Re: [PATCH] drm/cirrus: Fix NULL pointer dereference when registering the fbdev

[Eric W. Biederman]

Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> writes:

> cirrus_modeset_init() is initializing/registering the emulated fbdev
> and, since commit c61b93fe51b1 ("drm/atomic: Fix remaining places where
> !funcs->best_encoder is valid"), DRM internals can access/test some of
> the fields in mode_config->funcs as part of the fbdev registration
> process.
> Make sure dev->mode_config.funcs is properly set to avoid dereferencing
> a NULL pointer.

That fixes the issues I am seeing.

Tested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>
> Fixes: c61b93fe51b1 ("drm/atomic: Fix remaining places where !funcs->best_encoder is valid")
> ---
> Hi Dave,
>
> As discussed on IRC, I'm sending this patch in a proper format. That's
> probably better to wait for Eric's feeback before applying it though.

It is weird I didn't see either of your email messages directly only
through lkml.  Weird.

> Regards,
>
> Boris
> ---
>  drivers/gpu/drm/cirrus/cirrus_main.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/cirrus/cirrus_main.c b/drivers/gpu/drm/cirrus/cirrus_main.c
> index 80446e2d3ab6..76bcb43e7c06 100644
> --- a/drivers/gpu/drm/cirrus/cirrus_main.c
> +++ b/drivers/gpu/drm/cirrus/cirrus_main.c
> @@ -185,14 +185,23 @@ int cirrus_driver_load(struct drm_device *dev, unsigned long flags)
>  		goto out;
>  	}
>  
> +	/*
> +	 * cirrus_modeset_init() is initializing/registering the emulated fbdev
> +	 * and DRM internals can access/test some of the fields in
> +	 * mode_config->funcs as part of the fbdev registration process.
> +	 * Make sure dev->mode_config.funcs is properly set to avoid
> +	 * dereferencing a NULL pointer.
> +	 * FIXME: mode_config.funcs assignment should probably be done in
> +	 * cirrus_modeset_init() (that's a common pattern seen in other DRM
> +	 * drivers).
> +	 */
> +	dev->mode_config.funcs = &cirrus_mode_funcs;
>  	r = cirrus_modeset_init(cdev);
>  	if (r) {
>  		dev_err(&dev->pdev->dev, "Fatal error during modeset init: %d\n", r);
>  		goto out;
>  	}
>  
> -	dev->mode_config.funcs = (void *)&cirrus_mode_funcs;
> -
>  	return 0;
>  out:
>  	cirrus_driver_unload(dev);