linux-kernel - Re: [RFC 1/2] x86/tsc: add a timer to make sure tsc

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Message-ID: <20210410144856.GC22054@shbuild999.sh.intel.com>
Date:   Sat, 10 Apr 2021 22:48:56 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, rui.zhang@...el.com,
        andi.kleen@...el.com, dave.hansen@...el.com, len.brown@...el.com
Subject: Re: [RFC 1/2] x86/tsc: add a timer to make sure tsc_adjust is always
 checked

Hi Boris,

On Sat, Apr 10, 2021 at 11:47:52AM +0200, Borislav Petkov wrote:
> On Sat, Apr 10, 2021 at 11:27:11AM +0200, Thomas Gleixner wrote:
> > On Tue, Mar 30 2021 at 16:25, Feng Tang wrote:
> > > Normally the tsc_sync will be checked every time system enters idle state,
> > > but there is still caveat that a system won't enter idle, either because
> > > it's too busy or configured purposely to not enter idle. Setup a periodic
> > > timer to make sure the check is always on.
> > 
> > Bah. I really hate the fact that we don't have a knob to disable writes
> > to the TSC/TSC_ADJUST msrs. That would spare this business alltogether.
> 
> We have the MSR filtering and I'd *love* to add those MSRs to a
> permanent ban list of MSRs which will never ever be written to from
> luserspace.

Yep, I just tried changing TSC_ADJUST msr with 'wrmsr' command, and
it did succeed and trigger the warning of tsc_verify_tsc_adjust():

	[ 1135.387866] [Firmware Bug]: TSC ADJUST differs: CPU0 0 --> 4096. Restoring

And the bigger risk is still BIOS's writing to TSC_ADJUST MSR beneath
kernel.

Thanks,
Feng

> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette