Re: [PATCH v6] media: em28xx: Fix race condition between open and init function

From: Igor Matheus Andrade Torrente
Date: Mon Sep 13 2021 - 11:44:44 EST

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In reply to: Hans Verkuil: "Re: [PATCH v6] media: em28xx: Fix race condition between open and init function"
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Hi Hans,

On 9/13/21 6:35 AM, Hans Verkuil wrote:

On 14/06/2021 21:52, Igor Matheus Andrade Torrente wrote:

Fixes a race condition - for lack of a more precise term - between
em28xx_v4l2_open and em28xx_v4l2_init, by managing the em28xx_v4l2
and v4l2_dev life-time with the v4l2_dev->release() callback.

The race happens when a thread[1] - containing the em28xx_v4l2_init()
code - calls the v4l2_mc_create_media_graph(), and it return a error,
if a thread[2] - running v4l2_open() - pass the verification point
and reaches the em28xx_v4l2_open() before the thread[1] finishes
the deregistration of v4l2 subsystem, the thread[1] will free all
resources before the em28xx_v4l2_open() can process their things,
because the em28xx_v4l2_init() has the dev->lock. And all this lead
the thread[2] to cause a user-after-free.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Reported-and-tested-by: syzbot+b2391895514ed9ef4a8e@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@xxxxxxxxx>
---

V2: Add v4l2_i2c_new_subdev null check
Deal with v4l2 subdevs dependencies

V3: Fix link error when compiled as a module

V4: Remove duplicated v4l2_device_disconnect
in the em28xx_v4l2_fini

V5: Move all the v4l2 resources management
to the v4l2_dev->release() callback.

V6: Address some Hans comments regarding
video_unregister_device and struct v4l2_device
inside the struct v4l2_device.

I'm sending this v6 that way but I'm totally open
to the Hilt approach, if it is a more desirable
way to fix this issue.
---
drivers/media/usb/em28xx/em28xx-cards.c | 3 +-
drivers/media/usb/em28xx/em28xx-video.c | 232 +++++++++++++++---------
drivers/media/usb/em28xx/em28xx.h | 1 -
3 files changed, 151 insertions(+), 85 deletions(-)

diff --git a/drivers/media/usb/em28xx/em28xx-cards.c b/drivers/media/usb/em28xx/em28xx-cards.c
index ba9292e2a587..6e67cf0a1e04 100644
--- a/drivers/media/usb/em28xx/em28xx-cards.c
+++ b/drivers/media/usb/em28xx/em28xx-cards.c
@@ -4120,7 +4120,6 @@ static void em28xx_usb_disconnect(struct usb_interface *intf)
struct em28xx *dev;
dev = usb_get_intfdata(intf);
- usb_set_intfdata(intf, NULL);
if (!dev)
return;
@@ -4148,6 +4147,8 @@ static void em28xx_usb_disconnect(struct usb_interface *intf)
dev->dev_next = NULL;
}
kref_put(&dev->ref, em28xx_free_device);
+
+ usb_set_intfdata(intf, NULL);
}
static int em28xx_usb_suspend(struct usb_interface *intf,
diff --git a/drivers/media/usb/em28xx/em28xx-video.c b/drivers/media/usb/em28xx/em28xx-video.c
index 6b84c3413e83..abf9b325eae4 100644
--- a/drivers/media/usb/em28xx/em28xx-video.c
+++ b/drivers/media/usb/em28xx/em28xx-video.c
@@ -1897,7 +1897,7 @@ static int vidioc_g_chip_info(struct file *file, void *priv,
strscpy(chip->name, "ac97", sizeof(chip->name));
else
strscpy(chip->name,
- dev->v4l2->v4l2_dev.name, sizeof(chip->name));
+ dev->v4l2->v4l2_dev->name, sizeof(chip->name));
return 0;
}
@@ -2113,21 +2113,6 @@ static int radio_s_tuner(struct file *file, void *priv,
return 0;
}
-/*
- * em28xx_free_v4l2() - Free struct em28xx_v4l2
- *
- * @ref: struct kref for struct em28xx_v4l2
- *
- * Called when all users of struct em28xx_v4l2 are gone
- */
-static void em28xx_free_v4l2(struct kref *ref)
-{
- struct em28xx_v4l2 *v4l2 = container_of(ref, struct em28xx_v4l2, ref);
-
- v4l2->dev->v4l2 = NULL;
- kfree(v4l2);
-}
-
/*
* em28xx_v4l2_open()
* inits the device and starts isoc transfer
@@ -2153,12 +2138,21 @@ static int em28xx_v4l2_open(struct file *filp)
return -EINVAL;
}
+ /* To prevent the case when the v4l2_device_put() has already being called,
+ * the ref is now 0, we call a v4l2_device_get, and end up accessing freed
+ * resources. Or straigth accessing a freed v4l2.
+ */
+ if (!v4l2 || !kref_get_unless_zero(&v4l2->v4l2_dev.ref))
+ return -ENODEV;
+

This should not be needed: once the video devices are unregistered, it is no longer
possible to open the video device. So this can't happen.

