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Commit ba5ef6dc8a82 ("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: ba5ef6dc8a82 ("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>
b16ef427ad