Re: [PATCH] io_uring: fix possible use after free to sqd

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Oct 15, 2020 at 05:13:35PM +0800, Xiaoguang Wang wrote:
> Reading codes finds a possible use after free issue to sqd:
>           thread1              |       thread2
> ==> io_attach_sq_data()        |
> ===> sqd = ctx_attach->sq_data;|
>                                | ==> io_put_sq_data()
>                                | ===> refcount_dec_and_test(&sqd->refs)
>                                |     If sqd->refs is zero, will free sqd.
>                                |
> ===> refcount_inc(&sqd->refs); |
>                                |
>                                | ====> kfree(sqd);
> ===> now use after free to sqd |
> 

IIUC the io_attach_sq_data() is called only during the setup of an
io_uring context, before that the fd is returned to the user space.

So, I'm not sure a second thread can call io_put_sq_data() while the
first thread is in io_attach_sq_data().

Can you check if this situation can really happen?

Thanks,
Stefano

> Use refcount_inc_not_zero() to fix this issue.
> 
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>  fs/io_uring.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 33b5cf18bb51..48e230feb704 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6868,7 +6868,11 @@ static struct io_sq_data *io_attach_sq_data(struct io_uring_params *p)
>  		return ERR_PTR(-EINVAL);
>  	}
>  
> -	refcount_inc(&sqd->refs);
> +	if (!refcount_inc_not_zero(&sqd->refs)) {
> +		fdput(f);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
>  	fdput(f);
>  	return sqd;
>  }
> -- 
> 2.17.2
>