Re: [syzbot] [kvm?] [net?] [virt?] general protection fault in vhost_work_queue

[Stefano Garzarella]

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.

Should we protect dev->worker() with an RCU to be safe?

> One question about the behavior before my commit though and what we want in
> the end going forward. Before that patch we would just drop work if
> vhost_work_queue was called before VHOST_SET_OWNER. Was that correct/expected?

I think so, since we ask the guest to call VHOST_SET_OWNER, before any
other command.

> The call to vhost_work_queue in vhost_vsock_start was only seeing the
> works queued after VHOST_SET_OWNER. Did you want works queued before that?

Yes, for example if an application in the host has tried to connect and
is waiting for a timeout, we already have work queued up to flush as
soon as we start the device. (See commit 0b841030625c ("vhost: vsock:
kick send_pkt worker once device is started")).

Thanks,
Stefano