linux-kernel - Re: [PATCH v3] [next] pcmcia: synclink_cs: replace 1-element array with flex-array member

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Date:   Sat, 17 Dec 2022 13:43:40 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
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>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] [next] pcmcia: synclink_cs: replace 1-element array
 with flex-array member

On Sat, Dec 17, 2022 at 12:59 AM Paulo Miguel Almeida
<paulo.miguel.almeida.rodenas@...il.com> 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. While at it, fix an edge case which could cause
> rx_buf_count to be 0 when max_frame_size was set to the maximum
> allowed value (65535).
>
> It's worth mentioning that struct RXBUF was allocating 1 byte "too much"
> for what is required (ignoring bytes added by padding).
>
> 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].

...

>  static int rx_alloc_buffers(MGSLPC_INFO *info)
>  {
>         /* each buffer has header and data */
> -       info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +       if (check_add_overflow(sizeof(RXBUF), info->max_frame_size, &info->rx_buf_size))
> +               return -EINVAL;
>
> -       /* calculate total allocation size for 8 buffers */
> -       info->rx_buf_total_size = info->rx_buf_size * 8;

> +       /* try to alloc as many buffers that can fit within RXBUF_MAX_SIZE (up to 8) */
> +       if (check_mul_overflow(info->rx_buf_size, 8, &info->rx_buf_total_size))
> +               return -EINVAL;

This check is implied by kcalloc(). But to make it effective we
probably need to get a count first.

> -       /* limit total allocated memory */
> -       if (info->rx_buf_total_size > 0x10000)
> -               info->rx_buf_total_size = 0x10000;
> +       if (info->rx_buf_total_size > RXBUF_MAX_SIZE)
> +               info->rx_buf_total_size = RXBUF_MAX_SIZE;

If max_frame_size > 8192 - sizeof(RXBUF), we bump into this condition...

>         /* calculate number of buffers */
>         info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;

...which means that rx_buf_count < 8...
(and if max_frame_size > RXBUF_MAX_SIZE - sizeof(RXBUF), count becomes
0, I don't know if below clamp_val() is the only place to guarantee
that)

> -       info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);
> +       info->rx_buf = kcalloc(info->rx_buf_count, info->rx_buf_size, GFP_KERNEL);

...hence rx_buf size will be less than rx_buf_total_size.

That is probably not an issue per se, but I'm wondering if the
(bigger) value of rx_buf_total_size is the problem further in the
code.

>         if (info->rx_buf == NULL)
>                 return -ENOMEM;

Maybe something like

static int rx_alloc_buffers(MGSLPC_INFO *info)
{
    /* Prevent count from being 0 */
    if (->max_frame_size > MAX_FRAME_SIZE)
        return -EINVAL;
    ...
   count = ...;
   ...
   rx_total_size = ...
   rx_buf = kcalloc(...);

Then you don't need to check overflow with check_add_overflow() and
check_mul_overflow() will be inside the kcalloc.

...

> -       if (info->max_frame_size < 4096)
> -               info->max_frame_size = 4096;
> -       else if (info->max_frame_size > 65535)
> -               info->max_frame_size = 65535;
> +       if (info->max_frame_size < MGSLPC_MIN_FRAME_SIZE)
> +               info->max_frame_size = MGSLPC_MIN_FRAME_SIZE;
> +       else if (info->max_frame_size > MGSLPC_MAX_FRAME_SIZE)
> +               info->max_frame_size = MGSLPC_MAX_FRAME_SIZE;

You can use clamp_val() macro here.

-- 
With Best Regards,
Andy Shevchenko