linux-kernel - Re: [PATCH] [next] pcmcia: synclink_cs: replace 1-element array with flex-array member

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Date:   Wed, 14 Dec 2022 11:29:37 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Haowen Bai <baihaowen@...zu.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [next] pcmcia: synclink_cs: replace 1-element array with
 flex-array member

On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> One-element arrays are deprecated, and we are replacing them with
> flexible array members instead. So, replace one-element array with
> flexible-array member in struct RXBUF and refactor the rest of the code
> accordingly.
> 
> It's worth mentioning that doing a build before/after this patch
> results in no binary output differences.
> 
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
> 
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> 
> Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
> ---
>  drivers/char/pcmcia/synclink_cs.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> index b2735be81ab2..1ab2d552f498 100644
> --- a/drivers/char/pcmcia/synclink_cs.c
> +++ b/drivers/char/pcmcia/synclink_cs.c
> @@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
>  typedef struct {
>  	int count;
>  	unsigned char status;
> -	char data[1];
> +	char data[];
>  } RXBUF;
>  
>  /* The queue of BH actions to be performed */
> @@ -2611,7 +2611,8 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
>  static int rx_alloc_buffers(MGSLPC_INFO *info)
>  {
>  	/* each buffer has header and data */
> -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +	info->rx_buf_size = max(offsetof(typeof(RXBUF), data) + 1, sizeof(RXBUF))
> +		+ info->max_frame_size;

It seems like there is an existing size bug here, and likely should be
fixed separately?

i.e. this was already allocating 1 byte "too much". I'd expect this
first:

-	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
+	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;

and then the next patch:

-	char data[1];
+	char data[];
...
-	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
+	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;

The above would induce a binary output change, and the second would not.

Though this results in what you had for the v2 patch (but I can't
believe it had no binary changes...)

-- 
Kees Cook