Re: [PATCH] [v3] media: s3c-camif: fix out-of-bounds array access

From: Laurent Pinchart
Date: Tue Jan 16 2018 - 15:18:00 EST


Hi Arnd,

Thank you for the patch.

On Tuesday, 16 January 2018 18:47:24 EET Arnd Bergmann wrote:
> While experimenting with older compiler versions, I ran
> into a warning that no longer shows up on gcc-4.8 or newer:
>
> drivers/media/platform/s3c-camif/camif-capture.c: In function
> '__camif_subdev_try_format':
> drivers/media/platform/s3c-camif/camif-capture.c:1265:25: error: array
> subscript is below array bounds
>
> This is an off-by-one bug, leading to an access before the start of the
> array, while newer compilers silently assume this undefined behavior
> cannot happen and leave the loop at index 0 if no other entry matches.
>
> As Sylvester explains, we actually need to ensure that the
> value is within the range, so this reworks the loop to be
> easier to parse correctly, and an additional check to fall
> back on the first format value for any unexpected input.
>
> I found an existing gcc bug for it and added a reduced version
> of the function there.
>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69249#c3
> Fixes: babde1c243b2 ("[media] V4L: Add driver for S3C24XX/S3C64XX SoC series
> camera interface") Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> v3: fix newly introduced off-by-one bug.
> v2: rework logic rather than removing it.
> ---
> drivers/media/platform/s3c-camif/camif-capture.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/media/platform/s3c-camif/camif-capture.c
> b/drivers/media/platform/s3c-camif/camif-capture.c index
> 437395a61065..f51b92e94a32 100644
> --- a/drivers/media/platform/s3c-camif/camif-capture.c
> +++ b/drivers/media/platform/s3c-camif/camif-capture.c
> @@ -1256,16 +1256,19 @@ static void __camif_subdev_try_format(struct
> camif_dev *camif, {
> const struct s3c_camif_variant *variant = camif->variant;
> const struct vp_pix_limits *pix_lim;
> - int i = ARRAY_SIZE(camif_mbus_formats);
> + int i;
>
> /* FIXME: constraints against codec or preview path ? */
> pix_lim = &variant->vp_pix_limits[VP_CODEC];
>
> - while (i-- >= 0)
> + for (i = 0; i < ARRAY_SIZE(camif_mbus_formats); i++)
> if (camif_mbus_formats[i] == mf->code)
> break;
>
> - mf->code = camif_mbus_formats[i];
> + if (i == ARRAY_SIZE(camif_mbus_formats))
> + mf->code = camif_mbus_formats[0];
> + else
> + mf->code = camif_mbus_formats[i];

I might be missing something very obvious, but isn't mf->code already ==
camif_mbus_formats[i] in the else branch ? How about simply

unsigned int i;

/* FIXME: constraints against codec or preview path ? */
pix_lim = &variant->vp_pix_limits[VP_CODEC];

for (i = 0; i < ARRAY_SIZE(camif_mbus_formats); i++)
if (camif_mbus_formats[i] == mf->code)
break;

if (i == ARRAY_SIZE(camif_mbus_formats))
mf->code = camif_mbus_formats[0];

(I do love the for (...) { ... } else { ... } construct from Python, I miss it
so much in C.)

> if (pad == CAMIF_SD_PAD_SINK) {
> v4l_bound_align_image(&mf->width, 8, CAMIF_MAX_PIX_WIDTH,

--
Regards,

Laurent Pinchart