Re: [PATCH] drm/modes: reject out-of-range GTF blanking duty cycle

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Medium] Unchecked `vrefresh` parameter from kernel command line can cause multiple divide-by-zero panics.
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--- Patch [1]: [PATCH] drm/modes: reject out-of-range GTF blanking duty cycle ---
commit c78fc29b3d453bf33d5cd88746572b5f1e9a5c60
Author: Xiang Mei <xmei5@asu.edu>

drm/modes: reject out-of-range GTF blanking duty cycle

This commit rejects GTF blanking duty cycles of 100% or more to prevent a
divide-by-zero error that occurs when the divisor (100000 - ideal_duty_cycle)
evaluates to 0.

> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index e1eed13a8e94f..291037706b587 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -966,6 +966,10 @@ drm_gtf_mode_complex(struct drm_device *dev, int hdisplay, int vdisplay,
>  	/* 18.Find the ideal blanking duty cycle from blanking duty cycle */
>  	ideal_duty_cycle = GTF_C_PRIME * 1000 -
>  				(GTF_M_PRIME * 1000000 / hfreq_est);

[Severity: Medium]
This is a pre-existing issue, but while fixing divide-by-zero crashes here,
could an unvalidated vrefresh parameter cause other divide-by-zero panics
in this function?

For example, if an administrator provides a kernel command line parameter
with a refresh rate of @0 (e.g., video=1024x768@0), vfieldrate_rqd
evaluates to 0. This would cause a panic earlier in drm_gtf_mode_complex():

    hperiod = 1000000 / (vfieldrate_rqd * vtotal_lines);

Also, if the refresh rate is @1818, tmp1 evaluates to 0:

    tmp1 = (1000000  - MIN_VSYNC_PLUS_BP * vfieldrate_rqd) / 500;
    // (1000000 - 550 * 1818) / 500 = 0

This leads to another direct division by zero:

    hfreq_est = (tmp2 * 1000 * vfieldrate_rqd) / tmp1;

And if hfreq_est evaluates to 0, it causes a third divide-by-zero on the
ideal_duty_cycle assignment above.

Should drm_gtf_mode_complex() validate the vrefresh parameter before
performing these calculations?

> +	if (ideal_duty_cycle >= 100000) {
> +		drm_mode_destroy(dev, drm_mode);
> +		return NULL;
> +	}
>  	/* 19.Find the number of pixels in the blanking time to the nearest
>  	 * double character cell: */
>  	hblank = total_active_pixels * ideal_duty_cycle /

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260711230728.2250485-1-xmei5@asu.edu?part=1