hwmon: (pwm-fan) Ensure that calculation doesn't discard big period values

With MAX_PWM being defined to 255 the code

	unsigned long period;
	...
	period = ctx->pwm->args.period;
	state.duty_cycle = DIV_ROUND_UP(pwm * (period - 1), MAX_PWM);

calculates a too small value for duty_cycle if the configured period is
big (either by discarding the 64 bit value ctx->pwm->args.period or by
overflowing the multiplication). As this results in a too slow fan and
so maybe an overheating machine better be safe than sorry and error out
in .probe.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20201215092031.152243-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
index 777439f48c1471a1cbd02d5b32208367ddf44b2d..111a91dc6b798e31ae6ff9998e4eea0428943bc5 100644 (file)
--- a/drivers/hwmon/pwm-fan.c
+++ b/drivers/hwmon/pwm-fan.c
@@ -334,8 +334,18 @@ static int pwm_fan_probe(struct platform_device *pdev)
 
        ctx->pwm_value = MAX_PWM;
 
-       /* Set duty cycle to maximum allowed and enable PWM output */
        pwm_init_state(ctx->pwm, &state);
+       /*
+        * __set_pwm assumes that MAX_PWM * (period - 1) fits into an unsigned
+        * long. Check this here to prevent the fan running at a too low
+        * frequency.
+        */
+       if (state.period > ULONG_MAX / MAX_PWM + 1) {
+               dev_err(dev, "Configured period too big\n");
+               return -EINVAL;
+       }
+
+       /* Set duty cycle to maximum allowed and enable PWM output */
        state.duty_cycle = ctx->pwm->args.period - 1;
        state.enabled = true;