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

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Date:   Thu, 10 Aug 2023 22:36:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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.