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, 372 insertions, 0 deletions

diff --git a/Documentation/process/security-bugs.rst b/Documentation/process/security-bugs.rst
new file mode 100644
index 000000000..c7dfecc87
--- /dev/null
+++ b/Documentation/process/security-bugs.rst
@@ -0,0 +1,372 @@
+.. _securitybugs:
+
+Security bugs
+=============
+
+Linux kernel developers take security very seriously.  As such, we'd
+like to know when a security bug is found so that it can be fixed and
+disclosed as quickly as possible.
+
+Preparing your report
+---------------------
+
+Like with any bug report, a security bug report requires a lot of analysis work
+from the developers, so the more information you can share about the issue, the
+better.  Please review the procedure outlined in
+Documentation/admin-guide/reporting-issues.rst if you are unclear about what
+information is helpful.  The following information are absolutely necessary in
+**any** security bug report:
+
+  * **affected kernel version range**: with no version indication, your report
+    will not be processed.  Note that a time-dependent version such as "latest
+    mainline" is not acceptable.  A stable identifier such as a commit ID or an
+    exact version is required.  Versions designating kernels not coming from
+    kernel.org (such as distro kernels) are meaningless to maintainers and
+    will not be processed.  A significant part of reports are for bugs that
+    have already been fixed, so it is extremely important that vulnerabilities
+    are verified on recent versions (development tree or latest stable
+    version), at least by verifying that the code has not changed since the
+    version where it was detected.
+
+  * **description of the problem**: a detailed description of the problem, with
+    traces showing its manifestation, and why you consider that the observed
+    behavior as a problem in the kernel, is necessary.
+
+  * **reproducer**: developers will need to be able to reproduce the problem to
+    consider a fix as effective.  This includes both a way to trigger the issue
+    and a way to confirm it happens.  A reproducer with low complexity
+    dependencies will be needed (source code, shell script, sequence of
+    instructions, file-system image etc).  Binary-only executables are not
+    accepted.  Working exploits are extremely helpful and will not be released
+    without consent from the reporter, unless they are already public.  By
+    definition if an issue cannot be reproduced, it is not exploitable, thus it
+    is not a security bug.
+
+  * **conditions**: if the bug depends on certain configuration options,
+    sysctls, permissions, timing, code modifications etc, these should be
+    indicated.
+
+In addition, the following information are highly desirable:
+
+  * **suspected location of the bug**: the file names and functions where the
+    bug is suspected to be present are very important, at least to help forward
+    the report to the appropriate maintainers.  When not possible (for example,
+    "system freezes each time I run this command"), the security team will help
+    identify the source of the bug.
+
+  * **a proposed fix**: bug reporters who have analyzed the cause of a bug in
+    the source code almost always have an accurate idea on how to fix it,
+    because they spent a long time studying it and its implications.  Proposing
+    a tested fix will save maintainers a lot of time, even if the fix ends up
+    not being the right one, because it helps understand the bug.  When
+    proposing a tested fix, please always format it in a way that can be
+    immediately merged (see Documentation/process/submitting-patches.rst).
+    This will save some back-and-forth exchanges if it is accepted, and you
+    will be credited for finding and fixing this issue.  Note that in this case
+    only a ``Signed-off-by:`` tag is needed, without ``Reported-by:`` when the
+    reporter and author are the same.
+
+  * **mitigations**: very often during a bug analysis, some ways of mitigating
+    the issue appear. It is useful to share them, as they can be helpful to
+    keep end users protected during the time it takes them to apply the fix.
+
+What qualifies as a security bug
+--------------------------------
+
+It is important that most bugs are handled publicly so as to involve the widest
+possible audience and find the best solution.  By nature, bugs that are handled
+in closed discussions between a small set of participants are less likely to
+produce the best possible fix (e.g., risk of missing valid use cases, limited
+testing abilities).
+
+It turns out that the majority of the bugs reported via the security team are
+just regular bugs that have been improperly qualified as security bugs due to
+a lack of awareness of the Linux kernel's threat model, as described in
+Documentation/process/threat-model.rst, and ought to have been sent through
+the normal channels described in Documentation/admin-guide/reporting-issues.rst
+instead.
+
+The security list exists for urgent bugs that grant an attacker a capability
+they are not supposed to have on a correctly configured production system, and
+can be easily exploited, representing an imminent threat to many users.  Before
+reporting, consider whether the issue actually crosses a trust boundary on such
+a system.