If a thread[1] with the `open` syscall pass verification point at v4l2_open() before the deregistration in the `em28xx_v4l2_fini`.

And If for some reason (e.g page fault) the thread[1] get stuck and can't get the `dev->lock` before the thread[2](which is in `em28xx_v4l2_dev_release`).*

The thread[1] will end up accessing freed resources.

Thread[1] |
Open() |
| Time
Thread[1] Thread[2] |
if(vdev==NULL... em28xx_v4l2_fini(em28xx_v4l2_fini) |
▽
Thread[1] Thread[2]
em28xx_v4l2_open(...) b2_video_unregister_device(...)

Thread[1] Thread[2]
PAGE-FAULT v4l2_device_put(&v4l2->v4l2_dev)

Thread[1] Thread[2]
PAGE-FAULT if (refcount_dec_and_test(...))

Thread[1] Thread[2]
PAGE-FAULT em28xx_v4l2_dev_release(...)

Thread[1] Thread[2]
PAGE-FAULT mutex_lock(&dev->lock);

Thread[1] Thread[2]
mutex_lock_interruptible() em28xx_v4l2_dev_unregistr(dev);

Thread[1] Thread[2]
mutex_lock_interruptible() mutex_unlock(&dev->lock)0

Thread[1] Thread[2]
access freed resources kref_put(&dev->ref, em28xx_free_device)

The case above is possible, isn't it?

em28xx_videodbg("open dev=%s type=%s users=%d\n",
video_device_node_name(vdev), v4l2_type_names[fh_type],
v4l2->users);
- if (mutex_lock_interruptible(&dev->lock))
+ if (mutex_lock_interruptible(&dev->lock)) {
+ v4l2_device_put(&v4l2->v4l2_dev);

So this is also not needed...

return -ERESTARTSYS;
+ }
ret = v4l2_fh_open(filp);
if (ret) {
@@ -2166,6 +2160,7 @@ static int em28xx_v4l2_open(struct file *filp)
"%s: v4l2_fh_open() returned error %d\n",
__func__, ret);
mutex_unlock(&dev->lock);
+ v4l2_device_put(&v4l2->v4l2_dev);

...and neither is this.

return ret;
}
@@ -2187,8 +2182,6 @@ static int em28xx_v4l2_open(struct file *filp)
v4l2_device_call_all(&v4l2->v4l2_dev, 0, tuner, s_radio);
}
- kref_get(&dev->ref);
- kref_get(&v4l2->ref);
v4l2->users++;
mutex_unlock(&dev->lock);
@@ -2222,36 +2215,30 @@ static int em28xx_v4l2_fini(struct em28xx *dev)
mutex_lock(&dev->lock);
- v4l2_device_disconnect(&v4l2->v4l2_dev);
-
em28xx_uninit_usb_xfer(dev, EM28XX_ANALOG_MODE);
- em28xx_v4l2_media_release(dev);
-
if (video_is_registered(&v4l2->radio_dev)) {
- dev_info(&dev->intf->dev, "V4L2 device %s deregistered\n",
+ dev_info(&dev->intf->dev,
+ "V4L2 device %s deregistered\n",
video_device_node_name(&v4l2->radio_dev));
- video_unregister_device(&v4l2->radio_dev);
+ vb2_video_unregister_device(&v4l2->radio_dev);
}
if (video_is_registered(&v4l2->vbi_dev)) {
- dev_info(&dev->intf->dev, "V4L2 device %s deregistered\n",
+ dev_info(&dev->intf->dev,
+ "V4L2 device %s deregistered\n",
video_device_node_name(&v4l2->vbi_dev));
- video_unregister_device(&v4l2->vbi_dev);
+ vb2_video_unregister_device(&v4l2->vbi_dev);
}
if (video_is_registered(&v4l2->vdev)) {
- dev_info(&dev->intf->dev, "V4L2 device %s deregistered\n",
+ dev_info(&dev->intf->dev,
+ "V4L2 device %s deregistered\n",
video_device_node_name(&v4l2->vdev));
- video_unregister_device(&v4l2->vdev);
+ vb2_video_unregister_device(&v4l2->vdev);
}
- v4l2_ctrl_handler_free(&v4l2->ctrl_handler);
- v4l2_device_unregister(&v4l2->v4l2_dev);
-
- kref_put(&v4l2->ref, em28xx_free_v4l2);
-
mutex_unlock(&dev->lock);
- kref_put(&dev->ref, em28xx_free_device);
+ v4l2_device_put(&v4l2->v4l2_dev);
return 0;
}
@@ -2323,9 +2310,8 @@ static int em28xx_v4l2_close(struct file *filp)
exit:
v4l2->users--;
- kref_put(&v4l2->ref, em28xx_free_v4l2);
mutex_unlock(&dev->lock);
- kref_put(&dev->ref, em28xx_free_device);
+ v4l2_device_put(&v4l2->v4l2_dev);

