RE: [PATCH v5 1/2] drm/hyperv: validate resolution_count and fix WIN8 fallback

[Michael Kelley]

From: Berkant Koc <me@xxxxxxxxxx> Sent: Tuesday, May 19, 2026 1:08 PM
> 
> A SYNTHVID_RESOLUTION_RESPONSE with resolution_count > 64 walks past
> the supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT] array in the
> parse loop. Bound resolution_count against the array size, folded
> into the existing zero-check.
> 
> When the WIN10 resolution probe fails, the caller in
> hyperv_connect_vsp() left hv->screen_*_max / preferred_* unpopulated,
> which sets mode_config.max_width / max_height to 0 and makes
> drm_internal_framebuffer_create() reject every userspace framebuffer
> with -EINVAL. The pre-WIN10 branch had the same gap for
> preferred_width / preferred_height. Use a single post-probe fallback
> guarded by screen_width_max == 0 so both paths converge on the WIN8
> defaults.
> 
> Signed-off-by: Berkant Koc <me@xxxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-7 berkoc-pipeline
> Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
> Cc: stable@xxxxxxxxxxxxxxx # 5.14+
> Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

I ran a basic smoke-test on my local Hyper-V instance. I can
confirm that no error messages or failure were generated
in the "good" case where the resolution_count from Hyper-V is
valid. I did not simulate a bogus resolution_count and ensure
it is detected.

Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>  

> ---
>  drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> index 051ecc526..c3d0ff229 100644
> --- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
> @@ -391,8 +391,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
>  		return -ETIMEDOUT;
>  	}
> 
> -	if (msg->resolution_resp.resolution_count == 0) {
> -		drm_err(dev, "No supported resolutions\n");
> +	if (msg->resolution_resp.resolution_count == 0 ||
> +	    msg->resolution_resp.resolution_count >
> +	    SYNTHVID_MAX_RESOLUTION_COUNT) {
> +		drm_err(dev, "Invalid resolution count: %d\n",
> +			msg->resolution_resp.resolution_count);
>  		return -ENODEV;
>  	}
> 
> @@ -508,9 +511,13 @@ int hyperv_connect_vsp(struct hv_device *hdev)
>  		ret = hyperv_get_supported_resolution(hdev);
>  		if (ret)
>  			drm_err(dev, "Failed to get supported resolution from host, use default\n");
> -	} else {
> +	}
> +
> +	if (!hv->screen_width_max) {
>  		hv->screen_width_max = SYNTHVID_WIDTH_WIN8;
>  		hv->screen_height_max = SYNTHVID_HEIGHT_WIN8;
> +		hv->preferred_width = SYNTHVID_WIDTH_WIN8;
> +		hv->preferred_height = SYNTHVID_HEIGHT_WIN8;
>  	}
> 
>  	hv->mmio_megabytes = hdev->channel->offermsg.offer.mmio_megabytes;
> --
> 2.47.3
>