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

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Date:   Wed, 18 May 2022 00:16:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: [PATCH v2 0/3] x86/crash: Fix double NMI shootdown bug

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
-- 
2.36.0.550.gb090851708-goog