Re: [PATCH v3 2/2] drm: protect drm_master pointers in drm_lease.c

[Daniel Vetter <daniel@ffwll.ch>]

On Sun, Jun 20, 2021 at 07:03:27PM +0800, Desmond Cheong Zhi Xi wrote:
> Currently, direct copies of drm_file->master pointers should be
> protected by drm_device.master_mutex when being dereferenced. This is
> because drm_file->master is not invariant for the lifetime of
> drm_file. If drm_file is not the creator of master, then
> drm_file->is_master is false, and a call to drm_setmaster_ioctl will
> invoke drm_new_set_master, which then allocates a new master for
> drm_file and puts the old master.
> 
> Thus, without holding drm_device.master_mutex, the old value of
> drm_file->master could be freed while it is being used by another
> concurrent process.
> 
> In drm_lease.c, there are multiple instances where drm_file->master is
> accessed and dereferenced while drm_device.master_mutex is not
> held. This makes drm_lease.c vulnerable to use-after-free bugs.
> 
> We address this issue as follows:
> 
> 1. Clarify in the kerneldoc that drm_file->master is protected by
> drm_device.master_mutex.
> 
> 2. Add a new drm_file_get_master() function that calls drm_master_get
> on drm_file->master while holding on to drm_device.master_mutex. Since
> drm_master_get increments the reference count of master, this
> prevents master from being freed until we unreference it with
> drm_master_put.
> 
> 3. In each case where drm_file->master is directly accessed and
> eventually dereferenced in drm_lease.c, we wrap the access in a call
> to the new drm_file_get_master function, then unreference the master
> pointer once we are done using it.
> 
> Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

I think this approach looks much clearer than the previous. I've found a
few smaller things below still. I think at least ...

Cheers, Daniel

> ---
>  drivers/gpu/drm/drm_auth.c  | 22 ++++++++++++++
>  drivers/gpu/drm/drm_lease.c | 57 ++++++++++++++++++++++++++-----------
>  include/drm/drm_auth.h      |  1 +
>  include/drm/drm_file.h      | 15 ++++++++--
>  4 files changed, 75 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 86d4b72e95cb..0c64a77c67a6 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -384,6 +384,28 @@ struct drm_master *drm_master_get(struct drm_master *master)
>  }
>  EXPORT_SYMBOL(drm_master_get);
>  
> +/**
> + * drm_file_get_master - reference @file_priv->master
> + * @file_priv: DRM file private
> + *
> + * Increments the reference count of @file_priv->master and returns

Does this format correctly? I'd go with "&drm_file.master of @file_priv".

> + * @file_priv->master.
> + *
> + * Master pointers returned from this function should be unreferenced using
> + * drm_master_put().
> + */
> +struct drm_master *drm_file_get_master(struct drm_file *file_priv)
> +{
> +	struct drm_master *master;
> +
> +	mutex_lock(&file_priv->master->dev->master_mutex);
> +	master = drm_master_get(file_priv->master);
> +	mutex_unlock(&file_priv->master->dev->master_mutex);
> +
> +	return master;
> +}
> +EXPORT_SYMBOL(drm_file_get_master);
> +
>  static void drm_master_destroy(struct kref *kref)
>  {
>  	struct drm_master *master = container_of(kref, struct drm_master, refcount);
> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> index da4f085fc09e..65eab82f8acc 100644
> --- a/drivers/gpu/drm/drm_lease.c
> +++ b/drivers/gpu/drm/drm_lease.c
> @@ -107,10 +107,17 @@ static bool _drm_has_leased(struct drm_master *master, int id)
>   */
>  bool _drm_lease_held(struct drm_file *file_priv, int id)
>  {
> +	bool ret;
> +	struct drm_master *master;
> +
>  	if (!file_priv || !file_priv->master)

So here we still have a ->master access outside of the locked code
section. I think the best fix for that would be to move the NULL check
into drm_file_get_master (where we grab the lock already anyway), and
update the kerneldoc to state that it might return NULL.

Same with all the checks for ->master below.

>  		return true;
>  
> -	return _drm_lease_held_master(file_priv->master, id);
> +	master = drm_file_get_master(file_priv);
> +	ret = _drm_lease_held_master(master, id);
> +	drm_master_put(&master);
> +
> +	return ret;
>  }
>  
>  /**
> @@ -132,10 +139,11 @@ bool drm_lease_held(struct drm_file *file_priv, int id)
>  	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
>  		return true;

master->lessor dereferenced outside the lock or without holding a
reference.

>  
> -	master = file_priv->master;
> +	master = drm_file_get_master(file_priv);
>  	mutex_lock(&master->dev->mode_config.idr_mutex);
>  	ret = _drm_lease_held_master(master, id);
>  	mutex_unlock(&master->dev->mode_config.idr_mutex);
> +	drm_master_put(&master);
>  	return ret;
>  }
>  
> @@ -158,7 +166,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
>  	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
>  		return crtcs_in;

Same here.

>  
> -	master = file_priv->master;
> +	master = drm_file_get_master(file_priv);
>  	dev = master->dev;
>  
>  	count_in = count_out = 0;
> @@ -177,6 +185,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
>  		count_in++;
>  	}
>  	mutex_unlock(&master->dev->mode_config.idr_mutex);
> +	drm_master_put(&master);
>  	return crtcs_out;
>  }
>  
> @@ -490,7 +499,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	size_t object_count;
>  	int ret = 0;
>  	struct idr leases;
> -	struct drm_master *lessor = lessor_priv->master;
> +	struct drm_master *lessor;
>  	struct drm_master *lessee = NULL;
>  	struct file *lessee_file = NULL;
>  	struct file *lessor_file = lessor_priv->filp;
> @@ -502,12 +511,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> -	/* Do not allow sub-leases */
> -	if (lessor->lessor) {
> -		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
> -		return -EINVAL;
> -	}
> -
>  	/* need some objects */
>  	if (cl->object_count == 0) {
>  		DRM_DEBUG_LEASE("no objects in lease\n");
> @@ -519,12 +522,22 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  		return -EINVAL;
>  	}
>  
> +	lessor = drm_file_get_master(lessor_priv);
> +	/* Do not allow sub-leases */
> +	if (lessor->lessor) {

Here we check after grabbing the reference, so looks correct.

