Re: [v4.9-rc4] dvb-usb/cinergyT2 NULL pointer dereference

[Mauro Carvalho Chehab <mchehab@osg.samsung.com>]

Em Thu, 10 Nov 2016 12:15:39 +0100
Jörg Otte <jrg.otte@gmail.com> escreveu:

> 2016-11-10 9:40 GMT+01:00 Mauro Carvalho Chehab <mchehab@s-opensource.com>:
> > Em Wed, 9 Nov 2016 11:07:35 -0800
> > Linus Torvalds <torvalds@linux-foundation.org> escreveu:
> >
> >> On Wed, Nov 9, 2016 at 3:09 AM, Jörg Otte <jrg.otte@gmail.com> wrote:
> >> >
> >> > Tried patch with no success. Again a NULL ptr dereferece.
> >>
> >> I suspect a much simpler approach is to just miove the "data_mutex"
> >> away from the priv area and into "struct dvb_usb_device" and
> >> "dvb_usb_adapter". Sure, that grows those structures a tiny bit, and
> >> not every driver may need that mutex, but it simplifies things
> >> enormously. Mauro?
> >>
> >>              Linus

> The patch works for me.

Thanks for testing! That's the (hopefully) final version of it,
with the fix applied to the other dvb-usb drivers that use
data_mutex (except for the frontend ones, with uses a different
private structure, and where the mutex is initialized at attach).

Benjamin,

Could you please test it?

Thanks!
Mauro

-

[PATCH] dvb-usb: move data_mutex to struct dvb_usb_device

The data_mutex is initialized too late, as it is needed for
each device driver's power control, causing an OOPS:

	dvb-usb: found a 'TerraTec/qanu USB2.0 Highspeed DVB-T Receiver' in warm state.
	BUG: unable to handle kernel NULL pointer dereference at           (null)
	IP: [<ffffffff846617af>] __mutex_lock_slowpath+0x6f/0x100 PGD 0
	Oops: 0002 [#1] SMP
	Modules linked in: dvb_usb_cinergyT2(+) dvb_usb
	CPU: 0 PID: 2029 Comm: modprobe Not tainted 4.9.0-rc4-dvbmod #24
	Hardware name: FUJITSU LIFEBOOK A544/FJNBB35 , BIOS Version 1.17 05/09/2014
	task: ffff88020e943840 task.stack: ffff8801f36ec000
	RIP: 0010:[<ffffffff846617af>]  [<ffffffff846617af>] __mutex_lock_slowpath+0x6f/0x100
	RSP: 0018:ffff8801f36efb10  EFLAGS: 00010282
	RAX: 0000000000000000 RBX: ffff88021509bdc8 RCX: 00000000c0000100
	RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff88021509bdcc
	RBP: ffff8801f36efb58 R08: ffff88021f216320 R09: 0000000000100000
	R10: ffff88021f216320 R11: 00000023fee6c5a1 R12: ffff88020e943840
	R13: ffff88021509bdcc R14: 00000000ffffffff R15: ffff88021509bdd0
	FS:  00007f21adb86740(0000) GS:ffff88021f200000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 0000000000000000 CR3: 0000000215bce000 CR4: 00000000001406f0
	Stack:
	 ffff88021509bdd0 0000000000000000 0000000000000000 ffffffffc0137c80
	 ffff88021509bdc8 ffff8801f5944000 0000000000000001 ffffffffc0136b00
	 ffff880213e52000 ffff88021509bdc8 ffffffff84661856 ffff88021509bd80
	Call Trace:
	 [<ffffffff84661856>] ? mutex_lock+0x16/0x25
	 [<ffffffffc013616f>] ? cinergyt2_power_ctrl+0x1f/0x60 [dvb_usb_cinergyT2]
	 [<ffffffffc012e67e>] ? dvb_usb_device_init+0x21e/0x5d0 [dvb_usb]
	 [<ffffffffc0136021>] ? cinergyt2_usb_probe+0x21/0x50 [dvb_usb_cinergyT2]
	 [<ffffffff844326f3>] ? usb_probe_interface+0xf3/0x2a0
	 [<ffffffff8438e348>] ? driver_probe_device+0x208/0x2b0
	 [<ffffffff8438e477>] ? __driver_attach+0x87/0x90
	 [<ffffffff8438e3f0>] ? driver_probe_device+0x2b0/0x2b0
	 [<ffffffff8438c612>] ? bus_for_each_dev+0x52/0x80
	 [<ffffffff8438d983>] ? bus_add_driver+0x1a3/0x220
	 [<ffffffff8438ec06>] ? driver_register+0x56/0xd0
	 [<ffffffff84431527>] ? usb_register_driver+0x77/0x130
	 [<ffffffffc013a000>] ? 0xffffffffc013a000
	 [<ffffffff84000426>] ? do_one_initcall+0x46/0x180
	 [<ffffffff840eb2c8>] ? free_vmap_area_noflush+0x38/0x70
	 [<ffffffff840f3844>] ? kmem_cache_alloc+0x84/0xc0
	 [<ffffffff840b802c>] ? do_init_module+0x50/0x1be
	 [<ffffffff84095adb>] ? load_module+0x1d8b/0x2100
	 [<ffffffff84093020>] ? find_symbol_in_section+0xa0/0xa0
	 [<ffffffff84095fe9>] ? SyS_finit_module+0x89/0x90
	 [<ffffffff846637a0>] ? entry_SYSCALL_64_fastpath+0x13/0x94
	Code: e8 a7 1d 00 00 8b 03 83 f8 01 0f 84 97 00 00 00 48 8b 43 10 4c 8d 7b 08 48 89 63 10 4c 89 3c 24 41 be ff ff ff ff 48 89 44 24 08 <48> 89 20 4c 89 64 24 10 eb 1a 49 c7 44 24 08 02 00 00 00 c6 43 RIP  [<ffffffff846617af>] __mutex_lock_slowpath+0x6f/0x100 RSP <ffff8801f36efb10>
	CR2: 0000000000000000

