[PATCH] drm/fb-helper: Fix drm_fb_helper_firmware_config() NULL pointer deref

[Daniel Vetter]

On Tue, Apr 23, 2019 at 04:53:53PM +0200, Noralf Trønnes wrote:
> Non-atomic drivers like ast doesn't have connector->state set resulting
> in a NULL pointer deref:
> 
> [   29.609593] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
> [   29.609619] Call Trace:
> [   29.609630]  ? drm_helper_probe_single_connector_modes+0x27f/0x680
> [   29.609640]  drm_setup_crtcs+0x431/0xd80 [drm_kms_helper]
> [   29.753065]  __drm_fb_helper_initial_config_and_unlock+0x6f/0x6a0
> [   29.753160]  ? drm_modeset_unlock_all+0x31/0x50 [drm]
> [   29.765758]  ast_fbdev_init+0xa8/0xc0 [ast]
> [   29.765762]  ast_driver_load.cold.7+0x2b3/0xe11 [ast]
> [   29.765775]  drm_dev_register+0x111/0x150 [drm]
> 
> Fix by bailing out if the driver does not support atomic modesetting.
> 
> Fixes: 09ded8af57bc ("drm/i915/fbdev: Move intel_fb_initial_config() to fbdev helper")
> Reported-by: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 2339f0f8f5a8..899c2eca26d1 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -2588,6 +2588,9 @@ static bool drm_fb_helper_firmware_config(struct drm_fb_helper *fb_helper,
>  	int num_connectors_detected = 0;
>  	struct drm_modeset_acquire_ctx ctx;
>  
> +	if (!drm_drv_uses_atomic_modeset(dev))
> +		return false;

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

I think for merging we're already past feature freeze, but
drm-misc-next-fixes hasn't been rolled forward yet. I think you need to
wait for drm-misc maintainers to do that (I pinged them already), and then
put this one there.
-Daniel

> +
>  	save_enabled = kcalloc(count, sizeof(bool), GFP_KERNEL);
>  	if (!save_enabled)
>  		return false;
> -- 
> 2.20.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch