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

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Date:   Tue, 30 May 2023 18:11:58 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Mike Christie <michael.christie@...cle.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 Tue, May 30, 2023 at 11:01:11AM -0500, Mike Christie wrote:
>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

Yes, I came to the same conclusion (see my email sent at the same time
:-).

>> 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?

But should we pair smp_wmb() with an smp_rmb() wherever we check 
dev->worker?

Thanks,
Stefano