linux-hardening - Re: [PATCH v2] io_uring: Replace 0-length array with flexible array

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Date:   Fri, 6 Jan 2023 09:53:25 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        stable@...r.kernel.org, io-uring@...r.kernel.org,
        Dylan Yudaken <dylany@...com>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] io_uring: Replace 0-length array with flexible array

On Thu, Jan 05, 2023 at 11:05:11AM -0800, Kees Cook wrote:
> Zero-length arrays are deprecated[1]. Replace struct io_uring_buf_ring's
> "bufs" with a flexible array member. (How is the size of this array
> verified?) Detected with GCC 13, using -fstrict-flex-arrays=3:
> 
> In function 'io_ring_buffer_select',
>     inlined from 'io_buffer_select' at io_uring/kbuf.c:183:10:
> io_uring/kbuf.c:141:23: warning: array subscript 255 is outside the bounds of an interior zero-length array 'struct io_uring_buf[0]' [-Wzero-length-bounds]
>   141 |                 buf = &br->bufs[head];
>       |                       ^~~~~~~~~~~~~~~
> In file included from include/linux/io_uring.h:7,
>                  from io_uring/kbuf.c:10:
> include/uapi/linux/io_uring.h: In function 'io_buffer_select':
> include/uapi/linux/io_uring.h:628:41: note: while referencing 'bufs'
>   628 |                 struct io_uring_buf     bufs[0];
>       |                                         ^~~~
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Fixes: c7fb19428d67 ("io_uring: add support for ring mapped supplied buffers")
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Pavel Begunkov <asml.silence@...il.com>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: stable@...r.kernel.org
> Cc: io-uring@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks!
--
Gustavo

> ---
> v2: use helper since these flex arrays are in a union.
> v1: https://lore.kernel.org/lkml/20230105033743.never.628-kees@kernel.org
> ---
>  include/uapi/linux/io_uring.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 2780bce62faf..434f62e0fb72 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -625,7 +625,7 @@ struct io_uring_buf_ring {
>  			__u16	resv3;
>  			__u16	tail;
>  		};
> -		struct io_uring_buf	bufs[0];
> +		__DECLARE_FLEX_ARRAY(struct io_uring_buf, bufs);
>  	};
>  };
>  
> -- 
> 2.34.1
>