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, 48 insertions, 0 deletions

diff --git a/Documentation/admin-guide/init.rst b/Documentation/admin-guide/init.rst
new file mode 100644
index 000000000..41f06a091
--- /dev/null
+++ b/Documentation/admin-guide/init.rst
@@ -0,0 +1,48 @@
+Explaining the "No working init found." boot hang message
+=========================================================
+:Authors: Andreas Mohr <andi at lisas period de>
+          Cristian Souza <cristianmsbr at gmail period com>
+
+This document provides some high-level reasons for failure
+(listed roughly in order of execution) to load the init binary.
+
+1) **Unable to mount root FS**: Set "debug" kernel parameter (in bootloader
+   config file or CONFIG_CMDLINE) to get more detailed kernel messages.
+
+2) **init binary doesn't exist on rootfs**: Make sure you have the correct
+   root FS type (and ``root=`` kernel parameter points to the correct
+   partition), required drivers such as storage hardware (such as SCSI or
+   USB!) and filesystem (ext3, jffs2, etc.) are builtin (alternatively as
+   modules, to be pre-loaded by an initrd).
+
+3) **Broken console device**: Possibly a conflict in ``console= setup``
+   --> initial console unavailable. E.g. some serial consoles are unreliable
+   due to serial IRQ issues (e.g. missing interrupt-based configuration).
+   Try using a different ``console= device`` or e.g. ``netconsole=``.
+
+4) **Binary exists but dependencies not available**: E.g. required library
+   dependencies of the init binary such as ``/lib/ld-linux.so.2`` missing or
+   broken. Use ``readelf -d <INIT>|grep NEEDED`` to find out which libraries
+   are required.
+
+5) **Binary cannot be loaded**: Make sure the binary's architecture matches
+   your hardware. E.g. i386 vs. x86_64 mismatch, or trying to load x86 on ARM
+   hardware. In case you tried loading a non-binary file here (shell script?),
+   you should make sure that the script specifies an interpreter in its
+   shebang header line (``#!/...``) that is fully working (including its
+   library dependencies). And before tackling scripts, better first test a
+   simple non-script binary such as ``/bin/sh`` and confirm its successful
+   execution. To find out more, add code ``to init/main.c`` to display
+   kernel_execve()s return values.
+
+Please extend this explanation whenever you find new failure causes
+(after all loading the init binary is a CRITICAL and hard transition step
+which needs to be made as painless as possible), then submit a patch to LKML.
+Further TODOs:
+
+- Implement the various ``run_init_process()`` invocations via a struct array
+  which can then store the ``kernel_execve()`` result value and on failure
+  log it all by iterating over **all** results (very important usability fix).
+- Try to make the implementation itself more helpful in general, e.g. by
+  providing additional error messages at affected places.
+