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 13:11:25 +1300
From:   Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Andy Shevchenko <andy.shevchenko@...il.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 Fri, Dec 16, 2022 at 03:42:47PM -0800, Kees Cook wrote:
> 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
> 

With or without __packed__ attribute, sufficient space would be
allocated which is good :-)

In both cases there is still some "extra space" (1 byte on __packed and
4 bytes on non-packed) but that should be negligible. OTOH, if I'm asked
to cull those bytes I am happy to do it too.

pahole -C RXBUF non-packed/drivers/char/pcmcia/synclink_cs.o
typedef struct {
	int                        count;                /*     0     4 */
	unsigned char              status;               /*     4     1 */
	char                       data[];               /*     5     0 */

	/* size: 8, cachelines: 1, members: 3 */
	/* padding: 3 */
	/* last cacheline: 8 bytes */
} RXBUF;

pahole -C RXBUF packed/drivers/char/pcmcia/synclink_cs.o
typedef struct {
	int                        count;                /*     0     4 */
	unsigned char              status;               /*     4     1 */
	char                       data[];               /*     5     0 */

	/* size: 5, cachelines: 1, members: 3 */
	/* last cacheline: 5 bytes */
} __attribute__((__packed__)) RXBUF;

- Paulo A.

> > 
> > 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>
> > ---
> > 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