Re: [RESEND PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one maximum with default num_strings

From: Daniel Thompson <daniel.thompson@linaro.org>
To: Marijn Suijten <marijn.suijten@somainline.org>
Cc: phone-devel@vger.kernel.org, Andy Gross <agross@kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Rob Herring <robh+dt@kernel.org>,
	Lee Jones <lee.jones@linaro.org>,
	Jingoo Han <jingoohan1@gmail.com>,
	~postmarketos/upstreaming@lists.sr.ht,
	AngeloGioacchino Del Regno 
	<angelogioacchino.delregno@somainline.org>,
	Konrad Dybcio <konrad.dybcio@somainline.org>,
	Martin Botka <martin.botka@somainline.org>,
	Jami Kettunen <jami.kettunen@somainline.org>,
	Pavel Dubrova <pashadubrova@gmail.com>,
	Kiran Gunda <kgunda@codeaurora.org>,
	Bryan Wu <cooloney@gmail.com>,
	linux-arm-msm@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linux-fbdev@vger.kernel.org,
	Courtney Cavin <courtney.cavin@sonymobile.com>
Subject: Re: [RESEND PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one maximum with default num_strings
Date: Fri, 12 Nov 2021 12:08:39 +0000	[thread overview]
Message-ID: <20211112120839.i6g747vewg6bkyk7@maple.lan> (raw)
In-Reply-To: <20211112002706.453289-5-marijn.suijten@somainline.org>

On Fri, Nov 12, 2021 at 01:26:57AM +0100, Marijn Suijten wrote:
> When not specifying num-strings in the DT the default is used, but +1 is
> added to it which turns WLED3 into 4 and WLED4/5 into 5 strings instead
> of 3 and 4 respectively, causing out-of-bounds reads and register
> read/writes.  This +1 exists for a deficiency in the DT parsing code,
> and is simply omitted entirely - solving this oob issue - by parsing the
> property separately much like qcom,enabled-strings.
> 
> This also allows more stringent checks on the maximum value when
> qcom,enabled-strings is provided in the DT.  Note that num-strings is
> parsed after enabled-strings to give it final sign-off over the length,
> which DT currently utilizes to get around an incorrect fixed read of
> four elements from that array (has been addressed in a prior patch).
> 
> Fixes: 93c64f1ea1e8 ("leds: add Qualcomm PM8941 WLED driver")
> Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
> Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
> ---
>  drivers/video/backlight/qcom-wled.c | 51 +++++++++++------------------
>  1 file changed, 19 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/video/backlight/qcom-wled.c b/drivers/video/backlight/qcom-wled.c
> index 977cd75827d7..c5232478a343 100644
> --- a/drivers/video/backlight/qcom-wled.c
> +++ b/drivers/video/backlight/qcom-wled.c
> @@ -1552,6 +1520,25 @@ static int wled_configure(struct wled *wled)
>  		}
>  	}
> 
> +	rc = of_property_read_u32(dev->of_node, "qcom,num-strings", &val);
> +	if (!rc) {
> +		if (val < 1 || val > wled->max_string_count) {
> +			dev_err(dev, "qcom,num-strings must be between 1 and %d\n",
> +				wled->max_string_count);
> +			return -EINVAL;
> +		}
> +
> +		if (string_len > 0) {
> +			dev_warn(dev, "qcom,num-strings and qcom,enabled-strings are ambiguous\n");

This warning occurs even when there is no ambiguity.

This could be:

	if (string_len > 0 && val != string_len)

The warning should also be below the error message on the next if statement.
Combined these changes allows us to give a much more helpful and assertive
warning message:

qcom,num-strings mis-matches and will partially override
qcom,enabled-strings (remove qcom,num-strings?)

> +			if (val > string_len) {
> +				dev_err(dev, "qcom,num-strings exceeds qcom,enabled-strings\n");
> +				return -EINVAL;
> +			}
> +		}

Daniel.