Re: [PATCH] drm: Fix an unwanted master inheritance v2

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 02 Dec 2015, Thomas Hellstrom <thellstrom@vmware.com> wrote:
> A client calling drmSetMaster() using a file descriptor that was opened
> when another client was master would inherit the latter client's master
> object and all its authenticated clients.
>
> This is unwanted behaviour, and when this happens, instead allocate a
> brand new master object for the client calling drmSetMaster().
>
> Fixes a BUG() throw in vmw_master_set().
>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

Drive-by bikeshedding, the actual change might look neater (and easier
to revert if something falls apart) if you extracted the
drm_new_set_master() abstraction as a separate non-functional prep
patch. Not insisting, just a thought.

BR,
Jani.



> ---
>  drivers/gpu/drm/drm_drv.c  |  5 +++
>  drivers/gpu/drm/drm_fops.c | 84 ++++++++++++++++++++++++++++++----------------
>  include/drm/drmP.h         |  6 ++++
>  3 files changed, 67 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 9362609..7dd6728 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -160,6 +160,11 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
>  		goto out_unlock;
>  	}
>  
> +	if (!file_priv->allowed_master) {
> +		ret = drm_new_set_master(dev, file_priv);
> +		goto out_unlock;
> +	}
> +
>  	file_priv->minor->master = drm_master_get(file_priv->master);
>  	file_priv->is_master = 1;
>  	if (dev->driver->master_set) {
> diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
> index c59ce4d..6b5625e 100644
> --- a/drivers/gpu/drm/drm_fops.c
> +++ b/drivers/gpu/drm/drm_fops.c
> @@ -126,6 +126,60 @@ static int drm_cpu_valid(void)
>  }
>  
>  /**
> + * drm_new_set_master - Allocate a new master object and become master for the
> + * associated master realm.
> + *
> + * @dev: The associated device.
> + * @fpriv: File private identifying the client.
> + *
> + * This function must be called with dev::struct_mutex held.
> + * Returns negative error code on failure. Zero on success.
> + */
> +int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
> +{
> +	struct drm_master *old_master;
> +	int ret;
> +
> +	lockdep_assert_held_once(&dev->master_mutex);
> +
> +	/* create a new master */
> +	fpriv->minor->master = drm_master_create(fpriv->minor);
> +	if (!fpriv->minor->master)
> +		return -ENOMEM;
> +
> +	/* take another reference for the copy in the local file priv */
> +	old_master = fpriv->master;
> +	fpriv->master = drm_master_get(fpriv->minor->master);
> +
> +	if (dev->driver->master_create) {
> +		ret = dev->driver->master_create(dev, fpriv->master);
> +		if (ret)
> +			goto out_err;
> +	}
> +	if (dev->driver->master_set) {
> +		ret = dev->driver->master_set(dev, fpriv, true);
> +		if (ret)
> +			goto out_err;
> +	}
> +
> +	fpriv->is_master = 1;
> +	fpriv->allowed_master = 1;
> +	fpriv->authenticated = 1;
> +	if (old_master)
> +		drm_master_put(&old_master);
> +
> +	return 0;
> +
> +out_err:
> +	/* drop both references and restore old master on failure */
> +	drm_master_put(&fpriv->minor->master);
> +	drm_master_put(&fpriv->master);
> +	fpriv->master = old_master;
> +
> +	return ret;
> +}
> +
> +/**
>   * Called whenever a process opens /dev/drm.
>   *
>   * \param filp file pointer.
> @@ -189,35 +243,9 @@ static int drm_open_helper(struct file *filp, struct drm_minor *minor)
>  	mutex_lock(&dev->master_mutex);
>  	if (drm_is_primary_client(priv) && !priv->minor->master) {
>  		/* create a new master */
> -		priv->minor->master = drm_master_create(priv->minor);
> -		if (!priv->minor->master) {
> -			ret = -ENOMEM;
> +		ret = drm_new_set_master(dev, priv);
> +		if (ret)
>  			goto out_close;
> -		}
> -
> -		priv->is_master = 1;
> -		/* take another reference for the copy in the local file priv */
> -		priv->master = drm_master_get(priv->minor->master);
> -		priv->authenticated = 1;
> -
> -		if (dev->driver->master_create) {
> -			ret = dev->driver->master_create(dev, priv->master);
> -			if (ret) {
> -				/* drop both references if this fails */
> -				drm_master_put(&priv->minor->master);
> -				drm_master_put(&priv->master);
> -				goto out_close;
> -			}
> -		}
> -		if (dev->driver->master_set) {
> -			ret = dev->driver->master_set(dev, priv, true);
> -			if (ret) {
> -				/* drop both references if this fails */
> -				drm_master_put(&priv->minor->master);
> -				drm_master_put(&priv->master);
> -				goto out_close;
> -			}
> -		}
>  	} else if (drm_is_primary_client(priv)) {
>  		/* get a reference to the master */
>  		priv->master = drm_master_get(priv->minor->master);
> diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> index 0b921ae..441b26e 100644
> --- a/include/drm/drmP.h
> +++ b/include/drm/drmP.h
> @@ -309,6 +309,11 @@ struct drm_file {
>  	unsigned universal_planes:1;
>  	/* true if client understands atomic properties */
>  	unsigned atomic:1;
> +	/*
> +	 * This client is allowed to gain master privileges for @master.
> +	 * Protected by struct drm_device::master_mutex.
> +	 */
> +	unsigned allowed_master:1;
>  
>  	struct pid *pid;
>  	kuid_t uid;
> @@ -910,6 +915,7 @@ extern int drm_open(struct inode *inode, struct file *filp);
>  extern ssize_t drm_read(struct file *filp, char __user *buffer,
>  			size_t count, loff_t *offset);
>  extern int drm_release(struct inode *inode, struct file *filp);
> +extern int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv);
>  
>  				/* Mapping support (drm_vm.h) */
>  extern unsigned int drm_poll(struct file *filp, struct poll_table_struct *wait);

-- 
Jani Nikula, Intel Open Source Technology Center