Re: [PATCH v2 20/27] drm/dp_mst: Protect drm_dp_mst_port members with connection_mutex

[Lyude Paul]

On Wed, 2019-09-25 at 16:00 -0400, Sean Paul wrote:
> On Tue, Sep 03, 2019 at 04:45:58PM -0400, Lyude Paul wrote:
> > Yes-you read that right. Currently there is literally no locking in
> > place for any of the drm_dp_mst_port struct members that can be modified
> > in response to a link address response, or a connection status response.
> > Which literally means if we're unlucky enough to have any sort of
> > hotplugging event happen before we're finished with reprobing link
> > addresses, we'll race and the contents of said struct members becomes
> > undefined. Fun!
> > 
> > So, finally add some simple locking protections to our MST helpers by
> > protecting any drm_dp_mst_port members which can be changed by link
> > address responses or connection status notifications under
> > drm_device->mode_config.connection_mutex.
> > 
> > Cc: Juston Li <juston.li@xxxxxxxxx>
> > Cc: Imre Deak <imre.deak@xxxxxxxxx>
> > Cc: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
> > Cc: Harry Wentland <hwentlan@xxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/drm_dp_mst_topology.c | 144 +++++++++++++++++++-------
> >  include/drm/drm_dp_mst_helper.h       |  39 +++++--
> >  2 files changed, 133 insertions(+), 50 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > index 5101eeab4485..259634c5d6dc 100644
> > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > @@ -1354,6 +1354,7 @@ static void drm_dp_free_mst_port(struct kref *kref)
> >  		container_of(kref, struct drm_dp_mst_port, malloc_kref);
> >  
> >  	drm_dp_mst_put_mstb_malloc(port->parent);
> > +	mutex_destroy(&port->lock);
> >  	kfree(port);
> >  }
> >  
> > @@ -1906,6 +1907,36 @@ void drm_dp_mst_connector_early_unregister(struct
> > drm_connector *connector,
> >  }
> >  EXPORT_SYMBOL(drm_dp_mst_connector_early_unregister);
> >  
> > +static void
> > +drm_dp_mst_port_add_connector(struct drm_dp_mst_branch *mstb,
> > +			      struct drm_dp_mst_port *port)
> > +{
> > +	struct drm_dp_mst_topology_mgr *mgr = port->mgr;
> > +	char proppath[255];
> > +	int ret;
> > +
> > +	build_mst_prop_path(mstb, port->port_num, proppath, sizeof(proppath));
> > +	port->connector = mgr->cbs->add_connector(mgr, port, proppath);
> > +	if (!port->connector) {
> > +		ret = -ENOMEM;
> > +		goto error;
> > +	}
> > +
> > +	if ((port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV ||
> > +	     port->pdt == DP_PEER_DEVICE_SST_SINK) &&
> > +	    port->port_num >= DP_MST_LOGICAL_PORT_0) {
> > +		port->cached_edid = drm_get_edid(port->connector,
> > +						 &port->aux.ddc);
> > +		drm_connector_set_tile_property(port->connector);
> > +	}
> > +
> > +	mgr->cbs->register_connector(port->connector);
> > +	return;
> > +
> > +error:
> > +	DRM_ERROR("Failed to create connector for port %p: %d\n", port, ret);
> > +}
> > +
> >  static void
> >  drm_dp_mst_handle_link_address_port(struct drm_dp_mst_branch *mstb,
> >  				    struct drm_device *dev,
> > @@ -1913,8 +1944,12 @@ drm_dp_mst_handle_link_address_port(struct
> > drm_dp_mst_branch *mstb,
> >  {
> >  	struct drm_dp_mst_topology_mgr *mgr = mstb->mgr;
> >  	struct drm_dp_mst_port *port;
> > -	bool created = false;
