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, 184 insertions, 0 deletions

diff --git a/Documentation/process/debugging/gdb-kernel-debugging.rst b/Documentation/process/debugging/gdb-kernel-debugging.rst
new file mode 100644
index 000000000..53e225760
--- /dev/null
+++ b/Documentation/process/debugging/gdb-kernel-debugging.rst
@@ -0,0 +1,184 @@
+.. highlight:: none
+
+Debugging kernel and modules via gdb
+====================================
+
+The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
+interfaces allow to debug the Linux kernel and its modules during runtime
+using gdb. Gdb comes with a powerful scripting interface for python. The
+kernel provides a collection of helper scripts that can simplify typical
+kernel debugging steps. This is a short tutorial about how to enable and use
+them. It focuses on QEMU/KVM virtual machines as target, but the examples can
+be transferred to the other gdb stubs as well.
+
+
+Requirements
+------------
+
+- gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
+  for distributions)
+
+
+Setup
+-----
+
+- Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
+  www.qemu.org for more details). For cross-development,
+  https://landley.net/aboriginal/bin keeps a pool of machine images and
+  toolchains that can be helpful to start from.
+
+- Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
+  CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
+  CONFIG_FRAME_POINTER, keep it enabled.
+
+- Install that kernel on the guest, turn off KASLR if necessary by adding
+  "nokaslr" to the kernel command line.
+  Alternatively, QEMU allows to boot the kernel directly using -kernel,
+  -append, -initrd command line switches. This is generally only useful if
+  you do not depend on modules. See QEMU documentation for more details on
+  this mode. In this case, you should build the kernel with
+  CONFIG_RANDOMIZE_BASE disabled if the architecture supports KASLR.
+
+- Build the gdb scripts (required on kernels v5.1 and above)::
+
+    make scripts_gdb
+
+- Enable the gdb stub of QEMU/KVM, either
+
+    - at VM startup time by appending "-s" to the QEMU command line
+
+  or
+
+    - during runtime by issuing "gdbserver" from the QEMU monitor
+      console
+
+- cd /path/to/linux-build
+
+- Start gdb: gdb vmlinux
+
+  Note: Some distros may restrict auto-loading of gdb scripts to known safe
+  directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::
+
+    add-auto-load-safe-path /path/to/linux-build
+
+  to ~/.gdbinit. See gdb help for more details.
+
+- Attach to the booted guest::
+
+    (gdb) target remote :1234
+
+
+Examples of using the Linux-provided gdb helpers
+------------------------------------------------
+
+- Load module (and main kernel) symbols::
+
+    (gdb) lx-symbols
+    loading vmlinux
+    scanning for modules in /home/user/linux/build
+    loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
+    loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
+    loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
+    loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
+    loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
+    ...
+    loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
+
+- Set a breakpoint on some not yet loaded module function, e.g.::
+
+    (gdb) b btrfs_init_sysfs
+    Function "btrfs_init_sysfs" not defined.
+    Make breakpoint pending on future shared library load? (y or [n]) y
+    Breakpoint 1 (btrfs_init_sysfs) pending.
+
+- Continue the target::
+
+    (gdb) c
+
+- Load the module on the target and watch the symbols being loaded as well as
+  the breakpoint hit::
+
+    loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
+    loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
+    loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
+    loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
+
+    Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
+    36              btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
+
+- Dump the log buffer of the target kernel::
+
+    (gdb) lx-dmesg
+    [     0.000000] Initializing cgroup subsys cpuset
+    [     0.000000] Initializing cgroup subsys cpu
+    [     0.000000] Linux version 3.8.0-rc4-dbg+ (...
+    [     0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
+    [     0.000000] e820: BIOS-provided physical RAM map:
+    [     0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
+    [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
+    ....
+
+- Examine fields of the current task struct(supported by x86 and arm64 only)::
+
+    (gdb) p $lx_current().pid
+    $1 = 4998
+    (gdb) p $lx_current().comm
+    $2 = "modprobe\000\000\000\000\000\000\000"
+
+- Make use of the per-cpu function for the current or a specified CPU::
+
+    (gdb) p $lx_per_cpu(runqueues).nr_running
+    $3 = 1
+    (gdb) p $lx_per_cpu(runqueues, 2).nr_running
+    $4 = 0
+
+- Dig into hrtimers using the container_of helper::
+
+    (gdb) set $leftmost = $lx_per_cpu(hrtimer_bases).clock_base[0].active.rb_root.rb_leftmost
+    (gdb) p *$container_of($leftmost, "struct hrtimer", "node")
+    $5 = {
+      node = {
+        node = {
+          __rb_parent_color = 18446612686384860673,
+          rb_right = 0xffff888231da8b00,
+          rb_left = 0x0
+        },
+        expires = 1228461000000
+      },
+      _softexpires = 1228461000000,
+      function = 0xffffffff8137ab20 <tick_nohz_handler>,
+      base = 0xffff888231d9b4c0,
+      state = 1 '\001',
+      is_rel = 0 '\000',
+      is_soft = 0 '\000',
+      is_hard = 1 '\001'
+    }
+
+
+List of commands and functions
+------------------------------
+
+The number of commands and convenience functions may evolve over the time,
+this is just a snapshot of the initial version::
+
+ (gdb) apropos lx
+ function lx_current -- Return current task
+ function lx_module -- Find module by name and return the module variable
+ function lx_per_cpu -- Return per-cpu variable
+ function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
+ function lx_thread_info -- Calculate Linux thread_info from task variable
+ lx-dmesg -- Print Linux kernel log buffer
+ lx-lsmod -- List currently loaded modules
+ lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
+
+Detailed help can be obtained via "help <command-name>" for commands and "help
+function <function-name>" for convenience functions.
+
+Debugging GDB scripts
+---------------------
+
+GDB does not enable a full Python backtrace which can make debugging GDB
+scripts more difficult than necessary. The following will allow for printing a
+full backtrace of the python environment::
+
+ (gdb) set python print-stack full