Date: Mon, 29 Jun 2015 11:58:53 +0100
From: Matt Fleming <matt@codeblueprint.co.uk>
To: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Prarit Bhargava <prarit@redhat.com>, Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
        Len Brown <len.brown@intel.com>,
        Dasaratharaman Chandramouli 
	<dasaratharaman.chandramouli@intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>
Subject: Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr
Message-ID: <20150629105853.GE28334@codeblueprint.co.uk>
References: <1435341131-3279-1-git-send-email-prarit@redhat.com>
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 <20150627083921.GA13074@gmail.com>
 <CALCETrXpiMWwiAfbxcKTDYCewrwq-UTwLV+xXTxdOkd+B3G-OQ@mail.gmail.com>
 <559005DD.3070003@redhat.com>
 <20150628151049.GB20989@khazad-dum.debian.net>
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On Sun, 28 Jun, at 12:10:49PM, Henrique de Moraes Holschuh wrote:
> On Sun, 28 Jun 2015, Prarit Bhargava wrote:
> > Is it easier to blacklist MSRs we don't want generally exposed, or only expose
> > the ones that we think are safe?  That's sort of a devil's advocate sort of
> > question ;) and I'm wondering what the shorter list is.
> 
> The only way to make MSR access safe is to allow it only by whitelisting.
> The x86 platform restricts all MSR access to ring 0 for a damn good reason.

Blacklisting also breaks horribly if you run old kernels on new
hardware.

We need to "fail-closed" if someone tries to access an MSR the kernel
doesn't know about.

-- 
Matt Fleming, Intel Open Source Technology Center
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