linux-kernel - [PATCH AUTOSEL 5.4 13/28] hwmon: (pwm-fan) Ensure that calculation doesn't discard big period values

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Message-Id: <20210112125645.70739-13-sashal@kernel.org>
Date:   Tue, 12 Jan 2021 07:56:29 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Guenter Roeck <linux@...ck-us.net>,
        Sasha Levin <sashal@...nel.org>, linux-hwmon@...r.kernel.org
Subject: [PATCH AUTOSEL 5.4 13/28] hwmon: (pwm-fan) Ensure that calculation doesn't discard big period values

From: Uwe Kleine-König <u.kleine-koenig@...gutronix.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@...gutronix.de>
Link: https://lore.kernel.org/r/20201215092031.152243-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Guenter Roeck <linux@...ck-us.net>
Signed-off-by: Sasha Levin <sashal@...nel.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