On 03/03/2023 09:39, Hans Verkuil wrote:
> On 01/02/2023 22:45, Sakari Ailus wrote:
>> Add a new helper data structure media_devnode_compat_ref, which is used to
>> prevent user space from calling IOCTLs or other system calls to the media
>> device that has been already unregistered.
>>
>> The media device's memory may of course still be released during the call
>> but there is only so much that can be done to this without the driver
>> managing the lifetime of the resources it needs somehow.
>>
>> This patch should be reverted once all drivers have been converted to manage
>> their resources' lifetime.
>>
>> Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
>> ---
>> drivers/media/mc/mc-device.c | 60 ++++++++++++++++++++++++++++++-----
>> drivers/media/mc/mc-devnode.c | 21 ++++++++----
>> include/media/media-devnode.h | 29 +++++++++++++++++
>> 3 files changed, 96 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/media/mc/mc-device.c b/drivers/media/mc/mc-device.c
>> index 3a1db5fdbba7..22fdaa6370ea 100644
>> --- a/drivers/media/mc/mc-device.c
>> +++ b/drivers/media/mc/mc-device.c
>> @@ -45,18 +45,34 @@ static inline void __user *media_get_uptr(__u64 arg)
>> return (void __user *)(uintptr_t)arg;
>> }
>>
>> +static void compat_ref_release(struct kref *kref)
>> +{
>> + struct media_devnode_compat_ref *ref =
>> + container_of_const(kref, struct media_devnode_compat_ref, kref);
>> +
>> + kfree(ref);
>> +}
>> +
>> static int media_device_open(struct media_devnode *devnode, struct file *filp)
>> {
>> struct media_device *mdev = to_media_device(devnode);
>> struct media_device_fh *fh;
>> unsigned long flags;
>>
>> + if (devnode->ref && (!atomic_read(&devnode->ref->registered) ||
>> + !kref_get_unless_zero(&devnode->ref->kref)))
>> + return -ENXIO;
>> +
>
> This seems pointless: if the media device is unregistered, then the device
> node disappears and it can't be opened anymore.
>
> I'm confused by this patch in general: when media_device_unregister() is called,
> it is no longer possible to call ioctls and basically do anything except close
> the open fh.
>
> So what am I missing here? It all looks odd.
I read up on this a bit more, and I think this patch is bogus: drivers not
converted to the release() callback will indeed just crash, but that's no
different than many existing drivers, media or otherwise, when you forcibly
unbind them. It's broken today, and since you have to be root to unbind, I
would say that we can just leave it as-is rather than introducing a rather
ugly workaround. I don't think it will help anyway, since most likely
such drivers will also fails if the application has a video device open
when the device is unbound.
The way to fix it is converting driver to use the new release() callback.
Where this becomes really important is for usb devices which, by their
nature, are hotpluggable. So uvc at least should be converted to this,
I think.
Regards,
Hans
>
> Regards,
>
> Hans
>
>> fh = kzalloc(sizeof(*fh), GFP_KERNEL);
>> - if (!fh)
>> + if (!fh) {
>> + if (devnode->ref)
>> + kref_put(&devnode->ref->kref, compat_ref_release);
>> return -ENOMEM;
>> + }
>>
>> - filp->private_data = &fh->fh;
>> + fh->fh.ref = devnode->ref;
>>
>> + filp->private_data = &fh->fh;
>> spin_lock_irqsave(&mdev->fh_list_lock, flags);
>> list_add(&fh->mdev_list, &mdev->fh_list);
>> spin_unlock_irqrestore(&mdev->fh_list_lock, flags);
>> @@ -71,9 +87,14 @@ static int media_device_close(struct file *filp)
>> struct media_device_fh *fh = media_device_fh(filp);
