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Commitba5ef6dc8a("io_uring: fortify tctx/io_wq cleanup") introduced setting tctx->io_wq to NULL a bit earlier. This has caused KCSAN to detect a data race between accesses to tctx->io_wq: write to 0xffff88811d8df330 of 8 bytes by task 3709 on cpu 1: io_uring_clean_tctx fs/io_uring.c:9042 [inline] __io_uring_cancel fs/io_uring.c:9136 io_uring_files_cancel include/linux/io_uring.h:16 [inline] do_exit kernel/exit.c:781 do_group_exit kernel/exit.c:923 get_signal kernel/signal.c:2835 arch_do_signal_or_restart arch/x86/kernel/signal.c:789 handle_signal_work kernel/entry/common.c:147 [inline] exit_to_user_mode_loop kernel/entry/common.c:171 [inline] ... read to 0xffff88811d8df330 of 8 bytes by task 6412 on cpu 0: io_uring_try_cancel_iowq fs/io_uring.c:8911 [inline] io_uring_try_cancel_requests fs/io_uring.c:8933 io_ring_exit_work fs/io_uring.c:8736 process_one_work kernel/workqueue.c:2276 ... With the config used, KCSAN only reports data races with value changes: this implies that in the case here we also know that tctx->io_wq was non-NULL. Therefore, depending on interleaving, we may end up with: [CPU 0] | [CPU 1] io_uring_try_cancel_iowq() | io_uring_clean_tctx() if (!tctx->io_wq) // false | ... ... | tctx->io_wq = NULL io_wq_cancel_cb(tctx->io_wq, ...) | ... -> NULL-deref | Note: It is likely that thus far we've gotten lucky and the compiler optimizes the double-read into a single read into a register -- but this is never guaranteed, and can easily change with a different config! Fix the data race by restoring the previous behaviour, where both setting io_wq to NULL and put of the wq are _serialized_ after concurrent io_uring_try_cancel_iowq() via acquisition of the uring_lock and removal of the node in io_uring_del_task_file(). Fixes:ba5ef6dc8a("io_uring: fortify tctx/io_wq cleanup") Suggested-by: Pavel Begunkov <asml.silence@gmail.com> Reported-by: syzbot+bf2b3d0435b9b728946c@syzkaller.appspotmail.com Signed-off-by: Marco Elver <elver@google.com> Cc: Jens Axboe <axboe@kernel.dk> Link: https://lore.kernel.org/r/20210527092547.2656514-1-elver@google.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
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