linux-kernel - dell-smm-hwmon: security problems

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Date:	Wed, 8 Jun 2016 11:57:22 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Jean Delvare <jdelvare@...e.com>,
	Guenter Roeck <linux@...ck-us.net>, Mario_Limonciello@...l.com,
	Gabriele Mazzotta <gabriele.mzt@...il.com>,
	Michał Kępień <kernel@...pniu.pl>
Cc:	linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: dell-smm-hwmon: security problems

Hello!

Mario wrote me about two I think security problems in dell-smm-hwmon 
driver and I would like to ask you, how to fix them.

1) File /proc/i8k (exists only when kernel is compiled with CONFIG_I8K) 
exports DMI_PRODUCT_SERIAL and it can be read by ordinary user, without 
root permission. Normally DMI_PRODUCT_SERIAL can be read from sysfs file 
/sys/class/dmi/id/product_serial but only by root user.

2) Via /proc/i8k ordinary user can set fan speed. This is because how 
"restricted" parameter and variable works. Setting fan speed by normal 
non-root user can be dangerous, e.g. malicious application under user 
"nobody" could take control of fans.

Do you have idea how to fix these problems? Just to note that /proc/i8k 
has stable kernel ABI and changing it will break all existing i8k* 
applications. But /proc/i8k is there only for old legacy laptops (year 
2000).

There is module parameter "restricted" with default value false and 
description: "Allow fan control if SYS_ADMIN capability set". Current 
code do:

	case I8K_SET_FAN:
		if (restricted && !capable(CAP_SYS_ADMIN))
			return -EPERM;

For me description is a bit ambiguous. What about setting "restricted" 
by default to true and updating description to something like this?

"Disallow fan control when SYS_ADMIN capability is not set (default: 1)"

-- 
Pali Rohár
pali.rohar@...il.com

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