+
+**If you resorted to AI assistance to identify a bug, you must treat it as
+public**. While you may have valid reasons to believe it is not, the security
+team's experience shows that bugs discovered this way systematically surface
+simultaneously across multiple researchers, often on the same day. In this
+case, do not publicly share a reproducer, as this could cause unintended harm;
+just mention that one is available and maintainers might ask for it privately
+if they need it.
+
+If you are unsure whether an issue qualifies, err on the side of reporting
+privately: the security team would rather triage a borderline report than miss
+a real vulnerability.  Reporting ordinary bugs to the security list, however,
+does not make them move faster and consumes triage capacity that other reports
+need.
+
+Identifying contacts
+--------------------
+
+The most effective way to report a security bug is to send it directly to the
+affected subsystem's maintainers and Cc: the Linux kernel security team.  Do
+not send it to a public list at this stage, unless you have good reasons to
+consider the issue as being public or trivial to discover (e.g. result of a
+widely available automated vulnerability scanning tool that can be repeated by
+anyone, or use of AI-based tools).
+
+If you're sending a report for issues affecting multiple parts in the kernel,
+even if they're fairly similar issues, please send individual messages (think
+that maintainers will not all work on the issues at the same time). The only
+exception is when an issue concerns closely related parts maintained by the
+exact same subset of maintainers, and these parts are expected to be fixed all
+at once by the same commit, then it may be acceptable to report them at once.
+
+One difficulty for most first-time reporters is to figure the right list of
+recipients to send a report to.  In the Linux kernel, all official maintainers
+are trusted, so the consequences of accidentally including the wrong maintainer
+are essentially a bit more noise for that person, i.e. nothing dramatic.  As
+such, a suitable method to figure the list of maintainers (which kernel
+security officers use) is to rely on the get_maintainer.pl script, tuned to
+only report maintainers.  This script, when passed a file name, will look for
+its path in the MAINTAINERS file to figure a hierarchical list of relevant
+maintainers.  Calling it a first time with the finest level of filtering will
+most of the time return a short list of this specific file's maintainers::
+
+  $ ./scripts/get_maintainer.pl --no-l --no-r --pattern-depth 1 \
+    drivers/example.c
+  Developer One <dev1@example.com> (maintainer:example driver)
+  Developer Two <dev2@example.org> (maintainer:example driver)
+
+These two maintainers should then receive the message.  If the command does not
+return anything, it means the affected file is part of a wider subsystem, so we
+should be less specific::
+
+  $ ./scripts/get_maintainer.pl --no-l --no-r drivers/example.c
+  Developer One <dev1@example.com> (maintainer:example subsystem)
+  Developer Two <dev2@example.org> (maintainer:example subsystem)
+  Developer Three <dev3@example.com> (maintainer:example subsystem [GENERAL])
+  Developer Four <dev4@example.org> (maintainer:example subsystem [GENERAL])
+
+Here, picking the first, most specific ones, is sufficient.  When the list is
+long, it is possible to produce a comma-delimited e-mail address list on a
+single line suitable for use in the To: field of a mailer like this::
+
+  $ ./scripts/get_maintainer.pl --no-tree --no-l --no-r --no-n --m \
+    --no-git-fallback --no-substatus --no-rolestats --no-multiline \
+    --pattern-depth 1 drivers/example.c
+  dev1@example.com, dev2@example.org
+
+or this for the wider list::
+
+  $ ./scripts/get_maintainer.pl --no-tree --no-l --no-r --no-n --m \
+    --no-git-fallback --no-substatus --no-rolestats --no-multiline \
+    drivers/example.c
+  dev1@example.com, dev2@example.org, dev3@example.com, dev4@example.org
+
+If at this point you're still facing difficulties spotting the right
+maintainers, **and only in this case**, it's possible to send your report to
+the Linux kernel security team only.  Your message will be triaged, and you
+will receive instructions about whom to contact, if needed.  Your message may
+equally be forwarded as-is to the relevant maintainers.