> > -	int old_ddps = 0;
> > +	struct drm_dp_mst_branch *child_mstb = NULL;
> > +	struct drm_connector *connector_to_destroy = NULL;
> > +	int old_ddps = 0, ret;
> > +	u8 new_pdt = DP_PEER_DEVICE_NONE;
> > +	bool created = false, send_link_addr = false,
> > +	     create_connector = false;
> >  
> >  	port = drm_dp_get_port(mstb, port_msg->port_number);
> >  	if (!port) {
> > @@ -1923,6 +1958,7 @@ drm_dp_mst_handle_link_address_port(struct
> > drm_dp_mst_branch *mstb,
> >  			return;
> >  		kref_init(&port->topology_kref);
> >  		kref_init(&port->malloc_kref);
> > +		mutex_init(&port->lock);
> >  		port->parent = mstb;
> >  		port->port_num = port_msg->port_number;
> >  		port->mgr = mgr;
> > @@ -1937,11 +1973,17 @@ drm_dp_mst_handle_link_address_port(struct
> > drm_dp_mst_branch *mstb,
> >  		drm_dp_mst_get_mstb_malloc(mstb);
> >  
> >  		created = true;
> > -	} else {
> > -		old_ddps = port->ddps;
> >  	}
> >  
> > +	mutex_lock(&port->lock);
> > +	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> > +
> > +	if (!created)
> > +		old_ddps = port->ddps;
> > +
> >  	port->input = port_msg->input_port;
> > +	if (!port->input)
> > +		new_pdt = port_msg->peer_device_type;
> >  	port->mcs = port_msg->mcs;
> >  	port->ddps = port_msg->ddps;
> >  	port->ldps = port_msg->legacy_device_plug_status;
> > @@ -1969,44 +2011,58 @@ drm_dp_mst_handle_link_address_port(struct
> > drm_dp_mst_branch *mstb,
> >  		}
> >  	}
> >  
> > -	if (!port->input) {
> > -		int ret = drm_dp_port_set_pdt(port,
> > -					      port_msg->peer_device_type);
> > -		if (ret == 1) {
> > -			drm_dp_send_link_address(mgr, port->mstb);
> > -		} else if (ret < 0) {
> > -			DRM_ERROR("Failed to change PDT on port %p: %d\n",
> > -				  port, ret);
> > -			goto fail;
> > +	ret = drm_dp_port_set_pdt(port, new_pdt);
> > +	if (ret == 1) {
> > +		send_link_addr = true;
> > +	} else if (ret < 0) {
> > +		DRM_ERROR("Failed to change PDT on port %p: %d\n",
> > +			  port, ret);
> > +		goto fail_unlock;
> > +	}
> > +
> > +	if (send_link_addr) {
> > +		mutex_lock(&mgr->lock);
> > +		if (port->mstb) {
> > +			child_mstb = port->mstb;
> > +			drm_dp_mst_get_mstb_malloc(child_mstb);
> >  		}
> > +		mutex_unlock(&mgr->lock);
> >  	}
> >  
> > -	if (created && !port->input) {
> > -		char proppath[255];
> > +	/*
> > +	 * We unset port->connector before dropping connection_mutex so that
> > +	 * there's no chance any of the atomic MST helpers can accidentally
> > +	 * associate a to-be-destroyed connector with a port.
> > +	 */
> > +	if (port->connector && port->input) {
> > +		connector_to_destroy = port->connector;
> > +		port->connector = NULL;
> > +	} else if (!port->connector && !port->input) {
> > +		create_connector = true;
> > +	}
> >  
> > -		build_mst_prop_path(mstb, port->port_num, proppath,
> > -				    sizeof(proppath));
> > -		port->connector = (*mgr->cbs->add_connector)(mgr, port,
> > -							     proppath);
> > -		if (!port->connector)
> > -			goto fail;
> > +	drm_modeset_unlock(&dev->mode_config.connection_mutex);
> 
> Do you drop this early b/c it deadlocks with something upstack? If so, I
> wonder if you could plumb an acquire context through the appropriate
> functions to avoid needing the port->lock at all?

