linux-kernel - Re: [PATCH] drm/msm/dpu: reorder pointer operations after sanity checks to avoid NULL deref

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Message-ID: <5x4bmy52gxc6y3m6qswqxoxvjsk52zz7lvjpudqgr4d2hwaz4b@bdf2dzgji7gf>
Date: Sun, 9 Mar 2025 10:16:31 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Qasim Ijaz <qasdev00@...il.com>
Cc: robdclark@...il.com, quic_abhinavk@...cinc.com, sean@...rly.run, 
	marijn.suijten@...ainline.org, airlied@...il.com, simona@...ll.ch, quic_jesszhan@...cinc.com, 
	linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/dpu: reorder pointer operations after sanity
 checks to avoid NULL deref

On Sat, Mar 08, 2025 at 02:48:39PM +0000, Qasim Ijaz wrote:
> _dpu_encoder_trigger_start dereferences "struct dpu_encoder_phys *phys" 
> before the sanity checks which can lead to a NULL pointer dereference if 
> phys is NULL. 
> 
> Fix this by reordering the dereference after the sanity checks.
> 
> Signed-off-by: Qasim Ijaz <qasdev00@...il.com>
> Fixes: 8144d17a81d9 ("drm/msm/dpu: Skip trigger flush and start for CWB")

Your SoB should be the last tag. Fixes comes before it.

> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> index 0eed93a4d056..ba8b2a163232 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> @@ -1667,8 +1667,6 @@ static void _dpu_encoder_trigger_flush(struct drm_encoder *drm_enc,
>   */
>  static void _dpu_encoder_trigger_start(struct dpu_encoder_phys *phys)
>  {
> -	struct dpu_encoder_virt *dpu_enc = to_dpu_encoder_virt(phys->parent);
> -
>  	if (!phys) {
>  		DPU_ERROR("invalid argument(s)\n");
>  		return;
> @@ -1679,6 +1677,8 @@ static void _dpu_encoder_trigger_start(struct dpu_encoder_phys *phys)
>  		return;
>  	}
>  
> +	struct dpu_encoder_virt *dpu_enc = to_dpu_encoder_virt(phys->parent);
> +

I'm not a fan of having variable defs in the middle of the code. Please
keep the def at the top and assign it here.

>  	if (phys->parent->encoder_type == DRM_MODE_ENCODER_VIRTUAL &&
>  	    dpu_enc->cwb_mask) {
>  		DPU_DEBUG("encoder %d CWB enabled, skipping\n", DRMID(phys->parent));
> -- 
> 2.39.5
> 

-- 
With best wishes
Dmitry