linux-kernel - Re: [patch 26/30] x86/microcode: Protect against instrumentation

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Message-ID: <877cq2hr6s.ffs@tglx>
Date:   Fri, 11 Aug 2023 11:19:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Ashok Raj <ashok.raj@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [patch 26/30] x86/microcode: Protect against instrumentation

On Thu, Aug 10 2023 at 22:36, Peter Zijlstra wrote:
> On Thu, Aug 10, 2023 at 08:38:04PM +0200, Thomas Gleixner wrote:
>> From: Thomas Gleixner <tglx@...utronix.de>
>> 
>> The wait for control loop in which the siblings are waiting for the
>> microcode update on the primary thread must be protected against
>> instrumentation as instrumentation can end up in #INT3, #DB or #PF, which
>> then returns with IRET. That IRET reenables NMI which is the opposite of
>> what the NMI rendezvouz is trying to achieve.
>
> asm_exc_nmi will re-enable NMIs when DEBUG_ENTRY before we reach C.
> Late loading had better depend on !DEBUG_ENTRY.

Good point