linux-kernel - Re: [patch 4/8] x86/tsc: Verify TSC

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Date:   Sun, 20 Nov 2016 14:10:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [patch 4/8] x86/tsc: Verify TSC_ADJUST from idle

On Sat, Nov 19, 2016 at 01:47:37PM -0000, Thomas Gleixner wrote:
> When entering idle, it's a good oportunity to verify that the TSC_ADJUST
> MSR has not been tampered with (BIOS hiding SMM cycles). If tampering is
> detected, emit a warning and restore it to the previous value.

> +++ b/arch/x86/kernel/process.c
> @@ -277,6 +277,7 @@ void exit_idle(void)
>  
>  void arch_cpu_idle_enter(void)
>  {
> +	tsc_verify_tsc_adjust();
>  	local_touch_nmi();
>  	enter_idle();
>  }

Doing a RDMSR on the idle path isn't going to be popular. That path is
already way too slow.