Re: [PATCH] io_uring: always let io_iopoll_complete() complete polled io.

[Joseph Qi <joseph.qi@linux.alibaba.com>]

On 12/2/20 7:31 PM, Xiaoguang Wang wrote:
> Abaci Fuzz reported a double-free or invalid-free BUG in io_commit_cqring():
> [   95.504842] BUG: KASAN: double-free or invalid-free in io_commit_cqring+0x3ec/0x8e0
> [   95.505921]
> [   95.506225] CPU: 0 PID: 4037 Comm: io_wqe_worker-0 Tainted: G    B
> W         5.10.0-rc5+ #1
> [   95.507434] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> [   95.508248] Call Trace:
> [   95.508683]  dump_stack+0x107/0x163
> [   95.509323]  ? io_commit_cqring+0x3ec/0x8e0
> [   95.509982]  print_address_description.constprop.0+0x3e/0x60
> [   95.510814]  ? vprintk_func+0x98/0x140
> [   95.511399]  ? io_commit_cqring+0x3ec/0x8e0
> [   95.512036]  ? io_commit_cqring+0x3ec/0x8e0
> [   95.512733]  kasan_report_invalid_free+0x51/0x80
> [   95.513431]  ? io_commit_cqring+0x3ec/0x8e0
> [   95.514047]  __kasan_slab_free+0x141/0x160
> [   95.514699]  kfree+0xd1/0x390
> [   95.515182]  io_commit_cqring+0x3ec/0x8e0
> [   95.515799]  __io_req_complete.part.0+0x64/0x90
> [   95.516483]  io_wq_submit_work+0x1fa/0x260
> [   95.517117]  io_worker_handle_work+0xeac/0x1c00
> [   95.517828]  io_wqe_worker+0xc94/0x11a0
> [   95.518438]  ? io_worker_handle_work+0x1c00/0x1c00
> [   95.519151]  ? __kthread_parkme+0x11d/0x1d0
> [   95.519806]  ? io_worker_handle_work+0x1c00/0x1c00
> [   95.520512]  ? io_worker_handle_work+0x1c00/0x1c00
> [   95.521211]  kthread+0x396/0x470
> [   95.521727]  ? _raw_spin_unlock_irq+0x24/0x30
> [   95.522380]  ? kthread_mod_delayed_work+0x180/0x180
> [   95.523108]  ret_from_fork+0x22/0x30
> [   95.523684]
> [   95.523985] Allocated by task 4035:
> [   95.524543]  kasan_save_stack+0x1b/0x40
> [   95.525136]  __kasan_kmalloc.constprop.0+0xc2/0xd0
> [   95.525882]  kmem_cache_alloc_trace+0x17b/0x310
> [   95.533930]  io_queue_sqe+0x225/0xcb0
> [   95.534505]  io_submit_sqes+0x1768/0x25f0
> [   95.535164]  __x64_sys_io_uring_enter+0x89e/0xd10
> [   95.535900]  do_syscall_64+0x33/0x40
> [   95.536465]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [   95.537199]
> [   95.537505] Freed by task 4035:
> [   95.538003]  kasan_save_stack+0x1b/0x40
> [   95.538599]  kasan_set_track+0x1c/0x30
> [   95.539177]  kasan_set_free_info+0x1b/0x30
> [   95.539798]  __kasan_slab_free+0x112/0x160
> [   95.540427]  kfree+0xd1/0x390
> [   95.540910]  io_commit_cqring+0x3ec/0x8e0
> [   95.541516]  io_iopoll_complete+0x914/0x1390
> [   95.542150]  io_do_iopoll+0x580/0x700
> [   95.542724]  io_iopoll_try_reap_events.part.0+0x108/0x200
> [   95.543512]  io_ring_ctx_wait_and_kill+0x118/0x340
> [   95.544206]  io_uring_release+0x43/0x50
> [   95.544791]  __fput+0x28d/0x940
> [   95.545291]  task_work_run+0xea/0x1b0
> [   95.545873]  do_exit+0xb6a/0x2c60
> [   95.546400]  do_group_exit+0x12a/0x320
> [   95.546967]  __x64_sys_exit_group+0x3f/0x50
> [   95.547605]  do_syscall_64+0x33/0x40
> [   95.548155]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> The reason is that once we got a non EAGAIN error in io_wq_submit_work(),
> we'll complete req by calling io_req_complete(), which will hold completion_lock
> to call io_commit_cqring(), but for polled io, io_iopoll_complete() won't
> hold completion_lock to call io_commit_cqring(), then there maybe concurrent
> access to ctx->defer_list, double free may happen.
> 
> To fix this bug, we always let io_iopoll_complete() complete polled io.
> 
> Reported-by: Abaci Fuzz <abaci@linux.alibaba.com>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>  fs/io_uring.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index a8c136a..901ca67 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6074,8 +6074,19 @@ static struct io_wq_work *io_wq_submit_work(struct io_wq_work *work)
>  	}
>  
>  	if (ret) {
> -		req_set_fail_links(req);
> -		io_req_complete(req, ret);
> +		/*
> +		 * io_iopoll_complete() does not hold completion_lock to complete
> +		 * polled io, so here for polled io, just mark it done and still let
> +		 * io_iopoll_complete() complete it.
> +		 */
> +		if (req->ctx->flags & IORING_SETUP_IOPOLL) {
> +			struct kiocb *kiocb = &req->rw.kiocb;
> +
> +			kiocb_done(kiocb, ret, NULL);
> +		} else {
> +			req_set_fail_links(req);
> +			io_req_complete(req, ret);
> +		}
>  	}
>  
>  	return io_steal_work(req);
> 

This patch can also fix another BUG I'm looking at:

[   61.359713] BUG: KASAN: double-free or invalid-free in io_dismantle_req+0x938/0xf40
...
[   61.409315] refcount_t: underflow; use-after-free.
[   61.410261] WARNING: CPU: 1 PID: 1022 at lib/refcount.c:28 refcount_warn_saturate+0x266/0x2a0
...

It blames io_put_identity() has been called more than once and then
identity->count is underflow.

Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>