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, 34 insertions, 0 deletions

diff --git a/Documentation/userspace-api/ELF.rst b/Documentation/userspace-api/ELF.rst
new file mode 100644
index 000000000..ac8aeacd4
--- /dev/null
+++ b/Documentation/userspace-api/ELF.rst
@@ -0,0 +1,34 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+Linux-specific ELF idiosyncrasies
+=================================
+
+Definitions
+===========
+
+"First" program header is the one with the smallest offset in the file:
+e_phoff.
+
+"Last" program header is the one with the biggest offset in the file:
+e_phoff + (e_phnum - 1) * sizeof(Elf_Phdr).
+
+PT_INTERP
+=========
+
+First PT_INTERP program header is used to locate the filename of ELF
+interpreter. Other PT_INTERP headers are ignored (since Linux 2.4.11).
+
+PT_GNU_STACK
+============
+
+Last PT_GNU_STACK program header defines userspace stack executability
+(since Linux 2.6.6). Other PT_GNU_STACK headers are ignored.
+
+PT_GNU_PROPERTY
+===============
+
+ELF interpreter's last PT_GNU_PROPERTY program header is used (since
+Linux 5.8). If interpreter doesn't have one, then the last PT_GNU_PROPERTY
+program header of an executable is used. Other PT_GNU_PROPERTY headers
+are ignored.