...and neither is this v4l2_device_put.

return 0;
}
@@ -2517,6 +2503,28 @@ static void em28xx_tuner_setup(struct em28xx *dev, unsigned short tuner_addr)
v4l2_device_call_all(v4l2_dev, 0, tuner, s_frequency, &f);
}
+static void em28xx_v4l2_dev_unregister(struct em28xx *dev)
+{
+ struct em28xx_v4l2 *v4l2 = dev->v4l2;
+
+ v4l2_device_unregister(&v4l2->v4l2_dev);
+ em28xx_v4l2_media_release(dev);
+ v4l2_ctrl_handler_free(&v4l2->ctrl_handler);
+}
+
+static void em28xx_v4l2_dev_release(struct v4l2_device *v4l2_dev)
+{
+ struct em28xx *dev = v4l2_dev->dev->driver_data;
+
+ mutex_lock(&dev->lock);
+ em28xx_v4l2_dev_unregister(dev);
+ kfree(dev->v4l2);
+ dev->v4l2 = NULL;
+ mutex_unlock(&dev->lock);
+
+ kref_put(&dev->ref, em28xx_free_device);
+}
+
static int em28xx_v4l2_init(struct em28xx *dev)
{
u8 val;
@@ -2524,6 +2532,7 @@ static int em28xx_v4l2_init(struct em28xx *dev)
unsigned int maxw;
struct v4l2_ctrl_handler *hdl;
struct em28xx_v4l2 *v4l2;
+ struct v4l2_subdev *sd;
if (dev->is_audio_only) {
/* Shouldn't initialize IR for this interface */
@@ -2541,12 +2550,13 @@ static int em28xx_v4l2_init(struct em28xx *dev)
v4l2 = kzalloc(sizeof(*v4l2), GFP_KERNEL);
if (!v4l2) {
- mutex_unlock(&dev->lock);
- return -ENOMEM;
+ ret = -ENOMEM;
+ goto err;
}
- kref_init(&v4l2->ref);
+
v4l2->dev = dev;
dev->v4l2 = v4l2;
+ v4l2->v4l2_dev.release = em28xx_v4l2_dev_release;
#ifdef CONFIG_MEDIA_CONTROLLER
v4l2->v4l2_dev.mdev = dev->media_dev;
@@ -2555,7 +2565,7 @@ static int em28xx_v4l2_init(struct em28xx *dev)
if (ret < 0) {
dev_err(&dev->intf->dev,
"Call to v4l2_device_register() failed!\n");
- goto err;
+ goto free_v4l2;
}
hdl = &v4l2->ctrl_handler;
@@ -2574,25 +2584,53 @@ static int em28xx_v4l2_init(struct em28xx *dev)
/* request some modules */
- if (dev->has_msp34xx)
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "msp3400", 0, msp3400_addrs);
+ if (dev->has_msp34xx) {
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "msp3400", 0, msp3400_addrs);
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering 'msp34xx' v4l2 subdevice!\n");
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+ }
- if (dev->board.decoder == EM28XX_SAA711X)
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "saa7115_auto", 0, saa711x_addrs);
+ if (dev->board.decoder == EM28XX_SAA711X) {
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "saa7115_auto", 0, saa711x_addrs);
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering 'EM28XX_SAA711X' v4l2 subdevice!\n");
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+ }
- if (dev->board.decoder == EM28XX_TVP5150)
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "tvp5150", 0, tvp5150_addrs);
+ if (dev->board.decoder == EM28XX_TVP5150) {
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "tvp5150", 0, tvp5150_addrs);
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering 'EM28XX_TVP5150' v4l2 subdevice!\n");
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+ }
- if (dev->board.adecoder == EM28XX_TVAUDIO)
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "tvaudio", dev->board.tvaudio_addr, NULL);
+ if (dev->board.adecoder == EM28XX_TVAUDIO) {
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "tvaudio", dev->board.tvaudio_addr, NULL);
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering 'EM28XX_TVAUDIO' v4l2 subdevice!\n");
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+ }
/* Initialize tuner and camera */
@@ -2600,33 +2638,63 @@ static int em28xx_v4l2_init(struct em28xx *dev)
unsigned short tuner_addr = dev->board.tuner_addr;
int has_demod = (dev->board.tda9887_conf & TDA9887_PRESENT);
- if (dev->board.radio.type)
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "tuner", dev->board.radio_addr,
- NULL);
-
