[PATCH 5.4 35/76] hwmon: (pwm-fan) Ensure that calculation doesnt discard big period values

Greg Kroah-Hartman
2021-01-18T19:14:29Z

From: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

[ Upstream commit 1eda52334e6d13eb1a85f713ce06dd39342b5020 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/hwmon/pwm-fan.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
index c88ce77fe6763..df6f042fb605e 100644
--- a/drivers/hwmon/pwm-fan.c
+++ b/drivers/hwmon/pwm-fan.c
@@ -330,8 +330,18 @@ 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;

--
2.27.0