[PATCH v2] media: dvb-usb-v2: set active_fe before the demux goes live

[Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>]

dvb_usbv2_adapter_dvb_init() registers the demux devices before
dvb_usbv2_adapter_frontend_init() runs. Only the latter set
adap->active_fe to -1, so until then it holds the kzalloc'ed 0 while
adap->fe[0] is still NULL.

A process that opens the demux and starts a filter in that window, which
lasts as long as frontend_attach() takes, gets past the active_fe == -1
check in dvb_usb_start_feed() and passes the NULL adap->fe[0] to
streaming_ctrl() (or get_stream_config()). mxl111sf_ep6_streaming_ctrl()
then dereferences it:

  Oops: general protection fault, probably for non-canonical address
  0xdffffc0000000061
  KASAN: null-ptr-deref in range [0x0000000000000308-0x000000000000030f]
  RIP: 0010:mxl111sf_ep6_streaming_ctrl+0x2e/0x2e0
   dvb_usb_start_feed+0x57a/0xc70
   dmx_ts_feed_start_filtering+0x129/0x220
   dvb_dmxdev_filter_start+0x434/0x10d0
   dvb_demux_do_ioctl+0x48b/0x540

Initialise active_fe to -1 before the demux is registered so early
feeds fail with -EINVAL.

Reported-by: syzbot+c35815bad054e6ad396e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c35815bad054e6ad396e
Fixes: ad6b9e517b39 ("[media] dvb_usb_v2: move from dvb-usb to dvb-usb-v2")
Cc: stable@vger.kernel.org
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
 drivers/media/usb/dvb-usb-v2/dvb_usb_core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/media/usb/dvb-usb-v2/dvb_usb_core.c b/drivers/media/usb/dvb-usb-v2/dvb_usb_core.c
index bd86d250433d..557451b541e6 100644
--- a/drivers/media/usb/dvb-usb-v2/dvb_usb_core.c
+++ b/drivers/media/usb/dvb-usb-v2/dvb_usb_core.c
@@ -637,7 +637,6 @@ static int dvb_usbv2_adapter_frontend_init(struct dvb_usb_adapter *adap)
 	dev_dbg(&d->udev->dev, "%s: adap=%d\n", __func__, adap->id);
 
 	memset(adap->fe, 0, sizeof(adap->fe));
-	adap->active_fe = -1;
 
 	if (d->props->frontend_attach) {
 		ret = d->props->frontend_attach(adap);
@@ -795,6 +794,9 @@ static int dvb_usbv2_adapter_init(struct dvb_usb_device *d)
 			adap->max_feed_count = adap->props->pid_filter_count;
 		}
 
+		/* demux is live before frontends attach: no feed until then */
+		adap->active_fe = -1;
+
 		ret = dvb_usbv2_adapter_stream_init(adap);
 		if (ret)
 			goto err;
-- 
2.43.0