thunderbolt: Fix NULL dereference in tb_remove_work()grafted

There is a slight race between tb_remove_work() and tb_domain_remove()
which leads to dereferencing a NULL tb->root_switch pointer inside
tb_free_unplugged_xdomains():

        Thread A                            Thread B

  tb_remove_work()
                                    tb_domain_remove()
                                      mutex_lock(&tb->lock)
                                      tb_stop()
                                        /* doesn't cancel a running callback */
                                        cancel_delayed_work(&tcm->remove_work)
                                        ...
                                        tb_switch_remove(tb->root_switch)
                                        tb->root_switch = NULL
                                      mutex_unlock(&tb->lock)
    mutex_lock(&tb->lock)
    ...
    /* without checking ->root_switch */
    tb_free_unplugged_xdomains(tb->root_switch)
    mutex_unlock(&tb->lock)

Commit a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without
holding tb->lock") doesn't seem right to move tb_free_unplugged_xdomains()
out of the &tb->lock section and the check for tb->root_switch, in
particular.  It states:

  For this reason separate removing the XDomain from the topology data
  structures (where we need the lock) from unregistering the device from
  the bus (where remove callbacks of the drivers are being called).

tb_free_unplugged_xdomains() belongs to the former group of functions
requiring the lock.  And it also calls tb_xdomain_remove() which should
only be called with &tb->lock held.

Found by Linux Verification Center (linuxtesting.org) with Svace static
analysis tool.

Fixes: a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without holding tb->lock")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

1 files changed, 129 insertions, 0 deletions

diff --git a/Documentation/driver-api/mailbox.rst b/Documentation/driver-api/mailbox.rst
new file mode 100644
index 000000000..d46c60bd5
--- /dev/null
+++ b/Documentation/driver-api/mailbox.rst
@@ -0,0 +1,129 @@
+============================
+The Common Mailbox Framework
+============================
+
+:Author: Jassi Brar <jaswinder.singh@linaro.org>
+
+This document aims to help developers write client and controller
+drivers for the API. But before we start, let us note that the
+client (especially) and controller drivers are likely going to be
+very platform specific because the remote firmware is likely to be
+proprietary and implement non-standard protocol. So even if two
+platforms employ, say, PL320 controller, the client drivers can't
+be shared across them. Even the PL320 driver might need to accommodate
+some platform specific quirks. So the API is meant mainly to avoid
+similar copies of code written for each platform. Having said that,
+nothing prevents the remote f/w to also be Linux based and use the
+same api there. However none of that helps us locally because we only
+ever deal at client's protocol level.
+
+Some of the choices made during implementation are the result of this
+peculiarity of this "common" framework.
+
+
+
+Controller Driver (See include/linux/mailbox_controller.h)
+==========================================================
+
+
+Allocate mbox_controller and the array of mbox_chan.
+Populate mbox_chan_ops, except flush() and peek_data() all are mandatory.
+The controller driver might know a message has been consumed
+by the remote by getting an IRQ or polling some hardware flag
+or it can never know (the client knows by way of the protocol).
+The method in order of preference is IRQ -> Poll -> None, which
+the controller driver should set via 'txdone_irq' or 'txdone_poll'
+or neither.
+
+
+Client Driver (See include/linux/mailbox_client.h)
+==================================================
+
+
+The client might want to operate in blocking mode (synchronously
+send a message through before returning) or non-blocking/async mode (submit
+a message and a callback function to the API and return immediately).
+
+::
+
+	struct demo_client {
+		struct mbox_client cl;
+		struct mbox_chan *mbox;
+		struct completion c;
+		bool async;
+		/* ... */
+	};
+
+	/*
+	* This is the handler for data received from remote. The behaviour is purely
+	* dependent upon the protocol. This is just an example.
+	*/
+	static void message_from_remote(struct mbox_client *cl, void *mssg)
+	{
+		struct demo_client *dc = container_of(cl, struct demo_client, cl);
+		if (dc->async) {
+			if (is_an_ack(mssg)) {
+				/* An ACK to our last sample sent */
+				return; /* Or do something else here */
+			} else { /* A new message from remote */
+				queue_req(mssg);
+			}
+		} else {
+			/* Remote f/w sends only ACK packets on this channel */
+			return;
+		}
+	}
+
+	static void sample_sent(struct mbox_client *cl, void *mssg, int r)
+	{
+		struct demo_client *dc = container_of(cl, struct demo_client, cl);
+		complete(&dc->c);
+	}
+
+	static void client_demo(struct platform_device *pdev)
+	{
+		struct demo_client *dc_sync, *dc_async;
+		/* The controller already knows async_pkt and sync_pkt */
+		struct async_pkt ap;
+		struct sync_pkt sp;
+
+		dc_sync = kzalloc_obj(*dc_sync);
+		dc_async = kzalloc_obj(*dc_async);
+
+		/* Populate non-blocking mode client */
+		dc_async->cl.dev = &pdev->dev;
+		dc_async->cl.rx_callback = message_from_remote;
+		dc_async->cl.tx_done = sample_sent;
+		dc_async->cl.tx_block = false;
+		dc_async->cl.tx_tout = 0; /* doesn't matter here */
+		dc_async->cl.knows_txdone = false; /* depending upon protocol */
+		dc_async->async = true;
+		init_completion(&dc_async->c);
+
+		/* Populate blocking mode client */
+		dc_sync->cl.dev = &pdev->dev;
+		dc_sync->cl.rx_callback = message_from_remote;
+		dc_sync->cl.tx_done = NULL; /* operate in blocking mode */
+		dc_sync->cl.tx_block = true;
+		dc_sync->cl.tx_tout = 500; /* by half a second */
+		dc_sync->cl.knows_txdone = false; /* depending upon protocol */
+		dc_sync->async = false;
+
+		/* ASync mailbox is listed second in 'mboxes' property */
+		dc_async->mbox = mbox_request_channel(&dc_async->cl, 1);
+		/* Populate data packet */
+		/* ap.xxx = 123; etc */
+		/* Send async message to remote */
+		mbox_send_message(dc_async->mbox, &ap);
+
+		/* Sync mailbox is listed first in 'mboxes' property */
+		dc_sync->mbox = mbox_request_channel(&dc_sync->cl, 0);
+		/* Populate data packet */
+		/* sp.abc = 123; etc */
+		/* Send message to remote in blocking mode */
+		mbox_send_message(dc_sync->mbox, &sp);
+		/* At this point 'sp' has been sent */
+
+		/* Now wait for async chan to be done */
+		wait_for_completion(&dc_async->c);
+	}