linux-kernel - Re: RFC: Tainting the kernel on raw I/O access

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Message-ID: <20140904174340.24e8bdc0@alan.etchedpixels.co.uk>
Date:	Thu, 4 Sep 2014 17:43:40 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <h.peter.anvin@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: RFC: Tainting the kernel on raw I/O access

> > As for the original purpose of taints, I'm not aware of any
> > problems with MSR access or port IO causing excessive 
> > kernel oops reports. Are you?

I'm not. From the bugzilla trends I don't think its a major cause, and we
can usually root out the "user with dumb external module" problem already.

> Really? Either one can be used to modify the running kernel (or
> microcode), and possibly even destroy hardware. 

At least on x86 I would hope not the latter at least on modern systems.
So the most irritating thing you can do is probably rootkit the box. It's
not as if you can't rootkit a typical distribution shipping Linux system
half a dozen other simpler ways than using I/O ports. Besides which once
someone has rootkitted your box it won't show the taint anyway !

As a security measure the tainting is next to useless.

As a debug aid it's potentially handy.

> > If there are none I don't think it makes sense.
> > 
> > At least personally I use MSR accesses quite frequently
> > for benign purposes.
> But how much of that is just reading MSR's, and of the writes, how much
> are either debugging or things that the average user isn't ever going to do?

Most of the uses are benign and sensible things like power monitoring
tools. 

Alan
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