Re: KASAN: use-after-free Read in percpu_ref_switch_to_atomic_rcu

[Jens Axboe <axboe@kernel.dk>]

On 3/6/20 7:35 AM, Dan Carpenter wrote:
> 
> There a bunch of similar bugs.  It's seems a common anti-pattern.
> 
> block/blk-cgroup.c:85 blkg_free() warn: freeing 'blkg' which has percpu_ref_exit()
> block/blk-core.c:558 blk_alloc_queue_node() warn: freeing 'q' which has percpu_ref_exit()
> drivers/md/md.c:5528 md_free() warn: freeing 'mddev' which has percpu_ref_exit()
> drivers/target/target_core_transport.c:583 transport_free_session() warn: freeing 'se_sess' which has percpu_ref_exit()
> fs/aio.c:592 free_ioctx() warn: freeing 'ctx' which has percpu_ref_exit()
> fs/aio.c:806 ioctx_alloc() warn: freeing 'ctx' which has percpu_ref_exit()
> fs/io_uring.c:6115 io_sqe_files_unregister() warn: freeing 'data' which has percpu_ref_exit()
> fs/io_uring.c:6431 io_sqe_files_register() warn: freeing 'ctx->file_data' which has percpu_ref_exit()
> fs/io_uring.c:7134 io_ring_ctx_free() warn: freeing 'ctx' which has percpu_ref_exit()
> kernel/cgroup/cgroup.c:4948 css_free_rwork_fn() warn: freeing 'css' which has percpu_ref_exit()
> mm/backing-dev.c:615 cgwb_create() warn: freeing 'wb' which has percpu_ref_exit()

The file table io_uring issue is using the ref in a funky way, switching
in and out of atomic if we need to quiesce it. That's different from
other use cases, that just use it as a "normal" reference. Hence for the
funky use case, you can potentially have a switch in progress when you
exit the ref. You really want to wait for that, the easiest solution is
to punt the exit + free to an RCU callback, if there's nothing else you
need to handle once the switch is done.

So I would not be so quick to assume that similar patterns (exit + free)
have similar issues.

-- 
Jens Axboe