>> unsigned long flags;
>>
>> - spin_lock_irqsave(&mdev->fh_list_lock, flags);
>> - list_del(&fh->mdev_list);
>> - spin_unlock_irqrestore(&mdev->fh_list_lock, flags);
>> + if (!fh->fh.ref || atomic_read(&fh->fh.ref->registered)) {
>> + spin_lock_irqsave(&mdev->fh_list_lock, flags);
>> + list_del(&fh->mdev_list);
>> + spin_unlock_irqrestore(&mdev->fh_list_lock, flags);
>> + }
>> +
>> + if (fh->fh.ref)
>> + kref_put(&fh->fh.ref->kref, compat_ref_release);
>>
>> kfree(fh);
>>
>> @@ -816,6 +837,8 @@ EXPORT_SYMBOL_GPL(media_device_init);
>>
>> void media_device_cleanup(struct media_device *mdev)
>> {
>> + if (mdev->devnode.ref)
>> + kref_put(&mdev->devnode.ref->kref, compat_ref_release);
>> __media_device_release(mdev);
>> media_device_put(mdev);
>> }
>> @@ -824,6 +847,7 @@ EXPORT_SYMBOL_GPL(media_device_cleanup);
>> int __must_check __media_device_register(struct media_device *mdev,
>> struct module *owner)
>> {
>> + struct media_devnode_compat_ref *ref = NULL;
>> int ret;
>>
>> /* Register the device node. */
>> @@ -833,19 +857,39 @@ int __must_check __media_device_register(struct media_device *mdev,
>> /* Set version 0 to indicate user-space that the graph is static */
>> mdev->topology_version = 0;
>>
>> + if (!mdev->ops || !mdev->ops->release) {
>> + ref = kzalloc(sizeof(*mdev->devnode.ref), GFP_KERNEL);
>> + if (!ref)
>> + return -ENOMEM;
>> +
>> + kref_init(&ref->kref);
>> + atomic_set(&ref->registered, 1);
>> + mdev->devnode.ref = ref;
>> + }
>> +
>> ret = media_devnode_register(&mdev->devnode, owner);
>> if (ret < 0)
>> - return ret;
>> + goto err_release_ref;
>>
>> ret = device_create_file(&mdev->devnode.dev, &dev_attr_model);
>> if (ret < 0) {
>> - media_devnode_unregister(&mdev->devnode);
>> - return ret;
>> + goto err_devnode_unregister;
>> }
>>
>> dev_dbg(mdev->dev, "Media device registered\n");
>>
>> return 0;
>> +
>> +err_devnode_unregister:
>> + media_devnode_unregister(&mdev->devnode);
>> +
>> +err_release_ref:
>> + if (ref) {
>> + kref_put(&ref->kref, compat_ref_release);
>> + mdev->devnode.ref = NULL;
>> + }
>> +
>> + return ret;
>> }
>> EXPORT_SYMBOL_GPL(__media_device_register);
>>
>> diff --git a/drivers/media/mc/mc-devnode.c b/drivers/media/mc/mc-devnode.c
>> index 760314dd22e1..f2cb3617df02 100644
>> --- a/drivers/media/mc/mc-devnode.c
>> +++ b/drivers/media/mc/mc-devnode.c
>> @@ -65,6 +65,14 @@ static struct bus_type media_bus_type = {
>> .name = MEDIA_NAME,
>> };
>>
>> +static bool media_devnode_is_registered_compat(struct media_devnode_fh *fh)
>> +{
>> + if (fh->ref)
>> + return atomic_read(&fh->ref->registered);
>> +
>> + return media_devnode_is_registered(fh->devnode);
>> +}
>> +
>> static ssize_t media_read(struct file *filp, char __user *buf,
>> size_t sz, loff_t *off)
>> {
>> @@ -72,7 +80,7 @@ static ssize_t media_read(struct file *filp, char __user *buf,
>>
>> if (!devnode->fops->read)
>> return -EINVAL;
>> - if (!media_devnode_is_registered(devnode))
>> + if (!media_devnode_is_registered_compat(filp->private_data))
>> return -EIO;
>> return devnode->fops->read(filp, buf, sz, off);
>> }
>> @@ -84,7 +92,7 @@ static ssize_t media_write(struct file *filp, const char __user *buf,
>>
>> if (!devnode->fops->write)
>> return -EINVAL;
>> - if (!media_devnode_is_registered(devnode))
>> + if (!media_devnode_is_registered_compat(filp->private_data))
>> return -EIO;
>> return devnode->fops->write(filp, buf, sz, off);
>> }
>> @@ -94,7 +102,7 @@ static __poll_t media_poll(struct file *filp,
>> {
>> struct media_devnode *devnode = media_devnode_data(filp);