+
+Responsible use of AI to find bugs
+----------------------------------
+
+A significant fraction of bug reports submitted to the security team are
+actually the result of code reviews assisted by AI tools. While this can be an
+efficient means to find bugs in rarely explored areas, it causes an overload on
+maintainers, who are sometimes forced to ignore such reports due to their poor
+quality or accuracy. As such, reporters must be particularly cautious about a
+number of points which tend to make these reports needlessly difficult to
+handle:
+
+  * **Length**: AI-generated reports tend to be excessively long, containing
+    multiple sections and excessive detail. This makes it difficult to spot
+    important information such as affected files, versions, and impact. Please
+    ensure that a clear summary of the problem and all critical details are
+    presented first. Do not require triage engineers to scan multiple pages of
+    text. Configure your tools to produce concise, human-style reports.
+
+  * **Formatting**: Most AI-generated reports are littered with Markdown tags.
+    These decorations complicate the search for important information and do
+    not survive the quoting processes involved in forwarding or replying.
+    Please **always convert your report to plain text** without any formatting
+    decorations before sending it.
+
+  * **Impact Evaluation**: Many AI-generated reports lack an understanding
+    of the kernel's threat model (see Documentation/process/threat-model.rst)
+    and go to great lengths inventing theoretical consequences. This adds
+    noise and complicates triage. Please stick to verifiable facts (e.g.,
+    "this bug permits any user to gain CAP_NET_ADMIN") without enumerating
+    speculative implications. Have your tool read this documentation as
+    part of the evaluation process.
+
+  * **Reproducer**: AI-based tools are often capable of generating reproducers.
+    Please always ensure your tool provides one and **test it thoroughly**. If
+    the reproducer does not work, or if the tool cannot produce one, the
+    validity of the report should be seriously questioned. Note that since the
+    report will be posted to a public list, the reproducer should only be
+    shared upon maintainers' request.
+
+  * **Propose a Fix**: Many AI tools are actually better at writing code than
+    evaluating it. Please ask your tool to propose a fix and **test it** before
+    reporting the problem. If the fix cannot be tested because it relies on
+    rare hardware or almost extinct network protocols, the issue is likely not
+    a security bug. In any case, if a fix is proposed, it must adhere to
+    Documentation/process/submitting-patches.rst and include a 'Fixes:' tag
+    designating the commit that introduced the bug.
+
+Failure to consider these points exposes your report to the risk of being
+ignored.
+
+Use common sense when evaluating the report. If the affected file has not been
+touched for more than one year and is maintained by a single individual, it is
+likely that usage has declined and exposed users are virtually non-existent
+(e.g., drivers for very old hardware, obsolete filesystems). In such cases,
+there is no need to consume a maintainer's time with an unimportant report. If
+the issue is clearly trivial and publicly discoverable, you should report it
+directly to the public mailing lists.
+
+At the very least, when using an AI assistant to find and report bugs, make the
+assistant read this file and threat-model.rst before proceeding, and have it
+follow the procedure described in coding-assistants.rst: work on an up-to-date
+mainline tree and note the commit ID, verify the bug is real, write a fix,
+build it warning-free and checkpatch-clean, commit it with a Fixes tag, and
+identify the maintainers with get_maintainer.pl.
+
+On top of that procedure, the AI assistant **MUST**:
+
+1. Prepare a plain-text report explaining the problem. It must contain the
+   four items listed at the top of this file as absolutely necessary: the
+   affected version or commit ID noted while following the procedure above,
+   the description of the problem, the reproducer or its status, and the
+   triggering conditions.
+2. Start the report with a temporary section listing the recipients' addresses
+   (maintainers+list for the patch, maintainers only for the report and
+   reproducer), and with instructions reminding the reporter to check that
+   their email client is properly setup (see email-clients.rst), and leave it
+   to the reporter to remove that temporary section.
+
+A more detailed process is covered at https://github.com/masoncl/kres.git.
+
+Sending the report
+------------------
+
+Reports are to be sent over e-mail exclusively.  Please use a working e-mail
+address, preferably the same that you want to appear in ``Reported-by`` tags
+if any.  If unsure, send your report to yourself first.
+
+The security team and maintainers almost always require additional
+information beyond what was initially provided in a report and rely on
+active and efficient collaboration with the reporter to perform further
+testing (e.g., verifying versions, configuration options, mitigations, or
+patches). Before contacting the security team, the reporter must ensure
+they are available to explain their findings, engage in discussions, and
+run additional tests.  Reports where the reporter does not respond promptly
+or cannot effectively discuss their findings may be abandoned if the
+communication does not quickly improve.
