netdev - Re: [syzbot] [kvm?] [net?] [virt?] general protection fault in vhost_work

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Date: Thu, 1 Jun 2023 11:33:09 -0500
From: Mike Christie <michael.christie@...cle.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>,
        syzbot <syzbot+d0d442c22fa8db45ff0e@...kaller.appspotmail.com>,
        jasowang@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        virtualization@...ts.linux-foundation.org, stefanha@...hat.com
Subject: Re: [syzbot] [kvm?] [net?] [virt?] general protection fault in
 vhost_work_queue

On 6/1/23 2:47 AM, Stefano Garzarella wrote:
>>
>> static void vhost_worker_free(struct vhost_dev *dev)
>> {
>> -    struct vhost_worker *worker = dev->worker;
>> +    struct vhost_task *vtsk = READ_ONCE(dev->worker.vtsk);
>>
>> -    if (!worker)
>> +    if (!vtsk)
>>         return;
>>
>> -    dev->worker = NULL;
>> -    WARN_ON(!llist_empty(&worker->work_list));
>> -    vhost_task_stop(worker->vtsk);
>> -    kfree(worker);
>> +    vhost_task_stop(vtsk);
>> +    WARN_ON(!llist_empty(&dev->worker.work_list));
>> +    WRITE_ONCE(dev->worker.vtsk, NULL);
> 
> The patch LGTM, I just wonder if we should set dev->worker to zero here,

We might want to just set kcov_handle to zero for now.

In 6.3 and older, I think we could do:

1. vhost_dev_set_owner could successfully set dev->worker.
2. vhost_transport_send_pkt runs vhost_work_queue and sees worker
is set and adds the vhost_work to the work_list.
3. vhost_dev_set_owner fails in vhost_attach_cgroups, so we stop
the worker before the work can be run and set worker to NULL.
4. We clear kcov_handle and return.

We leave the work on the work_list.

5. Userspace can then retry vhost_dev_set_owner. If that works, then the
work gets executed ok eventually.

OR

Userspace can just close the device. vhost_vsock_dev_release would
eventually call vhost_dev_cleanup (vhost_dev_flush won't see a worker
so will just return), and that will hit the WARN_ON but we would
proceed ok.

If I do a memset of the worker, then if userspace were to retry
VHOST_SET_OWNER, we would lose the queued work since the work_list would
get zero'd. I think it's unlikely this ever happens, but you know best
so let me know if this a real issue.