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, 71 insertions, 0 deletions

diff --git a/Documentation/process/debugging/index.rst b/Documentation/process/debugging/index.rst
new file mode 100644
index 000000000..357243e18
--- /dev/null
+++ b/Documentation/process/debugging/index.rst
@@ -0,0 +1,71 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+Debugging advice for Linux Kernel developers
+============================================
+
+general guides
+--------------
+
+.. toctree::
+   :maxdepth: 1
+
+   driver_development_debugging_guide
+   gdb-kernel-debugging
+   kgdb
+   userspace_debugging_guide
+
+subsystem specific guides
+-------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   media_specific_debugging_guide
+
+General debugging advice
+========================
+
+Depending on the issue, a different set of tools is available to track down the
+problem or even to realize whether there is one in the first place.
+
+As a first step you have to figure out what kind of issue you want to debug.
+Depending on the answer, your methodology and choice of tools may vary.
+
+Do I need to debug with limited access?
+---------------------------------------
+
+Do you have limited access to the machine or are you unable to stop the running
+execution?
+
+In this case your debugging capability depends on built-in debugging support of
+provided distribution kernel.
+The :doc:`/process/debugging/userspace_debugging_guide` provides a brief
+overview over a range of possible debugging tools in that situation. You can
+check the capability of your kernel, in most cases, by looking into config file
+within the /boot directory.
+
+Do I have root access to the system?
+------------------------------------
+
+Are you easily able to replace the module in question or to install a new
+kernel?
+
+In that case your range of available tools is a lot bigger, you can find the
+tools in the :doc:`/process/debugging/driver_development_debugging_guide`.
+
+Is timing a factor?
+-------------------
+
+It is important to understand if the problem you want to debug manifests itself
+consistently (i.e. given a set of inputs you always get the same, incorrect
+output), or inconsistently. If it manifests itself inconsistently, some timing
+factor might be at play. If inserting delays into the code does change the
+behavior, then quite likely timing is a factor.
+
+When timing does alter the outcome of the code execution using a simple
+printk() for debugging purposes may not work, a similar alternative is to use
+trace_printk() , which logs the debug messages to the trace file instead of the
+kernel log.
+
+**Copyright** ©2024 : Collabora