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

[Kees Cook <keescook@chromium.org>]

On Sat, Dec 17, 2022 at 11:59:06AM +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. 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).

What was the result of using __packed to make sure there wasn't a sizing
error?

-Kees

> 
> 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@gmail.com>
> ---
> Changelog:
> 
> - v3:
>   fix size calculation mistakes using overflow.h macros: (Req: Andy
>   Shevchenko, Kees Cook)
>   add notes struct RXBUF size (Kees Cook)
> 
> - v2: removed changes to how the size of RXBUF was calculated. I
>   changed my mind after thinking about the existing padding in the
>   struct. Happy to discuss it if anyone sees it differently.
> 
> - v1: https://lore.kernel.org/lkml/Y5mMWEtHWKOiPVU+@mail.google.com/
> ---
>  drivers/char/pcmcia/synclink_cs.c | 33 +++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> index b2735be81ab2..eee6772a0978 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 */
> @@ -229,12 +229,18 @@ typedef struct _mgslpc_info {
>  } MGSLPC_INFO;
>  
>  #define MGSLPC_MAGIC 0x5402
> +#define MGSLPC_MAX_FRAME_SIZE 65535
> +#define MGSLPC_MIN_FRAME_SIZE 4096
>  
>  /*
>   * The size of the serial xmit buffer is 1 page, or 4096 bytes
>   */
>  #define TXBUFSIZE 4096
>  
> +/*
> + * RXBUF accommodates at least 1 buffer (header+data) of MGSLPC_MAX_FRAME_SIZE
> + */
> +#define RXBUF_MAX_SIZE (sizeof(RXBUF) + MGSLPC_MAX_FRAME_SIZE)
>  
>  #define CHA     0x00   /* channel A offset */
>  #define CHB     0x40   /* channel B offset */
> @@ -529,7 +535,7 @@ static int mgslpc_probe(struct pcmcia_device *link)
>  	tty_port_init(&info->port);
>  	info->port.ops = &mgslpc_port_ops;
>  	INIT_WORK(&info->task, bh_handler);
> -	info->max_frame_size = 4096;
> +	info->max_frame_size = MGSLPC_MIN_FRAME_SIZE;
>  	init_waitqueue_head(&info->status_event_wait_q);
>  	init_waitqueue_head(&info->event_wait_q);
>  	spin_lock_init(&info->lock);
> @@ -2611,19 +2617,20 @@ 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;
> +	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;
>  
> -	/* 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;
>  
>  	/* calculate number of buffers */
>  	info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
>  
> -	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);
>  	if (info->rx_buf == NULL)
>  		return -ENOMEM;
>  
> @@ -2695,10 +2702,10 @@ static int mgslpc_add_device(MGSLPC_INFO *info)
>  		current_dev->next_device = info;
>  	}
>  
> -	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;
>  
>  	printk("SyncLink PC Card %s:IO=%04X IRQ=%d\n",
>  		info->device_name, info->io_base, info->irq_level);
> -- 
> 2.38.1
> 

-- 
Kees Cook