So, move it to the struct dvb_usb_device and initialize it
before calling the driver's callbacks.

Reported-by: Jörg Otte <jrg.otte@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

diff --git a/drivers/media/usb/dvb-usb/af9005.c b/drivers/media/usb/dvb-usb/af9005.c
index b257780fb380..7853261906b1 100644
--- a/drivers/media/usb/dvb-usb/af9005.c
+++ b/drivers/media/usb/dvb-usb/af9005.c
@@ -53,7 +53,6 @@ struct af9005_device_state {
 	u8 sequence;
 	int led_state;
 	unsigned char data[256];
-	struct mutex data_mutex;
 };
 
 static int af9005_generic_read_write(struct dvb_usb_device *d, u16 reg,
@@ -72,7 +71,7 @@ static int af9005_generic_read_write(struct dvb_usb_device *d, u16 reg,
 		return -EINVAL;
 	}
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = 14;		/* rest of buffer length low */
 	st->data[1] = 0;		/* rest of buffer length high */
 
@@ -140,7 +139,7 @@ static int af9005_generic_read_write(struct dvb_usb_device *d, u16 reg,
 			values[i] = st->data[8 + i];
 
 ret:
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 
 }
@@ -481,7 +480,7 @@ int af9005_send_command(struct dvb_usb_device *d, u8 command, u8 * wbuf,
 	}
 	packet_len = wlen + 5;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 
 	st->data[0] = (u8) (packet_len & 0xff);
 	st->data[1] = (u8) ((packet_len & 0xff00) >> 8);
@@ -512,7 +511,7 @@ int af9005_send_command(struct dvb_usb_device *d, u8 command, u8 * wbuf,
 			rbuf[i] = st->data[i + 7];
 	}
 
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 }
 
@@ -523,7 +522,7 @@ int af9005_read_eeprom(struct dvb_usb_device *d, u8 address, u8 * values,
 	u8 seq;
 	int ret, i;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 
 	memset(st->data, 0, sizeof(st->data));
 
@@ -559,7 +558,7 @@ int af9005_read_eeprom(struct dvb_usb_device *d, u8 address, u8 * values,
 		for (i = 0; i < len; i++)
 			values[i] = st->data[6 + i];
 	}
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 
 	return ret;
 }
@@ -847,7 +846,7 @@ static int af9005_rc_query(struct dvb_usb_device *d, u32 * event, int *state)
 		return 0;
 	}
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 
 	/* deb_info("rc_query\n"); */
 	st->data[0] = 3;		/* rest of packet length low */
@@ -890,7 +889,7 @@ static int af9005_rc_query(struct dvb_usb_device *d, u32 * event, int *state)
 	}
 
