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Date: Wed, 14 Dec 2022 12:26:01 -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 Thu, Dec 15, 2022 at 09:09:46AM +1300, Paulo Miguel Almeida 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].
> > >
> > > 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
>
> Hi Kees, Hi Andy, Thanks for taking the time to review this patch.
>
> 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;
Ah-ha, now I see.
>
> 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.
Yeah, I think your v2 is fine. Perhaps explicitly repeat the notes about
struct size padding in a v3 commit log?
--
Kees Cook
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