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

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Message-ID: <6p7pi6mf3db3gp3xqarap4uzrgwlzqiz7wgg5kn2ep7hvrw5pg@wxowhbw4e7w7>
Date:   Tue, 30 May 2023 18:17:00 +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 11:09:09AM -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