> +		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
> +		ret = -EINVAL;
> +		goto out_lessor;
> +	}
> +
>  	object_count = cl->object_count;
>  
>  	object_ids = memdup_user(u64_to_user_ptr(cl->object_ids),
>  			array_size(object_count, sizeof(__u32)));
> -	if (IS_ERR(object_ids))
> -		return PTR_ERR(object_ids);
> +	if (IS_ERR(object_ids)) {
> +		ret = PTR_ERR(object_ids);
> +		goto out_lessor;
> +	}
>  
>  	idr_init(&leases);
>  
> @@ -535,14 +548,15 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	if (ret) {
>  		DRM_DEBUG_LEASE("lease object lookup failed: %i\n", ret);
>  		idr_destroy(&leases);
> -		return ret;
> +		goto out_lessor;
>  	}
>  
>  	/* Allocate a file descriptor for the lease */
>  	fd = get_unused_fd_flags(cl->flags & (O_CLOEXEC | O_NONBLOCK));
>  	if (fd < 0) {
>  		idr_destroy(&leases);
> -		return fd;
> +		ret = fd;
> +		goto out_lessor;
>  	}
>  
>  	DRM_DEBUG_LEASE("Creating lease\n");
> @@ -578,6 +592,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	/* Hook up the fd */
>  	fd_install(fd, lessee_file);
>  
> +	drm_master_put(&lessor);
>  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
>  	return 0;
>  
> @@ -587,6 +602,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  out_leases:
>  	put_unused_fd(fd);
>  
> +out_lessor:
> +	drm_master_put(&lessor);
>  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
>  	return ret;
>  }
> @@ -609,7 +626,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  	struct drm_mode_list_lessees *arg = data;
>  	__u32 __user *lessee_ids = (__u32 __user *) (uintptr_t) (arg->lessees_ptr);
>  	__u32 count_lessees = arg->count_lessees;
> -	struct drm_master *lessor = lessor_priv->master, *lessee;
> +	struct drm_master *lessor, *lessee;
>  	int count;
>  	int ret = 0;
>  
> @@ -620,6 +637,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> +	lessor = drm_file_get_master(lessor_priv);
>  	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
> @@ -643,6 +661,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  		arg->count_lessees = count;
>  
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> +	drm_master_put(&lessor);
>  
>  	return ret;
>  }
> @@ -662,7 +681,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  	struct drm_mode_get_lease *arg = data;
>  	__u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
>  	__u32 count_objects = arg->count_objects;
> -	struct drm_master *lessee = lessee_priv->master;
> +	struct drm_master *lessee;
>  	struct idr *object_idr;
>  	int count;
>  	void *entry;
> @@ -676,6 +695,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> +	lessee = drm_file_get_master(lessee_priv);
>  	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
> @@ -703,6 +723,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  		arg->count_objects = count;
>  
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> +	drm_master_put(&lessee);
>  
>  	return ret;
>  }
> @@ -721,7 +742,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  				void *data, struct drm_file *lessor_priv)
>  {
>  	struct drm_mode_revoke_lease *arg = data;
> -	struct drm_master *lessor = lessor_priv->master;
> +	struct drm_master *lessor;
>  	struct drm_master *lessee;
>  	int ret = 0;
>  
> @@ -731,6 +752,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> +	lessor = drm_file_get_master(lessor_priv);
>  	mutex_lock(&dev->mode_config.idr_mutex);
>  
>  	lessee = _drm_find_lessee(lessor, arg->lessee_id);
> @@ -751,6 +773,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  
>  fail:
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> +	drm_master_put(&lessor);
>  
>  	return ret;
>  }
> diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h
> index 6bf8b2b78991..f99d3417f304 100644
> --- a/include/drm/drm_auth.h
> +++ b/include/drm/drm_auth.h
> @@ -107,6 +107,7 @@ struct drm_master {
>  };
>  
>  struct drm_master *drm_master_get(struct drm_master *master);
> +struct drm_master *drm_file_get_master(struct drm_file *file_priv);
>  void drm_master_put(struct drm_master **master);
>  bool drm_is_current_master(struct drm_file *fpriv);
>  
> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
> index b81b3bfb08c8..e9931fca4ab7 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -226,9 +226,18 @@ struct drm_file {
>  	/**
>  	 * @master:
>  	 *
> -	 * Master this node is currently associated with. Only relevant if
> -	 * drm_is_primary_client() returns true. Note that this only
> -	 * matches &drm_device.master if the master is the currently active one.
> +	 * Master this node is currently associated with. Protected by struct
> +	 * &drm_device.master_mutex.
> +	 *
> +	 * Only relevant if drm_is_primary_client() returns true. Note that
> +	 * this only matches &drm_device.master if the master is the currently
> +	 * active one.
> +	 *
> +	 * When obtaining a copy of this pointer, it is recommended to either
> +	 * hold struct &drm_device.master_mutex for the duration of the
> +	 * pointer's use, or to use drm_file_get_master() if struct
> +	 * &drm_device.master_mutex is not currently held and there is no other
> +	 * need to hold it. This prevents @master from being freed during use.
>  	 *
>  	 * See also @authentication and @is_master and the :ref:`section on
>  	 * primary nodes and authentication <drm_primary_node>`.
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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