Hi Mauro,
On Sat, May 07, 2016 at 12:12:09PM -0300, Mauro Carvalho Chehab wrote:
> struct media_devnode is currently embedded at struct media_device.
>
> While this works fine during normal usage, it leads to a race
> condition during devnode unregister. the problem is that drivers
> assume that, after calling media_device_unregister(), the struct
> that contains media_device can be freed. This is not true, as it
> can't be freed until userspace closes all opened /dev/media devnodes.
>
> In other words, if the media devnode is still open, and media_device
> gets freed, any call to an ioctl will make the core to try to access
> struct media_device, with will cause an use-after-free and even GPF.
>
> Fix this by dynamically allocating the struct media_devnode and only
> freeing it when it is safe.
A few general comments on the patch --- I agree we've had the problem from
the day one, but it's really started showing up recently. I agree with the
taken approach of separating the lifetimes of both media device and the
devnode. However, I don't think the patch as such is enough.
For one, there are some issue that remain. In particular, access to the data
structures (i.e. media_device and media_devnode) aren't serialised: the
IOCTL or a system call passes media-devnode framework asynchronously with a
possible unregister() call coming from media_device_unregister() (in
media-device.c). This may, unless I'm mistaken, to the following sequence:
process 1 process 2
fd = open(/dev/media0)
media_device_unregister()
(things are being cleaned up
here but the devnode isn't
unregistered yet)
...
ioctl(fd, ...)
__media_ioctl()
media_devnode_is_registered()
(returns true here)
...
media_devnode_unregister()
...
(driver releases the media device
memory)
media_device_ioctl()
(By this point
devnode->media_dev does not
point to allocated memory.
Bad things will happen here.)
You could try to serialise the operations. I wonder how ugly that might
be, and I'm not sure this would be a workable approach.
Secondly, a dependency is created from media devnode (i.e. media devnode
becomes aware of the media device). This is against the original design of
the two, as the media devnode was intended for other kinds of device nodes
as well and is more generic than media device. I'm not necessarily arguing
we have to keep it this way (as media devnode is only used by media device),
but if we're not keeping it then it'd make sense to unify the two to keep it
clean.
Have you thought about taking a reference to the said structs (by the means
of kref) where one is acquired?
That way, we should be able to rather easily keep around everything that's
needed until the last remaining user has gone away: opening a file handle
should get a reference to both media device and media devnode as well as
registering them (media_device_register() and media_devnode_register()).
>
> Signed-off-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxx>
> ---
> drivers/media/media-device.c | 44 +++++++++++++++++++++++-----------
> drivers/media/media-devnode.c | 7 +++++-
> drivers/media/usb/au0828/au0828-core.c | 4 ++--
> drivers/media/usb/uvc/uvc_driver.c | 2 +-
> include/media/media-device.h | 5 +---
> include/media/media-devnode.h | 13 +++++++++-
> 6 files changed, 52 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> index 47a99af5525e..8c520e064c34 100644
> --- a/drivers/media/media-device.c
> +++ b/drivers/media/media-device.c
> @@ -423,7 +423,7 @@ static long media_device_ioctl(struct file *filp, unsigned int cmd,
> unsigned long arg)
> {
> struct media_devnode *devnode = media_devnode_data(filp);
> - struct media_device *dev = to_media_device(devnode);
> + struct media_device *dev = devnode->media_dev;
> long ret;
>
> mutex_lock(&dev->graph_mutex);
> @@ -495,7 +495,7 @@ static long media_device_compat_ioctl(struct file *filp, unsigned int cmd,
> unsigned long arg)
> {
> struct media_devnode *devnode = media_devnode_data(filp);
> - struct media_device *dev = to_media_device(devnode);
