linux-kernel - Re: [PATCH] drm/msm/adreno/a5xx_debugfs: fix potential NULL pointer dereference

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Date:   Fri, 2 Feb 2018 09:30:06 -0700
From:   Jordan Crouse <jcrouse@...eaurora.org>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Cc:     Rob Clark <robdclark@...il.com>, David Airlie <airlied@...ux.ie>,
        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/adreno/a5xx_debugfs: fix potential NULL pointer
 dereference

On Fri, Feb 02, 2018 at 06:32:23AM -0600, Gustavo A. R. Silva wrote:
> _minor_ is being dereferenced before it is null checked, hence there
> is a potential null pointer dereference. Fix this by moving the pointer
> dereference after _minor_ has been null checked.
> 
> Fixes: 024ad8df763f ("drm/msm: add a5xx specific debugfs")
> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
> ---
> 
> I wonder if a better solution for this would be to WARN_ON in case _minor_
> happens to be NULL and return -EINVAL, instead of just returning zero.
> 
> Something like:
> 
> struct drm_device *dev;
> 
> if (WARN_ON(!minor)
> 	return -EINVAL;
> 
> dev = minor->dev;
> 
> What do you think?

In my opinion everything in debugfs is optional. I'm not sure if it is even
possible for dev->primary, dev->render or dev->control to be NULL from the DRM
core but if so I think the failure should be silent.

Jordan
> 
>  drivers/gpu/drm/msm/adreno/a5xx_debugfs.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c b/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c
> index 6b27941..059ec7d 100644
> --- a/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c
> +++ b/drivers/gpu/drm/msm/adreno/a5xx_debugfs.c
> @@ -159,13 +159,15 @@ DEFINE_SIMPLE_ATTRIBUTE(reset_fops, NULL, reset_set, "%llx\n");
>  
>  int a5xx_debugfs_init(struct msm_gpu *gpu, struct drm_minor *minor)
>  {
> -	struct drm_device *dev = minor->dev;
> +	struct drm_device *dev;
>  	struct dentry *ent;
>  	int ret;
>  
>  	if (!minor)
>  		return 0;
>  
> +	dev = minor->dev;
> +
>  	ret = drm_debugfs_create_files(a5xx_debugfs_list,
>  			ARRAY_SIZE(a5xx_debugfs_list),
>  			minor->debugfs_root, minor);
> -- 
> 2.7.4
> 
> --
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