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

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Date:   Wed, 14 Dec 2022 22:39:52 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
Cc:     Kees Cook <keescook@...omium.org>, 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] [next] pcmcia: synclink_cs: replace 1-element array with
 flex-array member

On Wed, Dec 14, 2022 at 10:09 PM Paulo Miguel Almeida
<paulo.miguel.almeida.rodenas@...il.com> wrote:
> On Wed, Dec 14, 2022 at 11:29:37AM -0800, Kees Cook wrote:
> > 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].

...

> > >  typedef struct {
> > >     int count;
> > >     unsigned char status;
> > > -   char data[1];
> > > +   char data[];
> > >  } RXBUF;

...

> As both of you had similar points, I will reply them here.
>
> The reasons why it had no binary changes was because of the combination
> of this 2 things:
>
> 1) Existing padding - so sizeof(RXBUF) returned 8 bytes in both cases.
>
> pahole -C RXBUF gcc/before/drivers/char/pcmcia/synclink_cs.ko
> typedef struct {
>         int                        count;                /*     0     4 */
>         unsigned char              status;               /*     4     1 */
>         char                       data[1];              /*     5     1 */
>
>         /* size: 8, cachelines: 1, members: 3 */
>         /* padding: 2 */
>         /* last cacheline: 8 bytes */
> } RXBUF;
>
> pahole -C RXBUF gcc/after/drivers/char/pcmcia/synclink_cs.ko
> 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;

Yes, and Try to make it work with __packed. As I said, the problem is
that the code is relying on something which is architecture dependent
strictly speaking. And hence I disagree with Kees that v2 is okay to
go.

> 2) RXBUF (as implemented now) is just  like a pair of lenses from which a
> developer can have access to one of the circular buffers in MGSLPC_INFO
> struct called 'rx_buf'.

> 2611 static int rx_alloc_buffers(MGSLPC_INFO *info)
> 2612 {
> 2613         /* each buffer has header and data */
> 2614         info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> 2615
> 2616         /* calculate total allocation size for 8 buffers */
> 2617         info->rx_buf_total_size = info->rx_buf_size * 8;
> 2618
> 2619         /* limit total allocated memory */
> 2620         if (info->rx_buf_total_size > 0x10000)
> 2621                 info->rx_buf_total_size = 0x10000;
> 2622
> 2623         /* calculate number of buffers */
> 2624         info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
> 2625
> 2626         info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);
>
> To be honest, char data[_1_] in RXBUF was never required to be there.
> The code base seems to make sure that it doesn't run past its limits by
> keeping track of size buffer on MGSLPC_INFO->rx_buf_size (and sometimes
> RXBUF->count)
>
> (Addressing one point made by Andy about using of of the macros in
> overflow.h)
>         struct_size(buf, data, 1) would return 9 bytes which could
>         potentially break the existing driver as it produces binary
>         changes.

You got it incorrectly. I believe you should use something different than 1.
In previous lines in the function it multiplies sizeof + max_frame_size by 8.

The full change should be something like

check_add(sizeof(), max_frame_size)
kcalloc(8, size)

Think about it.

> Let me know your thoughts

-- 
With Best Regards,
Andy Shevchenko