linux-kernel - Re: [PATCH v2] drm: udl: Destroy framebuffer only if it was initialized

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Date:   Tue, 29 May 2018 08:46:16 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Emil Lundmark <lndmrk@...omium.org>
Cc:     dri-devel@...ts.freedesktop.org, Dave Airlie <airlied@...hat.com>,
        Sean Paul <seanpaul@...omium.org>,
        linux-kernel@...r.kernel.org, Daniel Vetter <daniel@...ll.ch>,
        Stéphane Marchesin 
        <stephane.marchesin@...il.com>
Subject: Re: [PATCH v2] drm: udl: Destroy framebuffer only if it was
 initialized

On Mon, May 28, 2018 at 04:27:11PM +0200, Emil Lundmark wrote:
> This fixes a NULL pointer dereference that can happen if the UDL
> driver is unloaded before the framebuffer is initialized. This can
> happen e.g. if the USB device is unplugged right after it was plugged
> in.
> 
> As explained by Stéphane Marchesin:
> 
> It happens when fbdev is disabled (which is the case for Chrome OS).
> Even though intialization of the fbdev part is optional (it's done in
> udlfb_create which is the callback for fb_probe()), the teardown isn't
> optional (udl_driver_unload -> udl_fbdev_cleanup ->
> udl_fbdev_destroy).
> 
> Note that udl_fbdev_cleanup *tries* to be conditional (you can see it
> does if (!udl->fbdev)) but that doesn't work, because udl->fbdev is
> always set during udl_fbdev_init.
> 
> Suggested-by: Sean Paul <seanpaul@...omium.org>
> Signed-off-by: Emil Lundmark <lndmrk@...omium.org>

You lost the r-b from Sean when resending, I'll leave that to Sean to
readd when he merges. Anyway, lgtm now with the more detailed explanation.

Acked-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---
> Changes in v2:
> - Updated commit message with explanation from Stéphane Marchesin
> 
>  drivers/gpu/drm/udl/udl_fb.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/udl/udl_fb.c b/drivers/gpu/drm/udl/udl_fb.c
> index 2ebdc6d5a76e..5754e37f741b 100644
> --- a/drivers/gpu/drm/udl/udl_fb.c
> +++ b/drivers/gpu/drm/udl/udl_fb.c
> @@ -426,9 +426,11 @@ static void udl_fbdev_destroy(struct drm_device *dev,
>  {
>  	drm_fb_helper_unregister_fbi(&ufbdev->helper);
>  	drm_fb_helper_fini(&ufbdev->helper);
> -	drm_framebuffer_unregister_private(&ufbdev->ufb.base);
> -	drm_framebuffer_cleanup(&ufbdev->ufb.base);
> -	drm_gem_object_put_unlocked(&ufbdev->ufb.obj->base);
> +	if (ufbdev->ufb.obj) {
> +		drm_framebuffer_unregister_private(&ufbdev->ufb.base);
> +		drm_framebuffer_cleanup(&ufbdev->ufb.base);
> +		drm_gem_object_put_unlocked(&ufbdev->ufb.obj->base);
> +	}
>  }
>  
>  int udl_fbdev_init(struct drm_device *dev)
> -- 
> 2.17.0.921.gf22659ad46-goog
> 

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