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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat, 28 Mar 2020 17:31:34 +0100
Message-ID: <CAKMK7uF2mipUSr-XRESrRJ8JdOZCekNTCVsDPW5hNp-mWwPj6Q@mail.gmail.com>
Subject: Re: [PATCH] fbcon: fix null-ptr-deref in fbcon_switch
To:     Qiujun Huang <hqjagain@gmail.com>
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sam Ravnborg <sam@ravnborg.org>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        ghalat@redhat.com, dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sat, Mar 28, 2020 at 4:15 PM Qiujun Huang <hqjagain@gmail.com> wrote:
> Add check for vc_cons[logo_shown].d, as it can be released by
> vt_ioctl(VT_DISALLOCATE).

Can you pls link to the syzbot report and distill the essence of the
crash/issue here in the commit message? As-is a bit unclear what's
going on. Patch itself looks correct.

Thanks, Daniel

> Reported-by: syzbot+732528bae351682f1f27@syzkaller.appspotmail.com
> Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
> ---
>  drivers/video/fbdev/core/fbcon.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index bb6ae995c2e5..7ee0f7b55829 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2254,7 +2254,7 @@ static int fbcon_switch(struct vc_data *vc)
>                 fbcon_update_softback(vc);
>         }
>
> -       if (logo_shown >= 0) {
> +       if (logo_shown >= 0 && vc_cons_allocated(logo_shown)) {
>                 struct vc_data *conp2 = vc_cons[logo_shown].d;
>
>                 if (conp2->vc_top == logo_lines
> @@ -2852,7 +2852,7 @@ static void fbcon_scrolldelta(struct vc_data *vc, int lines)
>                         return;
>                 if (vc->vc_mode != KD_TEXT || !lines)
>                         return;
> -               if (logo_shown >= 0) {
> +               if (logo_shown >= 0 && vc_cons_allocated(logo_shown)) {
>                         struct vc_data *conp2 = vc_cons[logo_shown].d;
>
>                         if (conp2->vc_top == logo_lines
> --
> 2.17.1
>


--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch