linux-kernel - Re: [PATCH] lm85: Bounds check to_sensor_dev

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Date:   Sat, 4 Feb 2023 10:55:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] lm85: Bounds check to_sensor_dev_attr()->index usage

On Fri, Feb 03, 2023 at 05:57:00PM -0800, Guenter Roeck wrote:
> That line of argument would suggest that we should perform parameter checks
> on each function entry all over the place, no matter if the range is known
> to be valid or not. Maybe that is the way things are going, but I don't
> like it at all. I have seen that kind of code before, in the telco space,
> where it typically at least doubled code size and resulted in mediocre
> performance, just because of a rule that mandated checking all parameters
> at the beginning of each function.

Well, I doubt I'll be able to change your opinion of telco code, but I
do think robustness is not an unreasonable default state for software,
and that GCC and Clang do a pretty good job with optimization, etc.

> I assume this is just one of many many patches you plan to send to add
> parameter checks to similar hwmon code ? I _really_ don't want to have
> the hwmon code cluttered with such unnecessary checks.

I was trying to provide complete coverage inspired by the specific
complaint GCC had, but this would also silence the warning:

diff --git a/drivers/hwmon/lm85.c b/drivers/hwmon/lm85.c
index 8d33c2484755..87d2455e721f 100644
--- a/drivers/hwmon/lm85.c
+++ b/drivers/hwmon/lm85.c
@@ -1106,6 +1106,7 @@ static ssize_t pwm_auto_pwm_minctl_store(struct device *dev,
 	mutex_lock(&data->update_lock);
 	data->autofan[nr].min_off = val;
 	tmp = lm85_read_value(client, LM85_REG_AFAN_SPIKE1);
+	nr = clamp_t(int, nr, 0, ARRAY_SIZE(data->autofan) - 1);
 	tmp &= ~(0x20 << nr);
 	if (data->autofan[nr].min_off)
 		tmp |= 0x20 << nr;

What's happening is GCC see that "nr" is used as a shift argument, so it
believes (not unreasonably) that this otherwise unknown value could be
up to 32. Here we can give it the bounded range ahead of time, keeping
it happy.

-- 
Kees Cook