> When the VM is powered off, vhost_dev_stop() is invoked which in turn calls vhost_vdpa_reset_device() causing the irq_bypass producers to be unregistered.
>
> On the next run, when qemu opens the vhost device, the vhost_vdpa_open() file operation calls vhost_dev_init(). Here, call_ctx->producer memory is cleared in vhost_vring_call_reset().
>
> Further, when the virtqueues are initialized by vhost_virtqueue_init(), vhost_vdpa_setup_vq_irq() again registers the irq_bypass producer for each virtqueue. As the node member of struct irq_bypass_producer is also initialized to zero, traversal on the producers list causes crash due to NULL pointer dereference.
>

Thanks a lot for reporting this issue.


> Fix details:
>
> I think that this issue can be fixed by invoking vhost_vdpa_setup_vq_irq() only when vhost_vdpa_set_status() includes VIRTIO_CONFIG_S_DRIVER_OK in the new status value. This way, there won’t be any stale nodes in the irqbypass  module’s producers list which are reset in vhost_vring_call_reset().
>
> Patch:
>
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c index 62a9bb0efc55..fdad94e2fbf9 100644
>
> --- a/drivers/vhost/vdpa.c
>
> +++ b/drivers/vhost/vdpa.c
>
> @@ -409,7 +409,6 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
>
> cb.private = NULL;
>
> }
>
> ops->set_vq_cb(vdpa, idx, &cb);
>
> - vhost_vdpa_setup_vq_irq(v, idx);
>
> break;
>
> case VHOST_SET_VRING_NUM:
>
> We can also track this issue in Bugzilla ticket 21171 (https://bugzilla.kernel.org/show_bug.cgi?id=211711 <https://bugzilla.kernel.org/show_bug.cgi?id=211711>) and the complete patch is attached with this email.
>

So vhost supports to remove or switch eventfd through vhost_vdpa_vring_ioctl(). So if userspace want to switch to another eventfd, we should re-do the register and unregister.

I think we need to deal this issue in another way. Can we check whether or not the producer is initialized before?

Thanks


> Regards,
>
> Gautam Dawar
>
```