linux-kernel - Re: [PATCH v4 0/4] x86/crash: Fix double NMI shootdown bug

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Message-ID: <87lenppw93.ffs@tglx>
Date:   Fri, 02 Dec 2022 18:10:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <seanjc@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        "Guilherme G . Piccoli" <gpiccoli@...lia.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Tom Lendacky <thomas.lendacky@....com>, stable@...r.kernel.org
Subject: Re: [PATCH v4 0/4] x86/crash: Fix double NMI shootdown bug

On Wed, Nov 30 2022 at 23:36, Sean Christopherson wrote:

> Fix a double NMI shootdown bug found and debugged by Guilherme, who did all
> the hard work.  NMI shootdown is a one-time thing; the handler leaves NMIs
> blocked and enters halt.  At best, a second (or third...) shootdown is an
> expensive nop, at worst it can hang the kernel and prevent kexec'ing into
> a new kernel, e.g. prior to the hardening of register_nmi_handler(), a
> double shootdown resulted in a double list_add(), which is fatal when running
> with CONFIG_BUG_ON_DATA_CORRUPTION=y.
>
> With the "right" kexec/kdump configuration, emergency_vmx_disable_all() can
> be reached after kdump_nmi_shootdown_cpus() (currently the only two users
> of nmi_shootdown_cpus()).
>
> To fix, move the disabling of virtualization into crash_nmi_callback(),
> remove emergency_vmx_disable_all()'s callback, and do a shootdown for
> emergency_vmx_disable_all() if and only if a shootdown hasn't yet occurred.
> The only thing emergency_vmx_disable_all() cares about is disabling VMX/SVM
> (obviously), and since I can't envision a use case for an NMI shootdown that
> doesn't want to disable virtualization, doing that in the core handler means
> emergency_vmx_disable_all() only needs to ensure _a_ shootdown occurs, it
> doesn't care when that shootdown happened or what callback may have run.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>