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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Sun, 11 Apr 2021 09:03:34 -0700
Message-ID: <CALCETrXD6aKet8gpQUmcH0Z2AtHoGY-7MDK8WmCtH3_K=EYjtg@mail.gmail.com>
Subject: Re: [PATCH] x86/msr: Block writes to certain MSRs unconditionally
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Feng Tang <feng.tang@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Zhang, Rui" <rui.zhang@intel.com>,
        "Kleen, Andi" <andi.kleen@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Len Brown <len.brown@intel.com>
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On Sat, Apr 10, 2021 at 11:52 AM Andi Kleen <ak@linux.intel.com> wrote:
>
> Borislav Petkov <bp@alien8.de> writes:
>
> > From: Borislav Petkov <bp@suse.de>
> > Date: Sat, 10 Apr 2021 14:08:13 +0200
> >
> > There are a bunch of MSRs which luserspace has no business poking at,
> > whatsoever. Add a ban list and put the TSC-related MSRs in there. Issue
> > a big juicy splat to catch offenders.
>
> Have you ever seen any user programs actually write those MSRs?
> I don't see why they ever would, it's not that they have any motivation
> to do it (unlike SMM), and I don't know of any examples.

I have actually seen real user programs poke MSR_SYSCALL_MASK.