media: cx23885: Fix use-after-free when unregistering the i2c_client for the dvb demod

Unregistering the i2c_client of the demod driver destroys the frontend
object.
Calling vb2_dvb_unregister_bus later accesses the frontend (and with the
refcount_t) conversion the refcount_t code complains:

kernel: ------------[ cut here ]------------
kernel: WARNING: CPU: 0 PID: 7883 at lib/refcount.c:128 refcount_sub_and_test+0x70/0x80
kernel: refcount_t: underflow; use-after-free.
kernel: Modules linked in: bluetooth si2165(O) a8293(O) tda10071(O) tea5767(O) tuner(O) cx23885(O-) tda18271(O) videobuf2_dvb(O) videobuf2_dma_sg(O) m88ds3103(O) tveeprom(O) cx2341x(O) v4l2_common(O) dvb_core(O) rc_core(O) videobuf2_memops(O) videobuf2_v4l2(O) ums_realtek videobuf2_core(O) uas videodev(O) media(O) rtl8192cu i2c_mux usb_storage rtl_usb rtl8192c_common rtlwifi snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core x86_pkg_temp_thermal kvm_intel kvm irqbypass
kernel: CPU: 0 PID: 7883 Comm: rmmod Tainted: G        W  O    4.11.3-gentoo #3
kernel: Hardware name: MEDION E2050 2391/H81H3-EM2, BIOS H81EM2W08.308 08/25/2014
kernel: Call Trace:
kernel:  dump_stack+0x4d/0x66
kernel:  __warn+0xc6/0xe0
kernel:  warn_slowpath_fmt+0x46/0x50
kernel:  ? kobject_put+0x2f/0x60
kernel:  refcount_sub_and_test+0x70/0x80
kernel:  refcount_dec_and_test+0x11/0x20
kernel:  dvb_unregister_frontend+0x42/0x60 [dvb_core]
kernel:  vb2_dvb_dealloc_frontends+0x9e/0x100 [videobuf2_dvb]
kernel:  vb2_dvb_unregister_bus+0xd/0x20 [videobuf2_dvb]
kernel:  cx23885_dvb_unregister+0xc3/0x110 [cx23885]
kernel:  cx23885_dev_unregister+0xea/0x150 [cx23885]
kernel:  cx23885_finidev+0x4f/0x70 [cx23885]
kernel:  pci_device_remove+0x34/0xb0
kernel:  device_release_driver_internal+0x150/0x200
kernel:  driver_detach+0x33/0x70
kernel:  bus_remove_driver+0x47/0xa0
kernel:  driver_unregister+0x27/0x50
kernel:  pci_unregister_driver+0x34/0x90
kernel:  cx23885_fini+0x10/0x12 [cx23885]
kernel:  SyS_delete_module+0x166/0x220
kernel:  ? exit_to_usermode_loop+0x7b/0x80
kernel:  entry_SYSCALL_64_fastpath+0x17/0x98
kernel: RIP: 0033:0x7f5901680b07
kernel: RSP: 002b:00007ffdf6cdb028 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
kernel: RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f5901680b07
kernel: RDX: 000000000000000a RSI: 0000000000000800 RDI: 0000000001500258
kernel: RBP: 00000000015001f0 R08: 0000000000000000 R09: 1999999999999999
kernel: R10: 0000000000000884 R11: 0000000000000206 R12: 00007ffdf6cda010
kernel: R13: 0000000000000000 R14: 00000000015001f0 R15: 00000000014ff010
kernel: ---[ end trace c3a4659b89086061 ]---
Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/pci/cx23885/cx23885-dvb.c

@@ -2637,6 +2637,11 @@ int cx23885_dvb_unregister(struct cx23885_tsport *port)
 	struct vb2_dvb_frontend *fe0;
 	struct i2c_client *client;
 
+	fe0 = vb2_dvb_get_frontend(&port->frontends, 1);
+
+	if (fe0 && fe0->dvb.frontend)
+		vb2_dvb_unregister_bus(&port->frontends);
+
 	/* remove I2C client for CI */
 	client = port->i2c_client_ci;
 	if (client) {
@@ -2665,11 +2670,6 @@ int cx23885_dvb_unregister(struct cx23885_tsport *port)
 		i2c_unregister_device(client);
 	}
 
-	fe0 = vb2_dvb_get_frontend(&port->frontends, 1);
-
-	if (fe0 && fe0->dvb.frontend)
-		vb2_dvb_unregister_bus(&port->frontends);
-
 	switch (port->dev->board) {
 	case CX23885_BOARD_NETUP_DUAL_DVBS2_CI:
 		netup_ci_exit(port);