> + struct media_device *dev = devnode->media_dev;
> long ret;
>
> switch (cmd) {
> @@ -531,7 +531,8 @@ static const struct media_file_operations media_device_fops = {
> static ssize_t show_model(struct device *cd,
> struct device_attribute *attr, char *buf)
> {
> - struct media_device *mdev = to_media_device(to_media_devnode(cd));
> + struct media_devnode *devnode = to_media_devnode(cd);
> + struct media_device *mdev = devnode->media_dev;
>
> return sprintf(buf, "%.*s\n", (int)sizeof(mdev->model), mdev->model);
> }
> @@ -704,23 +705,34 @@ EXPORT_SYMBOL_GPL(media_device_cleanup);
> int __must_check __media_device_register(struct media_device *mdev,
> struct module *owner)
> {
> + struct media_devnode *devnode;
> int ret;
>
> + devnode = kzalloc(sizeof(*devnode), GFP_KERNEL);
> + if (!devnode)
> + return -ENOMEM;
> +
> /* Register the device node. */
> - mdev->devnode.fops = &media_device_fops;
> - mdev->devnode.parent = mdev->dev;
> - mdev->devnode.release = media_device_release;
> + mdev->devnode = devnode;
> + devnode->fops = &media_device_fops;
> + devnode->parent = mdev->dev;
> + devnode->release = media_device_release;
>
> /* Set version 0 to indicate user-space that the graph is static */
> mdev->topology_version = 0;
>
> - ret = media_devnode_register(&mdev->devnode, owner);
> - if (ret < 0)
> + ret = media_devnode_register(mdev, devnode, owner);
> + if (ret < 0) {
> + mdev->devnode = NULL;
> + kfree(devnode);
> return ret;
> + }
>
> - ret = device_create_file(&mdev->devnode.dev, &dev_attr_model);
> + ret = device_create_file(&devnode->dev, &dev_attr_model);
> if (ret < 0) {
> - media_devnode_unregister(&mdev->devnode);
> + mdev->devnode = NULL;
> + media_devnode_unregister(devnode);
> + kfree(devnode);
> return ret;
> }
>
> @@ -771,7 +783,7 @@ void media_device_unregister(struct media_device *mdev)
> mutex_lock(&mdev->graph_mutex);
>
> /* Check if mdev was ever registered at all */
> - if (!media_devnode_is_registered(&mdev->devnode)) {
> + if (!media_devnode_is_registered(mdev->devnode)) {
> mutex_unlock(&mdev->graph_mutex);
> return;
> }
> @@ -794,9 +806,13 @@ void media_device_unregister(struct media_device *mdev)
>
> mutex_unlock(&mdev->graph_mutex);
>
> - device_remove_file(&mdev->devnode.dev, &dev_attr_model);
> - dev_dbg(mdev->dev, "Media device unregistering\n");
> - media_devnode_unregister(&mdev->devnode);
> + dev_dbg(mdev->dev, "Media device unregistered\n");
> +
> + /* Check if mdev devnode was registered */
> + if (media_devnode_is_registered(mdev->devnode)) {
> + device_remove_file(&mdev->devnode->dev, &dev_attr_model);
> + media_devnode_unregister(mdev->devnode);
> + }
> }
> EXPORT_SYMBOL_GPL(media_device_unregister);
>
> diff --git a/drivers/media/media-devnode.c b/drivers/media/media-devnode.c
> index 7481c9610945..ecdc02d6ed83 100644
> --- a/drivers/media/media-devnode.c
> +++ b/drivers/media/media-devnode.c
> @@ -44,6 +44,7 @@
> #include <linux/uaccess.h>
>
> #include <media/media-devnode.h>
> +#include <media/media-device.h>
>
> #define MEDIA_NUM_DEVICES 256
> #define MEDIA_NAME "media"
> @@ -74,6 +75,8 @@ static void media_devnode_release(struct device *cd)
> /* Release media_devnode and perform other cleanups as needed. */
> if (devnode->release)
> devnode->release(devnode);
> +
> + kfree(devnode);
> }
>
> static struct bus_type media_bus_type = {
> @@ -219,7 +222,8 @@ static const struct file_operations media_devnode_fops = {
> .llseek = no_llseek,
> };
>
> -int __must_check media_devnode_register(struct media_devnode *devnode,
> +int __must_check media_devnode_register(struct media_device *mdev,
> + struct media_devnode *devnode,
> struct module *owner)
> {
> int minor;
> @@ -238,6 +242,7 @@ int __must_check media_devnode_register(struct media_devnode *devnode,
> mutex_unlock(&media_devnode_lock);
>
> devnode->minor = minor;
> + devnode->media_dev = mdev;
>
> /* Part 2: Initialize and register the character device */
> cdev_init(&devnode->cdev, &media_devnode_fops);
> diff --git a/drivers/media/usb/au0828/au0828-core.c b/drivers/media/usb/au0828/au0828-core.c