 ret:
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 }
 
@@ -1004,20 +1003,8 @@ static struct dvb_usb_device_properties af9005_properties;
 static int af9005_usb_probe(struct usb_interface *intf,
 			    const struct usb_device_id *id)
 {
-	struct dvb_usb_device *d;
-	struct af9005_device_state *st;
-	int ret;
-
-	ret = dvb_usb_device_init(intf, &af9005_properties,
-				  THIS_MODULE, &d, adapter_nr);
-
-	if (ret < 0)
-		return ret;
-
-	st = d->priv;
-	mutex_init(&st->data_mutex);
-
-	return 0;
+	return dvb_usb_device_init(intf, &af9005_properties,
+				  THIS_MODULE, NULL, adapter_nr);
 }
 
 enum af9005_usb_table_entry {
diff --git a/drivers/media/usb/dvb-usb/cinergyT2-core.c b/drivers/media/usb/dvb-usb/cinergyT2-core.c
index 8ac825413d5a..290275bc7fde 100644
--- a/drivers/media/usb/dvb-usb/cinergyT2-core.c
+++ b/drivers/media/usb/dvb-usb/cinergyT2-core.c
@@ -42,7 +42,6 @@ DVB_DEFINE_MOD_OPT_ADAPTER_NR(adapter_nr);
 struct cinergyt2_state {
 	u8 rc_counter;
 	unsigned char data[64];
-	struct mutex data_mutex;
 };
 
 /* We are missing a release hook with usb_device data */
@@ -56,12 +55,12 @@ static int cinergyt2_streaming_ctrl(struct dvb_usb_adapter *adap, int enable)
 	struct cinergyt2_state *st = d->priv;
 	int ret;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = CINERGYT2_EP1_CONTROL_STREAM_TRANSFER;
 	st->data[1] = enable ? 1 : 0;
 
 	ret = dvb_usb_generic_rw(d, st->data, 2, st->data, 64, 0);
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 
 	return ret;
 }
@@ -71,12 +70,12 @@ static int cinergyt2_power_ctrl(struct dvb_usb_device *d, int enable)
 	struct cinergyt2_state *st = d->priv;
 	int ret;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = CINERGYT2_EP1_SLEEP_MODE;
 	st->data[1] = enable ? 0 : 1;
 
 	ret = dvb_usb_generic_rw(d, st->data, 2, st->data, 3, 0);
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 
 	return ret;
 }
@@ -89,7 +88,7 @@ static int cinergyt2_frontend_attach(struct dvb_usb_adapter *adap)
 
 	adap->fe_adap[0].fe = cinergyt2_fe_attach(adap->dev);
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = CINERGYT2_EP1_GET_FIRMWARE_VERSION;
 
 	ret = dvb_usb_generic_rw(d, st->data, 1, st->data, 3, 0);
@@ -97,7 +96,7 @@ static int cinergyt2_frontend_attach(struct dvb_usb_adapter *adap)
 		deb_rc("cinergyt2_power_ctrl() Failed to retrieve sleep "
 			"state info\n");
 	}
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 
 	/* Copy this pointer as we are gonna need it in the release phase */
 	cinergyt2_usb_device = adap->dev;
@@ -166,7 +165,7 @@ static int cinergyt2_rc_query(struct dvb_usb_device *d, u32 *event, int *state)
 
 	*state = REMOTE_NO_KEY_PRESSED;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = CINERGYT2_EP1_GET_RC_EVENTS;
 
 	ret = dvb_usb_generic_rw(d, st->data, 1, st->data, 5, 0);
@@ -202,29 +201,17 @@ static int cinergyt2_rc_query(struct dvb_usb_device *d, u32 *event, int *state)
 	}
 
 ret:
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 }
 
 static int cinergyt2_usb_probe(struct usb_interface *intf,
 				const struct usb_device_id *id)
 {
-	struct dvb_usb_device *d;
-	struct cinergyt2_state *st;
-	int ret;
-
-	ret = dvb_usb_device_init(intf, &cinergyt2_properties,
-				  THIS_MODULE, &d, adapter_nr);
-	if (ret < 0)
-		return ret;
-
-	st = d->priv;
-	mutex_init(&st->data_mutex);
-
-	return 0;
+	return dvb_usb_device_init(intf, &cinergyt2_properties,
+				   THIS_MODULE, NULL, adapter_nr);
 }
 
