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, 72 insertions, 0 deletions

diff --git a/Documentation/process/maintainer-devicetree.rst b/Documentation/process/maintainer-devicetree.rst
new file mode 100644
index 000000000..ecc3ed638
--- /dev/null
+++ b/Documentation/process/maintainer-devicetree.rst
@@ -0,0 +1,72 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+Devicetree and Open Firmware Subsystem
+======================================
+
+Other Process Documents
+-----------------------
+
+Please see the documents in Documentation/devicetree/bindings/ for information
+on how to write proper Devicetree bindings and how to submit patches.
+
+Patch Review and Handling
+-------------------------
+
+Patches handled by Devicetree maintainers are processed differently depending
+on the patch type:
+
+1. Core OF driver code, e.g. drivers/of/:
+   patches are reviewed and applied by DT maintainers.
+
+2. Devicetree bindings:
+   patches are reviewed by DT maintainers, but should be applied by subsystem
+   maintainers except in certain cases.  See also *For kernel maintainers* in
+   Documentation/devicetree/bindings/submitting-patches.rst.
+
+3. DTS and drivers:
+   DT maintainers might provide comments, but review is generally not expected.
+   DTS must pass schema checks (dtbs_check) or at least do not add any
+   new warnings.
+
+Patchwork
+~~~~~~~~~
+
+Devicetree maintainers review patches using Patchwork, so the current status of
+a patch can be checked there. For typical driver submissions, Patchwork
+receives the entire patch set, but only a few patches are usually Devicetree
+bindings that are reviewed by DT maintainers.
+
+Explanation of Patchwork statuses:
+
+ - **New**: Not yet processed by the automation toolset.
+ - **Needs ACK**: Waiting for review by DT maintainers.
+ - **Handled Elsewhere**: Non-DT patch; not being reviewed here.
+ - **RFC**: Patch was likely ignored because it was an incomplete RFC.
+ - **Changes Requested**: Patch was reviewed and DT maintainers expect changes.
+ - **Accepted**: Patch was reviewed and applied by DT maintainers to their tree.
+ - **Not Applicable**: Patch was reviewed and is likely in good shape, with a
+   *Reviewed-by* or *Acked-by* tag provided, but DT maintainers expect someone
+   else to apply it.
+
+Patch Re-review and Pinging
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Due to the high volume of email traffic, Devicetree maintainers do not read
+every email they receive and instead rely on Patchwork during the review
+process. They also often skip patches that have already been reviewed.
+
+As a result, maintainers might miss:
+
+1. Questions about already reviewed patches.
+2. Pings, for example when a patch has been reviewed by DT maintainers but has
+   not been picked up by subsystem maintainers.
+
+Such cases can be addressed by:
+
+1. Pinging DT maintainers on the IRC channel.
+2. Dropping the DT maintainer’s *Acked-by* or *Reviewed-by* tag when sending a new
+   version of the patch set, together with an explanation in the patch
+   changelog describing why the tag was removed and what is expected from DT
+   maintainers.
+