+
+The report must be sent to maintainers.  If there are two or fewer
+recipients in your message, you must also always Cc: the Linux kernel
+security team who will ensure the message is delivered to the proper
+people, and will be able to assist small maintainer teams with processes
+they may not be familiar with.  For larger teams, Cc: the Linux kernel
+security team for your first few reports or when seeking specific help,
+such as when resending a message which got no response within a week.
+Once you have become comfortable with the process for a few reports, it is
+no longer necessary to Cc: the security list when sending to large teams.
+The Linux kernel security team can be contacted by email at
+<security@kernel.org>.  This is a private list of security officers
+who will help verify the bug report and assist developers working on a fix.
+It is possible that the security team will bring in extra help from area
+maintainers to understand and fix the security vulnerability.
+
+Please send **plain text** emails without attachments where possible.
+It is much harder to have a context-quoted discussion about a complex
+issue if all the details are hidden away in attachments.  Think of it like a
+:doc:`regular patch submission <../process/submitting-patches>`
+(even if you don't have a patch yet): describe the problem and impact, list
+reproduction steps, and follow it with a proposed fix, all in plain text.
+Markdown, HTML and RST formatted reports are particularly frowned upon since
+they're quite hard to read for humans and encourage to use dedicated viewers,
+sometimes online, which by definition is not acceptable for a confidential
+security report. Note that some mailers tend to mangle formatting of plain
+text by default, please consult Documentation/process/email-clients.rst for
+more info.
+
+Disclosure and embargoed information
+------------------------------------
+
+The security list is not a disclosure channel.  For that, see Coordination
+below.
+
+Once a robust fix has been developed, the release process starts.  Fixes
+for publicly known bugs are released immediately.
+
+Although our preference is to release fixes for publicly undisclosed bugs
+as soon as they become available, this may be postponed at the request of
+the reporter or an affected party for up to 7 calendar days from the start
+of the release process, with an exceptional extension to 14 calendar days
+if it is agreed that the criticality of the bug requires more time.  The
+only valid reason for deferring the publication of a fix is to accommodate
+the logistics of QA and large scale rollouts which require release
+coordination.
+
+While embargoed information may be shared with trusted individuals in
+order to develop a fix, such information will not be published alongside
+the fix or on any other disclosure channel without the permission of the
+reporter.  This includes but is not limited to the original bug report
+and followup discussions (if any), exploits, CVE information or the
+identity of the reporter.
+
+In other words our only interest is in getting bugs fixed.  All other
+information submitted to the security list and any followup discussions
+of the report are treated confidentially even after the embargo has been
+lifted, in perpetuity.
+
+Coordination with other groups
+------------------------------
+
+While the kernel security team solely focuses on getting bugs fixed,
+other groups focus on fixing issues in distros and coordinating
+disclosure between operating system vendors.  Coordination is usually
+handled by the "linux-distros" mailing list and disclosure by the
+public "oss-security" mailing list, both of which are closely related
+and presented in the linux-distros wiki:
+<https://oss-security.openwall.org/wiki/mailing-lists/distros>
+
+Please note that the respective policies and rules are different since
+the 3 lists pursue different goals.  Coordinating between the kernel
+security team and other teams is difficult since for the kernel security
+team occasional embargoes (as subject to a maximum allowed number of
+days) start from the availability of a fix, while for "linux-distros"
+they start from the initial post to the list regardless of the
+availability of a fix.
+
+As such, the kernel security team strongly recommends that as a reporter
+of a potential security issue you DO NOT contact the "linux-distros"
+mailing list UNTIL a fix is accepted by the affected code's maintainers
+and you have read the distros wiki page above and you fully understand
+the requirements that contacting "linux-distros" will impose on you and
+the kernel community.  This also means that in general it doesn't make
+sense to Cc: both lists at once, except maybe for coordination if and
+while an accepted fix has not yet been merged.  In other words, until a
+fix is accepted do not Cc: "linux-distros", and after it's merged do not
+Cc: the kernel security team.
+
+CVE assignment
+--------------
+
+The security team does not assign CVEs, nor do we require them for
+reports or fixes, as this can needlessly complicate the process and may
+delay the bug handling.  If a reporter wishes to have a CVE identifier
+assigned for a confirmed issue, they can contact the :doc:`kernel CVE
+assignment team<../process/cve>` to obtain one.
+
+Non-disclosure agreements
+-------------------------
+
+The Linux kernel security team is not a formal body and therefore unable
+to enter any non-disclosure agreements.