- if (has_demod)
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "tuner", 0,
- v4l2_i2c_tuner_addrs(ADDRS_DEMOD));
+ if (dev->board.radio.type) {
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "tuner", dev->board.radio_addr,
+ NULL);
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering '%s' v4l2 subdevice!\n",
+ dev->board.name);
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+ }
+
+ if (has_demod) {
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "tuner", 0,
+ v4l2_i2c_tuner_addrs(ADDRS_DEMOD));
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering '%s' v4l2 subdevice!\n",
+ dev->i2c_adap[dev->def_i2c_bus].name);
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+ }
+
if (tuner_addr == 0) {
enum v4l2_i2c_tuner_type type =
has_demod ? ADDRS_TV_WITH_DEMOD : ADDRS_TV;
- struct v4l2_subdev *sd;
sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
&dev->i2c_adap[dev->def_i2c_bus],
"tuner", 0,
v4l2_i2c_tuner_addrs(type));
-
- if (sd)
+ if (sd) {
tuner_addr = v4l2_i2c_subdev_addr(sd);
+ } else {
+ dev_err(&dev->intf->dev,
+ "Error while registering '%s' v4l2 subdevice!\n",
+ dev->i2c_adap[dev->def_i2c_bus].name);
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
+
} else {
- v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
- &dev->i2c_adap[dev->def_i2c_bus],
- "tuner", tuner_addr, NULL);
+ sd = v4l2_i2c_new_subdev(&v4l2->v4l2_dev,
+ &dev->i2c_adap[dev->def_i2c_bus],
+ "tuner", tuner_addr, NULL);
+ if (!sd) {
+ dev_err(&dev->intf->dev,
+ "Error while registering '%s' v4l2 subdevice!\n",
+ dev->i2c_adap[dev->def_i2c_bus].name);
+ ret = -EINVAL;
+ goto unregister_dev;
+ }
}
em28xx_tuner_setup(dev, tuner_addr);
@@ -2755,7 +2823,6 @@ static int em28xx_v4l2_init(struct em28xx *dev)
if (ret)
goto unregister_dev;
- /* allocate and fill video video_device struct */
em28xx_vdev_init(dev, &v4l2->vdev, &em28xx_video_template, "video");
mutex_init(&v4l2->vb_queue_lock);
mutex_init(&v4l2->vb_vbi_queue_lock);
@@ -2768,7 +2835,6 @@ static int em28xx_v4l2_init(struct em28xx *dev)
if (dev->tuner_type != TUNER_ABSENT)
v4l2->vdev.device_caps |= V4L2_CAP_TUNER;
-
/* disable inapplicable ioctls */
if (dev->is_webcam) {
v4l2_disable_ioctl(&v4l2->vdev, VIDIOC_QUERYSTD);
@@ -2889,26 +2955,26 @@ static int em28xx_v4l2_init(struct em28xx *dev)
dev_info(&dev->intf->dev,
"V4L2 device %s deregistered\n",
video_device_node_name(&v4l2->radio_dev));
- video_unregister_device(&v4l2->radio_dev);
+ vb2_video_unregister_device(&v4l2->radio_dev);
}
if (video_is_registered(&v4l2->vbi_dev)) {
dev_info(&dev->intf->dev,
"V4L2 device %s deregistered\n",
video_device_node_name(&v4l2->vbi_dev));
- video_unregister_device(&v4l2->vbi_dev);
+ vb2_video_unregister_device(&v4l2->vbi_dev);
}
if (video_is_registered(&v4l2->vdev)) {
dev_info(&dev->intf->dev,
"V4L2 device %s deregistered\n",
video_device_node_name(&v4l2->vdev));
- video_unregister_device(&v4l2->vdev);
+ vb2_video_unregister_device(&v4l2->vdev);
}
- v4l2_ctrl_handler_free(&v4l2->ctrl_handler);
- v4l2_device_unregister(&v4l2->v4l2_dev);
-err:
+ em28xx_v4l2_dev_unregister(dev);
+free_v4l2:
+ kfree(v4l2);
dev->v4l2 = NULL;
- kref_put(&v4l2->ref, em28xx_free_v4l2);
+err:
mutex_unlock(&dev->lock);
return ret;
}
diff --git a/drivers/media/usb/em28xx/em28xx.h b/drivers/media/usb/em28xx/em28xx.h
index ab167cd1f400..666b7eff55f4 100644
--- a/drivers/media/usb/em28xx/em28xx.h
+++ b/drivers/media/usb/em28xx/em28xx.h
@@ -549,7 +549,6 @@ struct em28xx_eeprom {
#define EM28XX_RESOURCE_VBI 0x02
struct em28xx_v4l2 {
- struct kref ref;
struct em28xx *dev;
struct v4l2_device v4l2_dev;

With the suggested changes it should be ready (pending testing on my side).

Regards,

Hans

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