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, 51 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/dm-ebs.rst b/Documentation/admin-guide/device-mapper/dm-ebs.rst
new file mode 100644
index 000000000..c09f66db5
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/dm-ebs.rst
@@ -0,0 +1,51 @@
+======
+dm-ebs
+======
+
+
+This target is similar to the linear target except that it emulates
+a smaller logical block size on a device with a larger logical block
+size.  Its main purpose is to provide emulation of 512 byte sectors on
+devices that do not provide this emulation (i.e. 4K native disks).
+
+Supported emulated logical block sizes 512, 1024, 2048 and 4096.
+
+Underlying block size can be set to > 4K to test buffering larger units.
+
+
+Table parameters
+----------------
+  <dev path> <offset> <emulated sectors> [<underlying sectors>]
+
+Mandatory parameters:
+
+    <dev path>:
+        Full pathname to the underlying block-device,
+        or a "major:minor" device-number.
+    <offset>:
+        Starting sector within the device;
+        has to be a multiple of <emulated sectors>.
+    <emulated sectors>:
+        Number of sectors defining the logical block size to be emulated;
+        1, 2, 4, 8 sectors of 512 bytes supported.
+
+Optional parameter:
+
+    <underlying sectors>:
+        Number of sectors defining the logical block size of <dev path>.
+        2^N supported, e.g. 8 = emulate 8 sectors of 512 bytes = 4KiB.
+        If not provided, the logical block size of <dev path> will be used.
+
+
+Examples:
+
+Emulate 1 sector = 512 bytes logical block size on /dev/sda starting at
+offset 1024 sectors with underlying devices block size automatically set:
+
+ebs /dev/sda 1024 1
+
+Emulate 2 sector = 1KiB logical block size on /dev/sda starting at
+offset 128 sectors, enforce 2KiB underlying device block size.
+This presumes 2KiB logical blocksize on /dev/sda or less to work:
+
+ebs /dev/sda 128 2 4