-
 static struct usb_device_id cinergyt2_usb_table[] = {
 	{ USB_DEVICE(USB_VID_TERRATEC, 0x0038) },
 	{ 0 }
diff --git a/drivers/media/usb/dvb-usb/cxusb.c b/drivers/media/usb/dvb-usb/cxusb.c
index 39772812269d..243403081fa5 100644
--- a/drivers/media/usb/dvb-usb/cxusb.c
+++ b/drivers/media/usb/dvb-usb/cxusb.c
@@ -68,7 +68,7 @@ static int cxusb_ctrl_msg(struct dvb_usb_device *d,
 
 	wo = (rbuf == NULL || rlen == 0); /* write-only */
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = cmd;
 	memcpy(&st->data[1], wbuf, wlen);
 	if (wo)
@@ -77,7 +77,7 @@ static int cxusb_ctrl_msg(struct dvb_usb_device *d,
 		ret = dvb_usb_generic_rw(d, st->data, 1 + wlen,
 					 rbuf, rlen, 0);
 
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 }
 
@@ -1461,43 +1461,36 @@ static struct dvb_usb_device_properties cxusb_mygica_t230_properties;
 static int cxusb_probe(struct usb_interface *intf,
 		       const struct usb_device_id *id)
 {
-	struct dvb_usb_device *d;
-	struct cxusb_state *st;
-
 	if (0 == dvb_usb_device_init(intf, &cxusb_medion_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_bluebird_lgh064f_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_bluebird_dee1601_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_bluebird_lgz201_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_bluebird_dtt7579_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_bluebird_dualdig4_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_bluebird_nano2_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf,
 				&cxusb_bluebird_nano2_needsfirmware_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_aver_a868r_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf,
 				     &cxusb_bluebird_dualdig4_rev2_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_d680_dmb_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_mygica_d689_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &cxusb_mygica_t230_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
-	    0) {
-		st = d->priv;
-		mutex_init(&st->data_mutex);
-
+				     THIS_MODULE, NULL, adapter_nr) ||
+	    0)
 		return 0;
-	}
 
 	return -EINVAL;
 }
diff --git a/drivers/media/usb/dvb-usb/cxusb.h b/drivers/media/usb/dvb-usb/cxusb.h
index 9f3ee0e47d5c..18acda19527a 100644
--- a/drivers/media/usb/dvb-usb/cxusb.h
+++ b/drivers/media/usb/dvb-usb/cxusb.h
@@ -37,7 +37,6 @@ struct cxusb_state {
 	struct i2c_client *i2c_client_tuner;
 
 	unsigned char data[MAX_XFER_SIZE];
-	struct mutex data_mutex;
 };
 
 #endif
diff --git a/drivers/media/usb/dvb-usb/dtt200u.c b/drivers/media/usb/dvb-usb/dtt200u.c
index f88572c7ae7c..fcbff7fb0c4e 100644
--- a/drivers/media/usb/dvb-usb/dtt200u.c
+++ b/drivers/media/usb/dvb-usb/dtt200u.c
@@ -22,7 +22,6 @@ DVB_DEFINE_MOD_OPT_ADAPTER_NR(adapter_nr);
 
 struct dtt200u_state {
 	unsigned char data[80];
-	struct mutex data_mutex;
 };
 
 static int dtt200u_power_ctrl(struct dvb_usb_device *d, int onoff)
@@ -30,23 +29,24 @@ static int dtt200u_power_ctrl(struct dvb_usb_device *d, int onoff)
 	struct dtt200u_state *st = d->priv;
 	int ret = 0;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 
 	st->data[0] = SET_INIT;
 
 	if (onoff)
 		ret = dvb_usb_generic_write(d, st->data, 2);
 
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 }
 
 static int dtt200u_streaming_ctrl(struct dvb_usb_adapter *adap, int onoff)
 {
-	struct dtt200u_state *st = adap->dev->priv;
+	struct dvb_usb_device *d = adap->dev;
+	struct dtt200u_state *st = d->priv;
 	int ret;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = SET_STREAMING;
 	st->data[1] = onoff;
 
