linux-kernel - Re: [PATCH] Fix use-after-free in io_wqe_inc_running() due to wq already being free'd

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Date:   Mon, 15 Mar 2021 17:58:52 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jordy Zomer <jordy@...ing.systems>, axboe@...nel.dk
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix use-after-free in io_wqe_inc_running() due to wq
 already being free'd

On 15/03/2021 17:44, Jordy Zomer wrote:
> My syzkaller instance reported a use-after-free bug in io_wqe_inc_running.
> I tried fixing this by checking if wq isn't NULL in io_wqe_worker.
> If it does; return an -EFAULT. This because create_io_worker() will clean-up the worker if there's an error.
> 
> If you want I could send you the syzkaller reproducer and crash-logs :)

Yes, please.

Haven't looked up properly, but looks that wq==NULL should
never happen, so the fix is a bit racy.

> 
> Best Regards,
> 
> Jordy Zomer
> 
> Signed-off-by: Jordy Zomer <jordy@...ing.systems>
> ---
>  fs/io-wq.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/io-wq.c b/fs/io-wq.c
> index 0ae9ecadf295..9ed92d88a088 100644
> --- a/fs/io-wq.c
> +++ b/fs/io-wq.c
> @@ -482,6 +482,10 @@ static int io_wqe_worker(void *data)
>  	char buf[TASK_COMM_LEN];
>  
>  	worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
> +
> +	if (wq == NULL)
> +		return -EFAULT;
> +
>  	io_wqe_inc_running(worker);
>  
>  	sprintf(buf, "iou-wrk-%d", wq->task_pid);
> 

-- 
Pavel Begunkov