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

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Date:   Fri, 12 Nov 2021 12:08:39 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
Cc:     phone-devel@...r.kernel.org, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Martin Botka <martin.botka@...ainline.org>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Pavel Dubrova <pashadubrova@...il.com>,
        Kiran Gunda <kgunda@...eaurora.org>,
        Bryan Wu <cooloney@...il.com>, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        Courtney Cavin <courtney.cavin@...ymobile.com>
Subject: Re: [RESEND PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one
 maximum with default num_strings

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@...ainline.org>
> Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@...ainline.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.