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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Wed, 6 Nov 2019 13:10:56 +0100
Message-ID: <CAAeHK+zKShqnZ=R8KQvVjsfOkAGrWW5jbsXRUnuEY8k4XN3+Fw@mail.gmail.com>
Subject: Re: [PATCH] usb: appledisplay: fix use-after-free in bl_get_brightness
To:     Phong Tran <tranmanphong@gmail.com>
Cc:     syzbot+495dab1f175edc9c2f13@syzkaller.appspotmail.com, 2pi@mok.nu,
        alex.theissen@me.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        linux-kernel-mentees@lists.linuxfoundation.org
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On Wed, Nov 6, 2019 at 12:37 AM Phong Tran <tranmanphong@gmail.com> wrote:
>
> In context of USB disconnect, the delaywork trigger and calling
> appledisplay_bl_get_brightness() and the msgdata was freed.
>
> add the checking return value of usb_control_msg() and only update the
> data while the retval is valid.
>
> Reported-by: syzbot+495dab1f175edc9c2f13@syzkaller.appspotmail.com
> Reported-and-tested-by:
> syzbot+495dab1f175edc9c2f13@syzkaller.appspotmail.com
>
> https://groups.google.com/d/msg/syzkaller-bugs/dRmkh2UYusY/l2a6Mg3FAQAJ

Hi Phong,

FYI, when testing patches with the usb-fuzzer instance, you need to
provide the same kernel commit id as the one where the bug was
triggered. Please see here for details:

>
> Signed-off-by: Phong Tran <tranmanphong@gmail.com>
> ---
>  drivers/usb/misc/appledisplay.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/usb/misc/appledisplay.c b/drivers/usb/misc/appledisplay.c
> index ac92725458b5..3e3dfa5a3954 100644
> --- a/drivers/usb/misc/appledisplay.c
> +++ b/drivers/usb/misc/appledisplay.c
> @@ -164,7 +164,8 @@ static int appledisplay_bl_get_brightness(struct backlight_device *bd)
>                 0,
>                 pdata->msgdata, 2,
>                 ACD_USB_TIMEOUT);
> -       brightness = pdata->msgdata[1];
> +       if (retval >= 0)
> +               brightness = pdata->msgdata[1];
>         mutex_unlock(&pdata->sysfslock);
>
>         if (retval < 0)
> --
> 2.20.1
>