[PATCH 0/2] Fix use-after-free errors when unregistering the i2c_client for the dvb demod

[PATCH 0/2] Fix use-after-free errors when unregistering the i2c_client for the dvb demod

[Matthias Schwarzott]

Hi!

There seem to be a general error in a lot of dvb bride drivers about the order of i2c_unregister_device
and the calls to dvb_unregister_frontend and dvb_frontend_detach.

As soon as the i2c_client for a demod driver is unregistered the memory for the frontend is kfreed.
But the calls to dvb_unregister_frontend and dvb_frontend_detach access it later.

I fixed the error in cx23885 and cx231xx driver as I have access to the hardware.

Further drivers that might show the same bug (but I cannot test):
* em28xx
* ddbride
* saa7164

Regards
Matthias

[PATCH 1/2] cx23885: Fix use-after-free when unregistering the i2c_client for the dvb demod

[Matthias Schwarzott]

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>
---
 drivers/media/pci/cx23885/cx23885-dvb.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/media/pci/cx23885/cx23885-dvb.c b/drivers/media/pci/cx23885/cx23885-dvb.c
index 979b66627f60..e795ddeb7fe2 100644
--- a/drivers/media/pci/cx23885/cx23885-dvb.c
+++ b/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);
-- 
2.13.3

[PATCH 2/2] cx231xx: fix use-after-free when unregistering the i2c_client for the dvb demod

[Matthias Schwarzott]

Calling i2c_unregister_device for a demod driver destroys the frontend object.
Later it is accessed by calling dvb_unregister_frontend and
dvb_frontend_detach.

In some cases this leads to a general protection fault with this
callstack:

  dvb_unregister_frontend+0x25/0x50 [dvb_core]
  dvb_fini+0xdb/0x160 [cx231xx_dvb]
  cx231xx_unregister_extension+0x3d/0xb0 [cx231xx]
  cx231xx_dvb_unregister+0x10/0x809 [cx231xx_dvb]
  SyS_delete_module+0x18a/0x240
  ? exit_to_usermode_loop+0x7b/0x80
  entry_SYSCALL_64_fastpath+0x17/0x98

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
---
 drivers/media/usb/cx231xx/cx231xx-dvb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/media/usb/cx231xx/cx231xx-dvb.c b/drivers/media/usb/cx231xx/cx231xx-dvb.c
index ee3eeeb600f8..c18bb33e060e 100644
--- a/drivers/media/usb/cx231xx/cx231xx-dvb.c
+++ b/drivers/media/usb/cx231xx/cx231xx-dvb.c
@@ -585,6 +585,9 @@ static void unregister_dvb(struct cx231xx_dvb *dvb)
 	dvb->demux.dmx.remove_frontend(&dvb->demux.dmx, &dvb->fe_hw);
 	dvb_dmxdev_release(&dvb->dmxdev);
 	dvb_dmx_release(&dvb->demux);
+	dvb_unregister_frontend(dvb->frontend);
+	dvb_frontend_detach(dvb->frontend);
+	dvb_unregister_adapter(&dvb->adapter);
 	/* remove I2C tuner */
 	client = dvb->i2c_client_tuner;
 	if (client) {
@@ -597,9 +600,6 @@ static void unregister_dvb(struct cx231xx_dvb *dvb)
 		module_put(client->dev.driver->owner);
 		i2c_unregister_device(client);
 	}
-	dvb_unregister_frontend(dvb->frontend);
-	dvb_frontend_detach(dvb->frontend);
-	dvb_unregister_adapter(&dvb->adapter);
 }
 
 static int dvb_init(struct cx231xx *dev)
-- 
2.13.3

Re: [PATCH 1/2] cx23885: Fix use-after-free when unregistering the i2c_client for the dvb demod

[Matthias Schwarzott]

Am 02.08.2017 um 18:45 schrieb Matthias Schwarzott:
> diff --git a/drivers/media/pci/cx23885/cx23885-dvb.c b/drivers/media/pci/cx23885/cx23885-dvb.c
> index 979b66627f60..e795ddeb7fe2 100644
> --- a/drivers/media/pci/cx23885/cx23885-dvb.c
> +++ b/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);
> -

The following code is after i2c_unregister_device.
>  	switch (port->dev->board) {
>  	case CX23885_BOARD_NETUP_DUAL_DVBS2_CI:
>  		netup_ci_exit(port);
> 
I wonder if the code above should be moved to before the
i2c_unregister_device block.
Currently these NETUP board drivers do not use "new style/i2c_client
based" frontend drivers. But if in future this switch is extended one
could get in trouble.

Regards
Matthias