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

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Message-ID: <c87f0768-027b-c192-1baf-05273aac382b@oracle.com>
Date:   Tue, 30 May 2023 11:01:11 -0500
From:   Mike Christie <michael.christie@...cle.com>
To:     Stefano Garzarella <sgarzare@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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 5/30/23 10:58 AM, Mike Christie wrote:
> On 5/30/23 8:44 AM, Stefano Garzarella wrote:
>>
>> From a first glance, it looks like an issue when we call vhost_work_queue().
>> @Mike, does that ring any bells since you recently looked at that code?
> 
> I see the bug. needed to have set the dev->worker after setting worker->vtsk
> like below:
> 
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a92af08e7864..7bd95984a501 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -564,7 +564,6 @@ static int vhost_worker_create(struct vhost_dev *dev)
>  	if (!worker)
>  		return -ENOMEM;
>  
> -	dev->worker = worker;
>  	worker->kcov_handle = kcov_common_handle();
>  	init_llist_head(&worker->work_list);
>  	snprintf(name, sizeof(name), "vhost-%d", current->pid);
> @@ -576,6 +575,7 @@ static int vhost_worker_create(struct vhost_dev *dev)
>  	}
>  
>  	worker->vtsk = vtsk;

Shoot, oh wait, I think I needed a smp_wmb to always make sure worker->vtask
is set before dev->worker or vhost_work_queue could still end up seeing
dev->worker set before worker->vtsk right?

> +	dev->worker = worker;>  	vhost_task_start(vtsk);
>  	return 0;
>