linux-kernel - Re: [PATCH v2] drm/virtio: Fix handling CONFIG_DRM_VIRTIO_GPU

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Date:   Mon, 6 Mar 2023 15:02:04 +0000
From:   Emil Velikov <emil.velikov@...labora.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>, Rob Clark <robdclark@...il.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>,
        Ryan Neph <ryanneph@...omium.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Javier Martinez Canillas <javierm@...hat.com>,
        David Airlie <airlied@...hat.com>, kernel@...labora.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2] drm/virtio: Fix handling CONFIG_DRM_VIRTIO_GPU_KMS
 option

On 2023/03/06, Dmitry Osipenko wrote:
> VirtIO-GPU got a new config option for disabling KMS. There were two
> problems left unnoticed during review when the new option was added:
> 
> 1. The IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS) check in the code was
> inverted, hence KMS was disabled when it should be enabled and vice versa.
> 
> 2. The disabled KMS crashed kernel with a NULL dereference in
> drm_kms_helper_hotplug_event(), which shall not be invoked with a
> disabled KMS.
> 
> Fix the inverted config option check in the code and skip handling the
> VIRTIO_GPU_EVENT_DISPLAY sent by host when KMS is disabled in guest to fix
> the crash.
> 
> Fixes: 72122c69d717 ("drm/virtio: Add option to disable KMS support")
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
> 
> Changelog:
> 
> v2: - Moved the "has_edid" under the "num_scanouts" condition, like was
>       suggested by Gerd Hoffmann.
> 

Hi Dmitry, I think there's more than one piece like that in the driver.

>  drivers/gpu/drm/virtio/virtgpu_kms.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
> index 874ad6c2621a..15f2519988e7 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
> @@ -43,11 +43,13 @@ static void virtio_gpu_config_changed_work_func(struct work_struct *work)
>  	virtio_cread_le(vgdev->vdev, struct virtio_gpu_config,
>  			events_read, &events_read);
>  	if (events_read & VIRTIO_GPU_EVENT_DISPLAY) {
> -		if (vgdev->has_edid)
> -			virtio_gpu_cmd_get_edids(vgdev);
> -		virtio_gpu_cmd_get_display_info(vgdev);
> -		virtio_gpu_notify(vgdev);
> -		drm_helper_hpd_irq_event(vgdev->ddev);
> +		if (vgdev->num_scanouts) {
> +			if (vgdev->has_edid)
> +				virtio_gpu_cmd_get_edids(vgdev);

Worth doing the same thing in virtio_gpu_init()? Aka move the has_edid
&& get_edids within the num_scanouts if block.

HTH
Emil