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Subject: Re: [PATCH] media: usbvision: Fix a use after free in v4l2_release()
To:     Dan Carpenter <dan.carpenter@oracle.com>,
        syzbot <syzbot+75287f75e2fedd69d680@syzkaller.appspotmail.com>,
        Hillf Danton <hdanton@sina.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Allison Randal <allison@lohutok.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Souptick Joarder <jrdr.linux@gmail.com>, andreyknvl@google.com,
        bnvandana@gmail.com, laurent.pinchart@ideasonboard.com,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linux-usb@vger.kernel.org, mchehab@kernel.org,
        syzkaller-bugs@googlegroups.com
References: <20200124141356.365bgzg2lp3tjedm@kili.mountain>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <d8663b81-e920-3e1d-11d0-f636ea52c6ef@xs4all.nl>
Date:   Fri, 14 Feb 2020 11:06:36 +0100
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On 1/24/20 3:13 PM, Dan Carpenter wrote:
> Syzbot triggered a use after free in v5.5-rc6:
> 
> BUG: KASAN: use-after-free in v4l2_release+0x2f1/0x390 drivers/media/v4l2-core/v4l2-dev.c:459
> 
> Allocated by task 94:
>  usbvision_alloc drivers/media/usb/usbvision/usbvision-video.c:1315 [inline]
>  usbvision_probe.cold+0x5c5/0x1f21 drivers/media/usb/usbvision/usbvision-video.c:1469
> 
> Freed by task 1913:
>  kfree+0xd5/0x300 mm/slub.c:3957
>  usbvision_release+0x181/0x1c0 drivers/media/usb/usbvision/usbvision-video.c:1364
>  usbvision_radio_close.cold+0x2b/0x74 drivers/media/usb/usbvision/usbvision-video.c:1130
>  v4l2_release+0x2e7/0x390 drivers/media/v4l2-core/v4l2-dev.c:455
> 
> The problem is that the v4l2_release() calls usbvision_release() which
> frees "usbvision" but v4l2_release() still wants to use
> "usbvision->vdev".  One solution is to make this devm_ allocated memory
> so the memory isn't freed until later.

devm_ allocated memory is freed after disconnect, so I doubt this will help, or at
best it will just move the problem elsewhere.

The right approach would be to use the release() callback from struct v4l2_device:
that's called when the very last open filehandle is closed.

But I'm not sure if it is worth the effort. The usbvision driver is a mess and
personally I think it should be deprecated.

Regards,

	Hans

> 
> Reported-by: syzbot+75287f75e2fedd69d680@syzkaller.appspotmail.com
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---
> I copied this idea from a different driver, but I haven't tested it.
> I wanted to try the #syz fix command to see if it works.
> 
>  drivers/media/usb/usbvision/usbvision-video.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/media/usb/usbvision/usbvision-video.c b/drivers/media/usb/usbvision/usbvision-video.c
> index 93d36aab824f..07b4763062c4 100644
> --- a/drivers/media/usb/usbvision/usbvision-video.c
> +++ b/drivers/media/usb/usbvision/usbvision-video.c
> @@ -1312,7 +1312,7 @@ static struct usb_usbvision *usbvision_alloc(struct usb_device *dev,
>  {
>  	struct usb_usbvision *usbvision;
>  
> -	usbvision = kzalloc(sizeof(*usbvision), GFP_KERNEL);
> +	usbvision = devm_kzalloc(&dev->dev, sizeof(*usbvision), GFP_KERNEL);
>  	if (!usbvision)
>  		return NULL;
>  
> @@ -1336,7 +1336,6 @@ static struct usb_usbvision *usbvision_alloc(struct usb_device *dev,
>  	v4l2_ctrl_handler_free(&usbvision->hdl);
>  	v4l2_device_unregister(&usbvision->v4l2_dev);
>  err_free:
> -	kfree(usbvision);
>  	return NULL;
>  }
>  
> @@ -1361,7 +1360,6 @@ static void usbvision_release(struct usb_usbvision *usbvision)
>  
>  	v4l2_ctrl_handler_free(&usbvision->hdl);
>  	v4l2_device_unregister(&usbvision->v4l2_dev);
> -	kfree(usbvision);
>  
>  	PDEBUG(DBG_PROBE, "success");
>  }
>