Re: [PATCH] media: usbvision: Fix a use after free in v4l2_release()

Hans Verkuil
2020-02-14T10:06:48Z

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");
> }
>