@@ -61,26 +61,27 @@ static int dtt200u_streaming_ctrl(struct dvb_usb_adapter *adap, int onoff)
 	ret = dvb_usb_generic_write(adap->dev, st->data, 1);
 
 ret:
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 
 	return ret;
 }
 
 static int dtt200u_pid_filter(struct dvb_usb_adapter *adap, int index, u16 pid, int onoff)
 {
-	struct dtt200u_state *st = adap->dev->priv;
+	struct dvb_usb_device *d = adap->dev;
+	struct dtt200u_state *st = d->priv;
 	int ret;
 
 	pid = onoff ? pid : 0;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = SET_PID_FILTER;
 	st->data[1] = index;
 	st->data[2] = pid & 0xff;
 	st->data[3] = (pid >> 8) & 0x1f;
 
 	ret = dvb_usb_generic_write(adap->dev, st->data, 4);
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 
 	return ret;
 }
@@ -91,7 +92,7 @@ static int dtt200u_rc_query(struct dvb_usb_device *d)
 	u32 scancode;
 	int ret;
 
-	mutex_lock(&st->data_mutex);
+	mutex_lock(&d->data_mutex);
 	st->data[0] = GET_RC_CODE;
 
 	ret = dvb_usb_generic_rw(d, st->data, 1, st->data, 5, 0);
@@ -126,7 +127,7 @@ static int dtt200u_rc_query(struct dvb_usb_device *d)
 		deb_info("st->data: %*ph\n", 5, st->data);
 
 ret:
-	mutex_unlock(&st->data_mutex);
+	mutex_unlock(&d->data_mutex);
 	return ret;
 }
 
@@ -145,24 +146,17 @@ static struct dvb_usb_device_properties wt220u_miglia_properties;
 static int dtt200u_usb_probe(struct usb_interface *intf,
 		const struct usb_device_id *id)
 {
-	struct dvb_usb_device *d;
-	struct dtt200u_state *st;
-
 	if (0 == dvb_usb_device_init(intf, &dtt200u_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &wt220u_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &wt220u_fc_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &wt220u_zl0353_properties,
-				     THIS_MODULE, &d, adapter_nr) ||
+				     THIS_MODULE, NULL, adapter_nr) ||
 	    0 == dvb_usb_device_init(intf, &wt220u_miglia_properties,
-				     THIS_MODULE, &d, adapter_nr)) {
-		st = d->priv;
-		mutex_init(&st->data_mutex);
-
+				     THIS_MODULE, NULL, adapter_nr))
 		return 0;
-	}
 
 	return -ENODEV;
 }
diff --git a/drivers/media/usb/dvb-usb/dvb-usb-init.c b/drivers/media/usb/dvb-usb/dvb-usb-init.c
index 3896ba9a4179..84308569e7dc 100644
--- a/drivers/media/usb/dvb-usb/dvb-usb-init.c
+++ b/drivers/media/usb/dvb-usb/dvb-usb-init.c
@@ -142,6 +142,7 @@ static int dvb_usb_init(struct dvb_usb_device *d, short *adapter_nums)
 {
 	int ret = 0;
 
+	mutex_init(&d->data_mutex);
 	mutex_init(&d->usb_mutex);
 	mutex_init(&d->i2c_mutex);
 
diff --git a/drivers/media/usb/dvb-usb/dvb-usb.h b/drivers/media/usb/dvb-usb/dvb-usb.h
index 1448c3d27ea2..67f898b6f6d0 100644
--- a/drivers/media/usb/dvb-usb/dvb-usb.h
+++ b/drivers/media/usb/dvb-usb/dvb-usb.h
@@ -404,8 +404,12 @@ struct dvb_usb_adapter {
  *  Powered is in/decremented for each call to modify the state.
  * @udev: pointer to the device's struct usb_device.
  *
- * @usb_mutex: semaphore of USB control messages (reading needs two messages)
- * @i2c_mutex: semaphore for i2c-transfers
+ * @data_mutex: mutex to protect the data structure used to store URB data
+ * @usb_mutex: mutex of USB control messages (reading needs two messages).
+ *	Please notice that this mutex is used internally at the generic
+ *	URB control functions. So, drivers using dvb_usb_generic_rw() and
+ *	derivated functions should not lock it internally.
+ * @i2c_mutex: mutex for i2c-transfers
  *
  * @i2c_adap: device's i2c_adapter if it uses I2CoverUSB
  *
@@ -433,6 +437,7 @@ struct dvb_usb_device {
 	int powered;
 
 	/* locking */
+	struct mutex data_mutex;
 	struct mutex usb_mutex;
 
 	/* i2c */