linux-kernel - Re: [PATCH] virtio_console: break out of buf poll on remove

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Message-ID: <20220114164549-mutt-send-email-mst@kernel.org>
Date:   Fri, 14 Jan 2022 16:48:57 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, Amit Shah <amit@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_console: break out of buf poll on remove

On Tue, Oct 05, 2021 at 03:33:42PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Oct 05, 2021 at 03:04:07AM -0400, Michael S. Tsirkin wrote:
> > A common pattern for device reset is currently:
> > vdev->config->reset(vdev);
> > .. cleanup ..
> > 
> > reset prevents new interrupts from arriving and waits for interrupt
> > handlers to finish.
> > 
> > However if - as is common - the handler queues a work request which is
> > flushed during the cleanup stage, we have code adding buffers / trying
> > to get buffers while device is reset. Not good.
> > 
> > This was reproduced by running
> > 	modprobe virtio_console
> > 	modprobe -r virtio_console
> > in a loop.
> 
> That's a pathological case that is not "in the field" except by people
> who want to abuse the system as root.  And they can do much worse things
> than that.
> 
> > Fixing this comprehensively needs some thought, and new APIs.
> > Let's at least handle the specific case of virtio_console
> > removal that was reported in the field.
> 
> Let's fix this correctly, don't just hack it up now.

Well I poked at it some more, and things are not as bad
as I thought. It's mostly just console and possibly virtio-mem.
Well and virtio-bt has a completely borken cleanup that
does not even bother to reset the device, but that's
a separate issue, discussing it with the maintainer.

So I wrote some patches to document the requirements better, added a
wrapper for reset and generally cleaned the API up a bit, and added a
patch for mem, but generally I no longer think we need a major API
change.


> > Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1786239
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  drivers/char/virtio_console.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> > index 7eaf303a7a86..c852ce0b4d56 100644
> > --- a/drivers/char/virtio_console.c
> > +++ b/drivers/char/virtio_console.c
> > @@ -1956,6 +1956,12 @@ static void virtcons_remove(struct virtio_device *vdev)
> >  	list_del(&portdev->list);
> >  	spin_unlock_irq(&pdrvdata_lock);
> >  
> > +	/* Device is going away, exit any polling for buffers */
> > +	virtio_break_device(vdev);
> > +	if (use_multiport(portdev))
> > +		flush_work(&portdev->control_work);
> > +	else
> > +		flush_work(&portdev->config_work);
> >  	/* Disable interrupts for vqs */
> 
> newline before comment?

sure

> thanks,
> 
> greg k-h