linux-kernel - Re: [PATCH] vhost/vsock: don't check owner in vhost_vsock

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Message-ID: <20220222090550.5wokee4tfynr36jn@sgarzare-redhat>
Date:   Tue, 22 Feb 2022 10:05:50 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Anirudh Rayabharam <mail@...rudhrb.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Mike Christie <michael.christie@...cle.com>,
        Jason Wang <jasowang@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        kvm <kvm@...r.kernel.org>, Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] vhost/vsock: don't check owner in vhost_vsock_stop()
 while releasing

On Tue, Feb 22, 2022 at 01:06:12AM +0530, Anirudh Rayabharam wrote:
>On Mon, Feb 21, 2022 at 07:26:28PM +0100, Stefano Garzarella wrote:
>> On Mon, Feb 21, 2022 at 11:33:11PM +0530, Anirudh Rayabharam wrote:
>> > On Mon, Feb 21, 2022 at 05:44:20PM +0100, Stefano Garzarella wrote:
>> > > On Mon, Feb 21, 2022 at 09:44:39PM +0530, Anirudh Rayabharam wrote:
>> > > > On Mon, Feb 21, 2022 at 02:59:30PM +0100, Stefano Garzarella wrote:
>> > > > > On Mon, Feb 21, 2022 at 12:49 PM Stefano Garzarella <sgarzare@...hat.com> wrote:
>> > > > > >
>> > > > > > vhost_vsock_stop() calls vhost_dev_check_owner() to check the device
>> > > > > > ownership. It expects current->mm to be valid.
>> > > > > >
>> > > > > > vhost_vsock_stop() is also called by vhost_vsock_dev_release() when
>> > > > > > the user has not done close(), so when we are in do_exit(). In this
>> > > > > > case current->mm is invalid and we're releasing the device, so we
>> > > > > > should clean it anyway.
>> > > > > >
>> > > > > > Let's check the owner only when vhost_vsock_stop() is called
>> > > > > > by an ioctl.
>> > > > > >
>> > > > > > Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
>> > > > > > Cc: stable@...r.kernel.org
>> > > > > > Reported-by: syzbot+1e3ea63db39f2b4440e0@...kaller.appspotmail.com
>> > > > > > Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
>> > > > > > ---
>> > > > > >  drivers/vhost/vsock.c | 14 ++++++++------
>> > > > > >  1 file changed, 8 insertions(+), 6 deletions(-)
>> > > > >
>> > > > > Reported-and-tested-by: syzbot+0abd373e2e50d704db87@...kaller.appspotmail.com
>> > > >
>> > > > I don't think this patch fixes "INFO: task hung in vhost_work_dev_flush"
>> > > > even though syzbot says so. I am able to reproduce the issue locally
>> > > > even with this patch applied.
>> > >
>> > > Are you using the sysbot reproducer or another test?
>> > > In that case, can you share it?
>> >
>> > I am using the syzbot reproducer.
>> >
>> > >
>> > > From the stack trace it seemed to me that the worker accesses a zone that
>> > > has been cleaned (iotlb), so it is invalid and fails.
>> >
>> > Would the thread hang in that case? How?
>>
>> Looking at this log [1] it seems that the process is blocked on the
>> wait_for_completion() in vhost_work_dev_flush().
>>
>> Since we're not setting the backend to NULL to stop the worker, it's likely
>> that the worker will keep running, preventing the flush work from
>> completing.
>
>The log shows that the worker thread is stuck in iotlb_access_ok(). How
>will setting the backend to NULL stop it? During my debugging I found
>that the worker is stuck in this while loop:

Okay, looking at your new patch, now I see. If we enter in this loop 
before setting the backend to NULL and we have start = 0 and end = (u64) 
-1 , we should be there forever.

I'll remove that tag in v2, but the test might fail without this patch 
applied, because for now we don't stop workers correctly.

Thanks,
Stefano