Re: [PATCH] hwmon: (adt7470) Prevent divide by zero in adt7470_fan_write()

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On 1/20/22 11:37 PM, Dan Carpenter wrote:
On Thu, Jan 20, 2022 at 10:10:45PM -0800, Guenter Roeck wrote:
On 1/20/22 9:39 PM, Dan Carpenter wrote:
The "val" variable is controlled by the user and comes from
hwmon_attr_store().  The FAN_RPM_TO_PERIOD() macro divides by "val"
so a zero will crash the system.  Check for that and return -EINVAL.

Fixes: fc958a61ff6d ("hwmon: (adt7470) Convert to devm_hwmon_device_register_with_info API")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
---
   drivers/hwmon/adt7470.c | 3 +++
   1 file changed, 3 insertions(+)

diff --git a/drivers/hwmon/adt7470.c b/drivers/hwmon/adt7470.c
index d519aca4a9d6..cd474584dc0b 100644
--- a/drivers/hwmon/adt7470.c
+++ b/drivers/hwmon/adt7470.c
@@ -662,6 +662,9 @@ static int adt7470_fan_write(struct device *dev, u32 attr, int channel, long val
   	struct adt7470_data *data = dev_get_drvdata(dev);
   	int err;
+	if (!val)
+		return -EINVAL;
+

Technically that restores old (pre-fc958a61ff6d) behavior, but it is still bad:
Userspace can provide a value of -1 (or any other negative number), and it will
translate to 5400000 RPM. So it should either be

	if (val <= 0)
		return -EINVAL;

or
	if (val <= 0)
		val = 1;

There is a clamp() which does already turn invalid values into something
valid.


Yes, but
	-1 -> -5400000 -> 1, which translates to 5400000 rpm.
This is in contrast to
	1 -> 5400000 -> 65534
which translates to a more reasonable 82 rpm.

	val = FAN_RPM_TO_PERIOD(val);
	val = clamp_val(val, 1, 65534);

But I will make the <= 0 return -EINVAL change and resend.

Thanks,
Guenter



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