Messages in this thread | | | Date | Wed, 18 May 2022 00:16:44 +0000 | Subject | [PATCH v2 0/3] x86/crash: Fix double NMI shootdown bug | From | Sean Christopherson <> |
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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
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