Issue #22196 has been updated by ioquatix (Samuel Williams). Description updated Status changed from Open to Assigned Assignee set to ioquatix (Samuel Williams) Thanks for your report, I'll investigate it. ---------------------------------------- Bug #22196: Heap-use-after-free in `fiber_switch` with transfer-terminated async tasks on 3.4.10 https://bugs.ruby-lang.org/issues/22196#change-118103 * Author: scohen-tines (seth cohen) * Status: Assigned * Assignee: ioquatix (Samuel Williams) * ruby -v: ruby 3.4.10 (2026-06-30 revision 2b0b7728dc) +PRISM [aarch64-linux] * Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN ---------------------------------------- Ruby 3.4.10 intermittently segfaults while an async-http workload terminates task fibers after truncated chunked HTTP responses. The equivalent workload completed 20 high-churn attempts on Ruby 3.4.9 without a crash. ASan shows a heap-use-after-free: the outgoing fiber's `rb_fiber_t` is freed by GC while `fiber_switch` still reads its status on the `Fiber#transfer` path. ## Environment ```text ruby 3.4.10 (2026-06-30 revision 2b0b7728dc) +PRISM [aarch64-linux] ``` - Linux `aarch64` Docker image. - `async-1.32.1`, `async-http-0.64.2`, and `async-pool-0.10.3`. - The ASan build uses `-fsanitize=address -fno-omit-frame-pointer -g -O0` and has YJIT disabled. ## Expected behavior An incomplete chunked HTTP response should fail its async task with `EOFError`; the VM should not crash while cleaning up the task fiber. ## Actual behavior Ruby exits with a segmentation fault in the fiber stack pool: ```text -- C level backtrace -- fiber_pool_stack_release fiber_stack_release fiber_switch rb_fiber_start ``` One GDB run on a source-built 3.4.10 reached: ```text #0 fiber_pool_stack_release (stack=0xffffc4012ea0) at cont.c:770 pool = 0x0 #1 fiber_stack_release (fiber=0xffffc4012b00) at cont.c:903 #2 fiber_switch (...) at cont.c:2728 ``` We first hit this in a production service; the harness below reproduces the same C-level frames using only async-http against a local server, with no application code. ## AddressSanitizer result An ASan build makes the bug deterministic. With 50 in-process request bursts, all five stock 3.4.10 runs report: ```text ERROR: AddressSanitizer: heap-use-after-free READ of size 1 #0 fiber_switch /src/ruby-3.4.10/cont.c:2727 ... freed by thread T2 here: cont_free /src/ruby-3.4.10/cont.c:1079 fiber_free /src/ruby-3.4.10/cont.c:1164 ... previously allocated by thread T3 here: fiber_t_alloc /src/ruby-3.4.10/cont.c:1986 SUMMARY: AddressSanitizer: heap-use-after-free /src/ruby-3.4.10/cont.c:2727 in fiber_switch ``` Line 2727 of `cont.c` is the new `FIBER_TERMINATED_P(fiber)` condition (see Regression candidate below). Thus the first invalid operation is reading the freed `rb_fiber_t` object's status. It occurs before `fiber_pool_stack_release`; the latter is a downstream manifestation when the stale status reads as terminated and the release branch continues. The full report is attached as `stock-1.log`. ## Minimal reproduction harness The attached harness consists of: - `repro.rb`: local raw HTTP server plus direct async-http client; - `Gemfile` and `Gemfile.lock`: pinned async dependencies; - `Dockerfile` and `run.sh`: repeat the workload on a stock Ruby image. Run: ```sh ./run.sh 3.4.10 20 ``` This is probabilistic; successful runs are expected. The harness retains a log for every attempt and reports the first interpreter crash. The workload synchronizes groups of ten failures by default: ```sh FAULT_BARRIER=10 ./run.sh 3.4.10 50 ``` Run the same source comparison under ASan (deterministic): ```sh BATCHES=50 ./run-asan.sh 5 ``` `./run.sh 3.4.9 20` completes all 20 attempts without a crash; 3.4.9 predates the `148f263a` backport. ## Regression candidate Ruby 3.4.10 includes ruby_3_4 commit [`148f263a`](https://github.com/ruby/ruby/commit/148f263a0d299e4532c753470a282144c9104820), the backport of [`dc1777d`](https://github.com/ruby/ruby/commit/dc1777d01770ab62ec99ff6fa4cf622098f44968) for [Bug #21955](https://bugs.ruby-lang.org/issues/21955). That changes `fiber_switch` as follows: ```diff - if (resuming_fiber && FIBER_TERMINATED_P(fiber)) { + if (FIBER_TERMINATED_P(fiber)) { fiber_stack_release(fiber); } ``` `Fiber#transfer` passes a null `resuming_fiber`, so the old condition short-circuited without reading `fiber->status`. The new condition dereferences the stale `fiber` pointer before deciding whether to release its stack. We have not tested ruby-head/3.5, but the same condition is present there since `dc1777d`, so we expect it to be affected. ## Controlled source comparison The harness also includes: ```sh BATCHES=20 ./run-ab.sh 20 ``` It builds the published Ruby 3.4.10 source twice with the same compiler and workload: 1. stock 3.4.10; 2. 3.4.10 with only the above condition restored to `resuming_fiber && FIBER_TERMINATED_P(fiber)`. | Build | Attempts | `fiber_pool_stack_release` crashes | | --- | ---: | ---: | | stock 3.4.10 | 20 | 4 | | 3.4.10 with Bug #21955 condition reverted | 20 | 0 | The stock run also had one additional process abort with: ```text malloc(): unaligned tcache chunk detected ``` All 20 reverted-build processes completed successfully. These runs used `BATCHES=20`, 100 requests per batch, concurrency 20, and a barrier of 10 simultaneous truncated responses. The image is built from the Ruby 3.4.10 release tarball with `-O0 -g3 -fno-omit-frame-pointer`; the only source difference between the two images is the condition shown above. The ASan A/B uses `BATCHES=50`: all 5 stock runs report the above heap-use-after-free, while all 5 reverted runs complete cleanly. ## Request Could the fiber maintainers review why a `rb_fiber_t` can be finalized by GC while `fiber_switch` still holds it as the outgoing fiber? Reinstating the `resuming_fiber` guard avoids the dereference on the transfer path, but a proper fix likely needs to ensure that the outgoing fiber remains live through the switch or otherwise avoid reading it after its lifetime ends. ---Files-------------------------------- ruby-3.4.10-fiber-uaf.tar.gz (19.7 KB) stock-1.log (58.4 KB) -- https://bugs.ruby-lang.org/