In the Linux kernel, the following vulnerability has been resolved:

af_unix: Set gc_in_progress to true in unix_gc().

Igor Ushakov reported that unix_gc() could run with gc_in_progress
being false if the work is scheduled while running:

  Thread 1         Thread 2                     Thread 3
  --------         --------                     --------
                   unix_schedule_gc()           unix_schedule_gc()
                   `- if (!gc_in_progress)      `- if (!gc_in_progress)
                      |- gc_in_progress = true     |
                      `- queue_work()              |
  unix_gc() <----------------/                     |
  |                                                |- gc_in_progress = true
  ...                                              `- queue_work()
  |                                                       |
  `- gc_in_progress = false                               |
                                                          |
  unix_gc() <---------------------------------------------'
  |
  ... /* gc_in_progress == false */
  |
  `- gc_in_progress = false

unix_peek_fpl() relies on gc_in_progress not to confuse GC
by MSG_PEEK.

Let's set gc_in_progress to true in unix_gc().