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From:   Sean Paul <seanpaul@chromium.org>
Date:   Mon, 10 Sep 2018 10:23:53 -0400
Message-ID: <CAOw6vb+3Y5bL+P8VqFuCmkpPYyySBtMzXc2dUOS88Hqz_8-B6g@mail.gmail.com>
Subject: Re: [PATCH v2] drm: udl: Destroy framebuffer only if it was initialized
To:     lndmrk@chromium.org
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        Dave Airlie <airlied@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        =?UTF-8?Q?St=C3=A9phane_Marchesin?= <stephane.marchesin@gmail.com>
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On Mon, May 28, 2018 at 10:27 AM Emil Lundmark <lndmrk@chromium.org> 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 St=C3=A9phane 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@chromium.org>
> Signed-off-by: Emil Lundmark <lndmrk@chromium.org>

Many apologies, Emil, I completely dropped the ball on this one. I've
just applied it to -misc-fixes.

Sean


> ---
> Changes in v2:
> - Updated commit message with explanation from St=C3=A9phane 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
>