I'll give this a shot, as it would definitely be nicer then having port->lock
> 
> Sean
> 
> >  
> > -		if ((port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV ||
> > -		     port->pdt == DP_PEER_DEVICE_SST_SINK) &&
> > -		    port->port_num >= DP_MST_LOGICAL_PORT_0) {
> > -			port->cached_edid = drm_get_edid(port->connector,
> > -							 &port->aux.ddc);
> > -			drm_connector_set_tile_property(port->connector);
> > -		}
> > +	if (connector_to_destroy)
> > +		mgr->cbs->destroy_connector(mgr, connector_to_destroy);
> > +	else if (create_connector)
> > +		drm_dp_mst_port_add_connector(mstb, port);
> > +
> > +	mutex_unlock(&port->lock);
> >  
> > -		(*mgr->cbs->register_connector)(port->connector);
> > +	if (send_link_addr && child_mstb) {
> > +		drm_dp_send_link_address(mgr, child_mstb);
> > +		drm_dp_mst_put_mstb_malloc(child_mstb);
> >  	}
> >  
> >  	/* put reference to this port */
> >  	drm_dp_mst_topology_put_port(port);
> >  	return;
> >  
> > -fail:
> > +fail_unlock:
> > +	drm_modeset_unlock(&dev->mode_config.connection_mutex);
> > +	mutex_unlock(&port->lock);
> > +
> >  	/* Remove it from the port list */
> >  	mutex_lock(&mgr->lock);
> >  	list_del(&port->next);
> > @@ -2022,6 +2078,7 @@ static void
> >  drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch *mstb,
> >  			    struct drm_dp_connection_status_notify *conn_stat)
> >  {
> > +	struct drm_device *dev = mstb->mgr->dev;
> >  	struct drm_dp_mst_port *port;
> >  	int old_ddps;
> >  	bool dowork = false;
> > @@ -2030,6 +2087,8 @@ drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch
> > *mstb,
> >  	if (!port)
> >  		return;
> >  
> > +	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> > +
> >  	old_ddps = port->ddps;
> >  	port->mcs = conn_stat->message_capability_status;
> >  	port->ldps = conn_stat->legacy_device_plug_status;
> > @@ -2055,6 +2114,7 @@ drm_dp_mst_handle_conn_stat(struct drm_dp_mst_branch
> > *mstb,
> >  		}
> >  	}
> >  
> > +	drm_modeset_unlock(&dev->mode_config.connection_mutex);
> >  	drm_dp_mst_topology_put_port(port);
> >  	if (dowork)
> >  		queue_work(system_long_wq, &mstb->mgr->work);
> > @@ -2147,28 +2207,34 @@ drm_dp_get_mst_branch_device_by_guid(struct
> > drm_dp_mst_topology_mgr *mgr,
> >  static void drm_dp_check_and_send_link_address(struct
> > drm_dp_mst_topology_mgr *mgr,
> >  					       struct drm_dp_mst_branch *mstb)
> >  {
> > +	struct drm_device *dev = mgr->dev;
> >  	struct drm_dp_mst_port *port;
> > -	struct drm_dp_mst_branch *mstb_child;
> > +
> >  	if (!mstb->link_address_sent)
> >  		drm_dp_send_link_address(mgr, mstb);
> >  
> >  	list_for_each_entry(port, &mstb->ports, next) {
> > -		if (port->input)
> > -			continue;
> > +		struct drm_dp_mst_branch *mstb_child = NULL;
> > +
> > +		drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> >  
> > -		if (!port->ddps)
> > +		if (port->input || !port->ddps) {
> > +			drm_modeset_unlock(&dev-
> > >mode_config.connection_mutex);
> >  			continue;
> > +		}
> >  
> >  		if (!port->available_pbn)
> >  			drm_dp_send_enum_path_resources(mgr, mstb, port);
> >  
> > -		if (port->mstb) {
> > +		if (port->mstb)
> >  			mstb_child = drm_dp_mst_topology_get_mstb_validated(
> >  			    mgr, port->mstb);
> > -			if (mstb_child) {
> > -				drm_dp_check_and_send_link_address(mgr,
> > mstb_child);
