linux-kernel - Re: [PATCH v2 4/4] hwmon: ltc2945: Fix possible overflows

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Message-ID: <20221228170110.GC1267483@roeck-us.net>
Date:   Wed, 28 Dec 2022 09:01:10 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Cormier, Jonathan" <jcormier@...ticallink.com>
Cc:     linux-hwmon@...r.kernel.org,
        John Pruitt <jpruitt@...ticallink.com>,
        Jean Delvare <jdelvare@...e.com>, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bob Duke <bduke@...ticallink.com>
Subject: Re: [PATCH v2 4/4] hwmon: ltc2945: Fix possible overflows

On Mon, Dec 19, 2022 at 07:04:57PM -0500, Cormier, Jonathan wrote:
> From: John Pruitt <jpruitt@...ticallink.com>
> 
> Use 64-bit values for intermediate calculations. Check for
> overflows and return INT_MAX if overflows happened.
> 
> Signed-off-by: John Pruitt <jpruitt@...ticallink.com>
> Signed-off-by: "Cormier, Jonathan" <jcormier@...ticallink.com>

The problems here are introduced with the previous patch
and thus would need a Fixes: tag. It just doesn't make sense
to submit that as separate patch.

> ---
>  drivers/hwmon/ltc2945.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hwmon/ltc2945.c b/drivers/hwmon/ltc2945.c
> index fc7d399b2c85..7239422fc6db 100644
> --- a/drivers/hwmon/ltc2945.c
> +++ b/drivers/hwmon/ltc2945.c
> @@ -126,6 +126,10 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
>  		}
>  		val *= 1000;
>  		val = DIV_ROUND_CLOSEST_ULL(val, shunt_resistor);
> +		/* check for overflow, use MAX value if it happened */
> +		if (val > INT_MAX)
> +			val = INT_MAX;
> +

ltc2945_reg_to_val returns long long, and the calling code expects long long.
How would this ever overflow ?

>  		break;
>  	case LTC2945_VIN_H:
>  	case LTC2945_MAX_VIN_H:
> @@ -159,12 +163,14 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
>  }
> 
>  static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> -			      unsigned long val)
> +			      unsigned long val_32)
>  {
>  	struct ltc2945_data *data = dev_get_drvdata(dev);
>  	struct regmap *regmap = data->regmap;
>  	u32 shunt_resistor = data->shunt_resistor;
>  	unsigned int control;
> +	/* use 64-bit val for intermediate calculations */
> +	unsigned long long val = val_32;

This is unnnecessary. The parameter can be unsigned long long,
making the conversion automatic.

>  	int ret;
> 
>  	switch (reg) {
> @@ -184,7 +190,7 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
>  		if (control & CONTROL_MULT_SELECT) {
>  			/* 25 mV * 25 uV = 0.625 uV resolution. */
>  			val *= shunt_resistor;
> -			val = DIV_ROUND_CLOSEST(val, 625 * 1000);
> +			val = DIV_ROUND_CLOSEST_ULL(val, 625LL * 1000LL);
>  		} else {
>  			/*
>  			 * 0.5 mV * 25 uV = 0.0125 uV resolution.
> @@ -192,7 +198,7 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
>  			 * accept loss of accuracy.
>  			 */
>  			val *= shunt_resistor;
> -			val = DIV_ROUND_CLOSEST(val, 25 * 1000) * 2;
> +			val = DIV_ROUND_CLOSEST_ULL(val, 25LL * 1000LL) * 2;
>  		}
>  		break;
>  	case LTC2945_VIN_H:
> @@ -201,7 +207,7 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
>  	case LTC2945_MAX_VIN_THRES_H:
>  	case LTC2945_MIN_VIN_THRES_H:
>  		/* 25 mV resolution. */
> -		val /= 25;
> +		val = DIV_ROUND_CLOSEST_ULL(val, 25LL);

Unrelated change causing behavioral change. Not that I mind, but it is
still unrelated and would have to be a separate patch.

>  		break;
>  	case LTC2945_ADIN_H:
>  	case LTC2945_MAX_ADIN_H:
> @@ -218,11 +224,15 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
>  	case LTC2945_MIN_SENSE_THRES_H:
>  		/* 25 uV resolution. Convert to  mA. */
>  		val *= shunt_resistor;
> -		val = DIV_ROUND_CLOSEST(val, 25 * 1000);
> +		val = DIV_ROUND_CLOSEST_ULL(val, 25LL * 1000LL);
>  		break;
>  	default:
>  		return -EINVAL;
>  	}
> +	/* If val is too large, just return the max value */
> +	if (val > INT_MAX)
> +		return INT_MAX;
> +

While the return value is declared as int, the calling code expects
unsigned long. It would be better to adjust the return value and clamp
against ULONG_MAX.

>  	return val;
>  }
> 
> --
> 2.25.1