[PATCH] hwmon (tmp401): fix overflow caused by default conversion rate value

[PATCH] hwmon (tmp401): fix overflow caused by default conversion rate value

[Alexey Simakov]

The driver computes conversion intervals using the formula:

    interval = (1 << (7 - rate)) * 125ms

where 'rate' is the sensor's conversion rate register value. According to
the datasheet, the power-on reset value of this register is 0x8, which
could be assigned to the register, after handling i2c general call.
Using this default value causes a result greater than the bit width of
left operand and an undefined behaviour in the calculation above, since
shifting by values larger than the bit width is undefined behaviour as
per C language standard.

Limit the maximum usable 'rate' value to 7 to prevent undefined
behaviour in calculations.

Found by Linux Verification Center (linuxtesting.org) with Svace.

Fixes: ca53e7640de7 ("hwmon: (tmp401) Convert to _info API")
Signed-off-by: Alexey Simakov <bigalex934@gmail.com>
---
 drivers/hwmon/tmp401.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwmon/tmp401.c b/drivers/hwmon/tmp401.c
index 02c5a3bb1071..84aaf817144c 100644
--- a/drivers/hwmon/tmp401.c
+++ b/drivers/hwmon/tmp401.c
@@ -401,7 +401,7 @@ static int tmp401_chip_read(struct device *dev, u32 attr, int channel, long *val
 		ret = regmap_read(data->regmap, TMP401_CONVERSION_RATE, &regval);
 		if (ret < 0)
 			return ret;
-		*val = (1 << (7 - regval)) * 125;
+		*val = (1 << (7 - min(regval, 7))) * 125;
 		break;
 	case hwmon_chip_temp_reset_history:
 		*val = 0;
-- 
2.34.1

Re: [PATCH] hwmon (tmp401): fix overflow caused by default conversion rate value

[Guenter Roeck]

On Thu, Dec 11, 2025 at 07:43:43PM +0300, Alexey Simakov wrote:
> The driver computes conversion intervals using the formula:
> 
>     interval = (1 << (7 - rate)) * 125ms
> 
> where 'rate' is the sensor's conversion rate register value. According to
> the datasheet, the power-on reset value of this register is 0x8, which
> could be assigned to the register, after handling i2c general call.
> Using this default value causes a result greater than the bit width of
> left operand and an undefined behaviour in the calculation above, since
> shifting by values larger than the bit width is undefined behaviour as
> per C language standard.
> 
> Limit the maximum usable 'rate' value to 7 to prevent undefined
> behaviour in calculations.
> 
> Found by Linux Verification Center (linuxtesting.org) with Svace.
> 
> Fixes: ca53e7640de7 ("hwmon: (tmp401) Convert to _info API")
> Signed-off-by: Alexey Simakov <bigalex934@gmail.com>

Applied.

It does not matter in practice, though, unless someone overwrites the chip
configuration from outside the driver. The register is initialized with a
value of 5 (500ms) when the driver is loaded, and the driver never writes
a bad value. I added that note to the commit message.

Thanks,
Guenter