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

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Date:	Wed, 8 Jun 2016 15:55:48 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Jean Delvare <jdelvare@...e.com>, Mario_Limonciello@...l.com,
	Gabriele Mazzotta <gabriele.mzt@...il.com>,
	Michał Kępień <kernel@...pniu.pl>,
	linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: dell-smm-hwmon: security problems

On Wednesday 08 June 2016 15:24:10 Guenter Roeck wrote:
> On 06/08/2016 02:57 AM, Pali Rohár wrote:
> > 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)"
> 
> Sure. I am sure that someone will complain (we learned just recently
> that people still use the old commands, after all), but then the old
> behavior can be restored by setting the flag to 0.

Either setting that flag to 0 or running that tool under root or with 
capability CAP_SYS_ADMIN.

> I would not use a double negative to describe it. Why not just
> something like "Allow fan control only if SYS_ADMIN capability set
> (default 1)" ?

I was thinking about that description too, but there is problem with 
meaning too...

0 means fan control is allowed for any user
1 means fan control is allowed only for CAP_SYS_ADMIN

Description should be unambiguous for situation when flag is set to 0.

===

And do you have idea what to do with problem 1)?

-- 
Pali Rohár
pali.rohar@...il.com

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