>>
>> - if (!media_devnode_is_registered(devnode))
>> + if (!media_devnode_is_registered_compat(filp->private_data))
>> return EPOLLERR | EPOLLHUP;
>> if (!devnode->fops->poll)
>> return DEFAULT_POLLMASK;
>> @@ -106,12 +114,10 @@ __media_ioctl(struct file *filp, unsigned int cmd, unsigned long arg,
>> long (*ioctl_func)(struct file *filp, unsigned int cmd,
>> unsigned long arg))
>> {
>> - struct media_devnode *devnode = media_devnode_data(filp);
>> -
>> if (!ioctl_func)
>> return -ENOTTY;
>>
>> - if (!media_devnode_is_registered(devnode))
>> + if (!media_devnode_is_registered_compat(filp->private_data))
>> return -EIO;
>>
>> return ioctl_func(filp, cmd, arg);
>> @@ -265,6 +271,9 @@ void media_devnode_unregister(struct media_devnode *devnode)
>> if (!media_devnode_is_registered(devnode))
>> return;
>>
>> + if (devnode->ref)
>> + atomic_set(&devnode->ref->registered, 0);
>> +
>> mutex_lock(&media_devnode_lock);
>> clear_bit(MEDIA_FLAG_REGISTERED, &devnode->flags);
>> mutex_unlock(&media_devnode_lock);
>> diff --git a/include/media/media-devnode.h b/include/media/media-devnode.h
>> index d21c13829072..9ea55c53e5cb 100644
>> --- a/include/media/media-devnode.h
>> +++ b/include/media/media-devnode.h
>> @@ -20,6 +20,7 @@
>> #include <linux/fs.h>
>> #include <linux/device.h>
>> #include <linux/cdev.h>
>> +#include <linux/kref.h>
>>
>> struct media_devnode;
>>
>> @@ -55,9 +56,31 @@ struct media_file_operations {
>> int (*release) (struct file *);
>> };
>>
>> +/**
>> + * struct media_devnode_compat_ref - Workaround for drivers not managing media
>> + * device lifetime
>> + *
>> + * The purpose if this struct is to support drivers that do not manage the
>> + * lifetime of their respective media devices to avoid the worst effects of
>> + * this, namely an IOCTL call on an open file handle to a device that has been
>> + * unbound causing a kernel oops systematically. This is not a fix, the proper,
>> + * reliable way to handle this is to manage the resources used by the
>> + * driver. This struct and its use can be removed once all drivers have been
>> + * converted.
>> + *
>> + * @kref: kref for the memory of this struct
>> + * @registered: is this device registered?
>> + */
>> +struct media_devnode_compat_ref {
>> + struct kref kref;
>> + atomic_t registered;
>> +};
>> +
>> /**
>> * struct media_devnode_fh - Media device node file handle
>> * @devnode: pointer to the media device node
>> + * @ref: media device compat ref, if the driver does not manage media
>> + * device lifetime
>> *
>> * This structure serves as a base for per-file-handle data storage. Media
>> * device node users embed media_devnode_fh in their custom file handle data
>> @@ -67,6 +90,7 @@ struct media_file_operations {
>> */
>> struct media_devnode_fh {
>> struct media_devnode *devnode;
>> + struct media_devnode_compat_ref *ref;
>> };
>>
>> /**
>> @@ -80,6 +104,8 @@ struct media_devnode_fh {
>> * @flags: flags, combination of the ``MEDIA_FLAG_*`` constants
>> * @release: release callback called at the end of ``media_devnode_release()``
>> * routine at media-device.c.
>> + * @ref: reference for providing best effort system call safety in device
>> + * removal
>> *
>> * This structure represents a media-related device node.
>> *
>> @@ -101,6 +127,9 @@ struct media_devnode {
>>
>> /* callbacks */
>> void (*release)(struct media_devnode *devnode);
>> +
>> + /* compat reference */
>> + struct media_devnode_compat_ref *ref;
>> };
>>
>> /* dev to media_devnode */
>
