Re: [PATCH] io_uring: fix data race to avoid potential NULL-deref

[Jens Axboe]

On 5/27/21 3:25 AM, Marco Elver wrote:
> 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().

Applied, thanks.

-- 
Jens Axboe