linux-kernel - Re: [PATCH v2 0/3] x86/crash: Fix double NMI shootdown bug

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Message-ID: <35593875-73d5-55dd-8f11-7c902023536d@igalia.com>
Date:   Mon, 3 Oct 2022 13:03:45 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org
Subject: Re: [PATCH v2 0/3] x86/crash: Fix double NMI shootdown bug

On 17/05/2022 21:16, 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.
> 
> Patch 2 is a related bug fix found while exploring ideas for patch 1.
> Patch 3 is a cleanup to try to prevent future "fixed VMX but not SVM"
> style bugs.
> 
> Guilherme and Vitaly, I dropped your Tested-by and Reviewed-by tags
> since the relevant patches changed a decent amount.
> 
> v2:
>   - Use a NULL handler and crash_ipi_issued instead of a magic nop
>     handler. [tglx]
>   - Add comments to call out that modifying the existing handler
>     once the NMI is sent may cause explosions.
>   - Add a patch to cleanup cpu_emergency_vmxoff().
> 
> v1: https://lore.kernel.org/all/20220511234332.3654455-1-seanjc@google.com
> 
> Sean Christopherson (3):
>   x86/crash: Disable virt in core NMI crash handler to avoid double
>     shootdown
>   x86/reboot: Disable virtualization in an emergency if SVM is supported
>   x86/virt: Fold __cpu_emergency_vmxoff() into its sole caller
> 
>  arch/x86/include/asm/reboot.h  |  1 +
>  arch/x86/include/asm/virtext.h | 14 +-----
>  arch/x86/kernel/crash.c        | 16 +-----
>  arch/x86/kernel/reboot.c       | 89 +++++++++++++++++++++++++---------
>  4 files changed, 69 insertions(+), 51 deletions(-)
> 
> 
> base-commit: a7fed5c0431dbfa707037848830f980e0f93cfb3

Hi folks, monthly ping!
Any news on this fix series? Just checked, still applies cleanly.

Thanks,


Guilherme