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

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Date:   Tue, 5 Oct 2021 15:33:42 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
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 03:04:07AM -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.

> 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?

thanks,

greg k-h