> index 321ea5cf1329..bf53553d2624 100644
> --- a/drivers/media/usb/au0828/au0828-core.c
> +++ b/drivers/media/usb/au0828/au0828-core.c
> @@ -142,7 +142,7 @@ static void au0828_unregister_media_device(struct au0828_dev *dev)
> struct media_device *mdev = dev->media_dev;
> struct media_entity_notify *notify, *nextp;
>
> - if (!mdev || !media_devnode_is_registered(&mdev->devnode))
> + if (!mdev || !media_devnode_is_registered(mdev->devnode))
> return;
>
> /* Remove au0828 entity_notify callbacks */
> @@ -482,7 +482,7 @@ static int au0828_media_device_register(struct au0828_dev *dev,
> if (!dev->media_dev)
> return 0;
>
> - if (!media_devnode_is_registered(&dev->media_dev->devnode)) {
> + if (!media_devnode_is_registered(dev->media_dev->devnode)) {
>
> /* register media device */
> ret = media_device_register(dev->media_dev);
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 451e84e962e2..302e284a95eb 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -1674,7 +1674,7 @@ static void uvc_delete(struct uvc_device *dev)
> if (dev->vdev.dev)
> v4l2_device_unregister(&dev->vdev);
> #ifdef CONFIG_MEDIA_CONTROLLER
> - if (media_devnode_is_registered(&dev->mdev.devnode))
> + if (media_devnode_is_registered(dev->mdev.devnode))
> media_device_unregister(&dev->mdev);
> media_device_cleanup(&dev->mdev);
> #endif
> diff --git a/include/media/media-device.h b/include/media/media-device.h
> index a9b33c47310d..f743ae2210ee 100644
> --- a/include/media/media-device.h
> +++ b/include/media/media-device.h
> @@ -347,7 +347,7 @@ struct media_entity_notify {
> struct media_device {
> /* dev->driver_data points to this struct. */
> struct device *dev;
> - struct media_devnode devnode;
> + struct media_devnode *devnode;
>
> char model[32];
> char driver_name[32];
> @@ -393,9 +393,6 @@ struct usb_device;
> #define MEDIA_DEV_NOTIFY_PRE_LINK_CH 0
> #define MEDIA_DEV_NOTIFY_POST_LINK_CH 1
>
> -/* media_devnode to media_device */
> -#define to_media_device(node) container_of(node, struct media_device, devnode)
> -
> /**
> * media_entity_enum_init - Initialise an entity enumeration
> *
> diff --git a/include/media/media-devnode.h b/include/media/media-devnode.h
> index e1d5af077adb..5bb3b0e86d73 100644
> --- a/include/media/media-devnode.h
> +++ b/include/media/media-devnode.h
> @@ -33,6 +33,8 @@
> #include <linux/device.h>
> #include <linux/cdev.h>
>
> +struct media_device;
> +
> /*
> * Flag to mark the media_devnode struct as registered. Drivers must not touch
> * this flag directly, it will be set and cleared by media_devnode_register and
> @@ -81,6 +83,8 @@ struct media_file_operations {
> * before registering the node.
> */
> struct media_devnode {
> + struct media_device *media_dev;
> +
> /* device ops */
> const struct media_file_operations *fops;
>
> @@ -103,6 +107,7 @@ struct media_devnode {
> /**
> * media_devnode_register - register a media device node
> *
> + * @media_dev: struct media_device we want to register a device node
> * @devnode: media device node structure we want to register
> * @owner: should be filled with %THIS_MODULE
> *
> @@ -116,7 +121,8 @@ struct media_devnode {
> * the media_devnode structure is *not* called, so the caller is responsible for
> * freeing any data.
> */
> -int __must_check media_devnode_register(struct media_devnode *devnode,
> +int __must_check media_devnode_register(struct media_device *mdev,
> + struct media_devnode *devnode,
> struct module *owner);
>
> /**
> @@ -146,9 +152,14 @@ static inline struct media_devnode *media_devnode_data(struct file *filp)
> * false otherwise.
> *
> * @devnode: pointer to struct &media_devnode.
> + *
> + * Note: If mdev is NULL, it also returns false.
> */
> static inline int media_devnode_is_registered(struct media_devnode *devnode)
> {
> + if (!devnode)
> + return false;
> +
> return test_bit(MEDIA_FLAG_REGISTERED, &devnode->flags);
> }
>
--
Kind regards,
Sakari Ailus
e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
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