Concurrent Ruby: `ReentrantReadWriteLock` read-count overflow grants a write lock without exclusivityCVE-2026-54905

### Summary `Concurrent::ReentrantReadWriteLock` can incorrectly grant a write lock after one thread acquires the read lock 32,768 times. The lock stores a thread's local read and write hold counts in one integer. The low 15 bits are used for the read hold count, and bit 15 is used as `WRITE_LOCK_HELD`. After 32,768 reentrant read acquisitions, the local read count crosses into the write-lock bit. `try_write_lock` then treats the thread as already holding a write lock and returns `true` without setting the global `RUNNING_WRITER` bit. This breaks the core mutual-exclusion guarantee: the caller is told it has a write lock, but other threads can still hold or acquire read locks at the same time. ### Version Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab ### Details The implementation uses a shared counter to track global readers/writers and a per-thread local counter to support reentrancy: ```ruby READER_BITS = 15 WRITER_BITS = 14 WAITING_WRITER = 1 << READER_BITS RUNNING_WRITER = 1 << (READER_BITS + WRITER_BITS) MAX_READERS = WAITING_WRITER - 1 MAX_WRITERS = RUNNING_WRITER - MAX_READERS - 1 WRITE_LOCK_HELD = 1 << READER_BITS READ_LOCK_MASK = WRITE_LOCK_HELD - 1 WRITE_LOCK_MASK = MAX_WRITERS ``` When a thread already holds a lock, `acquire_read_lock` increments `@HeldCount`: ```ruby if (held = @HeldCount.value) > 0 if held & READ_LOCK_MASK == 0 @Counter.update { |c| c + 1 } end @HeldCount.value = held + 1 return true end ``` After 32,768 read acquisitions, the per-thread held count becomes `32768`, which is equal to `WRITE_LOCK_HELD`. Then `try_write_lock` returns success through its "already have a write lock" branch: ```ruby def try_write_lock if (held = @HeldCount.value) >= WRITE_LOCK_HELD @HeldCount.value = held + WRITE_LOCK_HELD return true else # normal global writer acquisition path end end ``` This branch does not set the global `RUNNING_WRITER` bit. Other threads therefore do not observe an active writer and can continue holding or acquiring read locks while the caller believes it owns the write lock. ### PoC ```ruby #!/usr/bin/env ruby # frozen_string_literal: true require 'concurrent/atomic/reentrant_read_write_lock' require 'concurrent/version' require 'thread' def wait_for_queue(queue, timeout_seconds) deadline = Process.clock_gettime(Process::CLOCK_MONOTONIC) + timeout_seconds loop do return queue.pop(true) rescue ThreadError return nil if Process.clock_gettime(Process::CLOCK_MONOTONIC) >= deadline sleep 0.001 end end puts "ruby=#{RUBY_DESCRIPTION}" puts "concurrent_ruby_version=#{Concurrent::VERSION}" puts "poc=ReentrantReadWriteLock read-depth overflow grants write lock without exclusivity" lock = Concurrent::ReentrantReadWriteLock.new other_reader_ready = Queue.new other_reader_stop = Queue.new other_reader = Thread.new do lock.acquire_read_lock other_reader_ready << :held other_reader_stop.pop end wait_for_queue(other_reader_ready, 1) puts "other_thread_holds_read_lock=true" depth = Concurrent::ReentrantReadWriteLock::WRITE_LOCK_HELD depth.times { lock.acquire_read_lock } held_count = lock.instance_eval { @HeldCount.value } counter_before = lock.instance_eval { @Counter.value } puts "main_thread_read_acquisitions=#{depth}" puts "main_thread_held_count=#{held_count}" puts "counter_before_try_write=#{counter_before}" puts "running_writer_bit_before=#{(counter_before & Concurrent::ReentrantReadWriteLock::RUNNING_WRITER) != 0}" write_granted = lock.try_write_lock counter_after = lock.instance_eval { @Counter.value } puts "try_write_lock_returned=#{write_granted}" puts "counter_after_try_write=#{counter_after}" puts "running_writer_bit_after=#{(counter_after & Concurrent::ReentrantReadWriteLock::RUNNING_WRITER) != 0}" third_reader_ready = Queue.new third_reader = Thread.new do lock.acquire_read_lock third_reader_ready << :acquired end third_reader_acquired = wait_for_queue(third_reader_ready, 0.25) == :acquired puts "new_reader_acquired_while_write_claimed=#{third_reader_acquired}" if write_granted && third_reader_acquired && (counter_after & Concurrent::ReentrantReadWriteLock::RUNNING_WRITER).zero? puts 'result=REPRODUCED write lock granted without setting global writer state' else puts 'result=NOT_REPRODUCED' end third_reader.kill other_reader_stop << :stop other_reader.kill ``` ### Log evidence ```text ruby=ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.arm64e-darwin25] concurrent_ruby_version=1.3.6 poc=ReentrantReadWriteLock read-depth overflow grants write lock without exclusivity other_thread_holds_read_lock=true main_thread_read_acquisitions=32768 main_thread_held_count=32768 counter_before_try_write=2 running_writer_bit_before=false try_write_lock_returned=true counter_after_try_write=2 running_writer_bit_after=false new_reader_acquired_while_write_claimed=true result=REPRODUCED write lock granted without setting global writer state ``` ### Impact This breaks the write-lock exclusivity guarantee. After the overflow, a thread can be told it has acquired the write lock while other threads can still hold or acquire read locks, allowing races and inconsistent reads of protected mutable state. ### Credit Pranjali Thakur - depthfirst ([depthfirst.com](<http://depthfirst.com>))

Quoted source text, attributed separately from HOL analysis.