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

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Message-ID: <202301051003.27CF3DC@keescook>
Date:   Thu, 5 Jan 2023 10:04:19 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        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] io_uring: Replace 0-length array with flexible array

On Thu, Jan 05, 2023 at 08:39:48AM +0100, Greg KH wrote:
> On Wed, Jan 04, 2023 at 07:37:48PM -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
> 
> Build problem aside, why is this a stable kernel issue?

My thinking was that since this is technically a UAPI change, it'd be
best to get it changed as widely as possible.

-- 
Kees Cook