Re: [Linux-kernel-mentees] [PATCH] v4l2-tpg: Fix global-out-of-bounds read in precalculate_color()

[Hans Verkuil]

Hi Peilin,

On 10/08/2020 07:05, Peilin Ye wrote:
> precalculate_color() is reading out of `sin` since `tpg->hue` is not being
> properly checked. Fix it. `cos` is safe, as long as `tpg->hue` is higher
> than or equal to -192.

Thank you for this patch, but there is something I don't understand, namely
just *how* tpg->hue can be out-of-range.

>From what I can see vivid sets hue via tpg_s_hue() when the V4L2_CID_HUE control
is set. But that control has a range of -128...128, so ctrl->val should always be in
that range.

I would really like to know 1) what the value of tpg->hue actually is when it goes
out of range, and 2) who is changing it to that value. Can you do a bit more digging?

That said, it makes sense that precalculate_color() avoids reading out-of-bounds.

> 
> Fixes: 63881df94d3e ("[media] vivid: add the Test Pattern Generator")
> Reported-and-tested-by: syzbot+02d9172bf4c43104cd70@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://syzkaller.appspot.com/bug?id=db50123c788e2cc5a9d90de569c398b66293ee48
> Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
> ---
>  drivers/media/common/v4l2-tpg/v4l2-tpg-core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c b/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c
> index 50f1e0b28b25..52205fe096f7 100644
> --- a/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c
> +++ b/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c
> @@ -930,6 +930,9 @@ static void precalculate_color(struct tpg_data *tpg, int k)
>  		/* Implement these operations */
>  		int tmp_cb, tmp_cr;
>  
> +		if (tpg->hue < -128 || tpg->hue > 128)
> +			return;

Rather than returning here, I prefer to just clamp tpg->hue to the valid range.

I'd be willing to merge a patch that clamps tpg->hue (it certainly doesn't hurt),
but I also would like to understand how it can be out of range in the first place.
I have the feeling that this is a symptom of another problem elsewhere.

Regards,

	Hans

> +
>  		/* First convert to YCbCr */
>  
>  		color_to_ycbcr(tpg, r, g, b, &y, &cb, &cr);
>