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

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

[Arnd Bergmann]

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@arndb.de>
---
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];
 
 	if (pad == CAMIF_SD_PAD_SINK) {
 		v4l_bound_align_image(&mf->width, 8, CAMIF_MAX_PIX_WIDTH,
-- 
2.9.0

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

[Sakari Ailus]

On Tue, Jan 16, 2018 at 05:47:24PM +0100, 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@arndb.de>

Thanks!

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>

-- 
Sakari Ailus
sakari.ailus@linux.intel.com

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

[Laurent Pinchart]

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@arndb.de>
> ---
> 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

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

[Arnd Bergmann]

On Tue, Jan 16, 2018 at 9:17 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> 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@arndb.de>
>> ---
>> 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 ?

Ah, that must be what I was thinking back when I first
discussed it with Sylvester in
https://patchwork.kernel.org/patch/9950041/

Unfortunately, I hadn't given it as much thought today when
I tried to reconstruct the result to send a new version

> 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];

Yes, makes sense. I'll send a v4.

          Arnd