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

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Message-ID: <43f67549-fe4d-e3ca-fbb0-33bea6e2b534@oracle.com>
Date:   Wed, 31 May 2023 10:15:06 -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 5/31/23 2:27 AM, Stefano Garzarella wrote:
> On Tue, May 30, 2023 at 6:30 PM <michael.christie@...cle.com> wrote:
>>
>> On 5/30/23 11:17 AM, Stefano Garzarella wrote:
>>> On Tue, May 30, 2023 at 11:09:09AM -0500, Mike Christie wrote:
>>>> On 5/30/23 11:00 AM, Stefano Garzarella wrote:
>>>>> I think it is partially related to commit 6e890c5d5021 ("vhost: use
>>>>> vhost_tasks for worker threads") and commit 1a5f8090c6de ("vhost: move
>>>>> worker thread fields to new struct"). Maybe that commits just
>>>>> highlighted the issue and it was already existing.
>>>>
>>>> See my mail about the crash. Agree with your analysis about worker->vtsk
>>>> not being set yet. It's a bug from my commit where I should have not set
>>>> it so early or I should be checking for
>>>>
>>>> if (dev->worker && worker->vtsk)
>>>>
>>>> instead of
>>>>
>>>> if (dev->worker)
>>>
>>> Yes, though, in my opinion the problem may persist depending on how the
>>> instructions are reordered.
>>
>> Ah ok.
>>
>>>
>>> Should we protect dev->worker() with an RCU to be safe?
>>
>> For those multiple worker patchsets Jason had asked me about supporting
>> where we don't have a worker while we are swapping workers around. To do
>> that I had added rcu around the dev->worker. I removed it in later patchsets
>> because I didn't think anyone would use it.
>>
>> rcu would work for your case and for what Jason had requested.
> 
> Yeah, so you already have some patches?
> 
> Do you want to send it to solve this problem?
> 

Yeah, I'll break them out and send them later today when I can retest
rebased patches.