> > -				drm_dp_mst_topology_put_mstb(mstb_child);
> > -			}
> > +
> > +		drm_modeset_unlock(&dev->mode_config.connection_mutex);
> > +
> > +		if (mstb_child) {
> > +			drm_dp_check_and_send_link_address(mgr, mstb_child);
> > +			drm_dp_mst_topology_put_mstb(mstb_child);
> >  		}
> >  	}
> >  }
> > diff --git a/include/drm/drm_dp_mst_helper.h
> > b/include/drm/drm_dp_mst_helper.h
> > index 7d80c38ee00e..1efbb086f7ac 100644
> > --- a/include/drm/drm_dp_mst_helper.h
> > +++ b/include/drm/drm_dp_mst_helper.h
> > @@ -45,23 +45,34 @@ struct drm_dp_vcpi {
> >  /**
> >   * struct drm_dp_mst_port - MST port
> >   * @port_num: port number
> > - * @input: if this port is an input port.
> > - * @mcs: message capability status - DP 1.2 spec.
> > - * @ddps: DisplayPort Device Plug Status - DP 1.2
> > - * @pdt: Peer Device Type
> > - * @ldps: Legacy Device Plug Status
> > - * @dpcd_rev: DPCD revision of device on this port
> > - * @num_sdp_streams: Number of simultaneous streams
> > - * @num_sdp_stream_sinks: Number of stream sinks
> > - * @available_pbn: Available bandwidth for this port.
> > + * @input: if this port is an input port. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @mcs: message capability status - DP 1.2 spec. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @ddps: DisplayPort Device Plug Status - DP 1.2. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @pdt: Peer Device Type. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @ldps: Legacy Device Plug Status. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @dpcd_rev: DPCD revision of device on this port. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @num_sdp_streams: Number of simultaneous streams. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @num_sdp_stream_sinks: Number of stream sinks. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> > + * @available_pbn: Available bandwidth for this port. Protected by
> > + * &drm_device.mode_config.connection_mutex.
> >   * @next: link to next port on this branch device
> >   * @mstb: branch device on this port, protected by
> >   * &drm_dp_mst_topology_mgr.lock
> >   * @aux: i2c aux transport to talk to device connected to this port,
> > protected
> > - * by &drm_dp_mst_topology_mgr.lock
> > + * by &drm_device.mode_config.connection_mutex.
> >   * @parent: branch device parent of this port
> >   * @vcpi: Virtual Channel Payload info for this port.
> > - * @connector: DRM connector this port is connected to.
> > + * @connector: DRM connector this port is connected to. Protected by
> > @lock.
> > + * When there is already a connector registered for this port, this is
> > also
> > + * protected by &drm_device.mode_config.connection_mutex.
> >   * @mgr: topology manager this port lives under.
> >   *
> >   * This structure represents an MST port endpoint on a device somewhere
> > @@ -100,6 +111,12 @@ struct drm_dp_mst_port {
> >  	struct drm_connector *connector;
> >  	struct drm_dp_mst_topology_mgr *mgr;
> >  
> > +	/**
> > +	 * @lock: Protects @connector. If needed, this lock should be grabbed
> > +	 * before &drm_device.mode_config.connection_mutex.
> > +	 */
> > +	struct mutex lock;
> > +
> >  	/**
> >  	 * @cached_edid: for DP logical ports - make tiling work by ensuring
> >  	 * that the EDID for all connectors is read immediately.
> > -- 
> > 2.21.0
> > 
-- 
Cheers,
	Lyude Paul