x86/crash: Disable virt in core NMI crash handler to avoid double shootdown

x86/crash: Disable virt in core NMI crash handler to avoid double shootdown

Disable virtualization in crash_nmi_callback() and rework the
emergency_vmx_disable_all() path to do an NMI shootdown if and only if a
shootdown has not already occurred.   NMI crash shootdown fundamentally
can't support multiple invocations as responding CPUs are deliberately
put into halt state without unblocking NMIs.  But, the emergency reboot
path doesn't have any work of its own, it simply cares about disabling
virtualization, i.e. so long as a shootdown occurred, emergency reboot
doesn't care who initiated the shootdown, or when.

If "crash_kexec_post_notifiers" is specified on the kernel command line,
panic() will invoke crash_smp_send_stop() and result in a second call to
nmi_shootdown_cpus() during native_machine_emergency_restart().

Invoke the callback _before_ disabling virtualization, as the current
VMCS needs to be cleared before doing VMXOFF.  Note, this results in a
subtle change in ordering between disabling virtualization and stopping
Intel PT on the responding CPUs.  While VMX and Intel PT do interact,
VMXOFF and writes to MSR_IA32_RTIT_CTL do not induce faults between one
another, which is all that matters when panicking.

Harden nmi_shootdown_cpus() against multiple invocations to try and
capture any such kernel bugs via a WARN instead of hanging the system
during a crash/dump, e.g. prior to the recent hardening of
register_nmi_handler(), re-registering the NMI handler would trigger a
double list_add() and hang the system if CONFIG_BUG_ON_DATA_CORRUPTION=y.

 list_add double add: new=ffffffff82220800, prev=ffffffff8221cfe8, next=ffffffff82220800.
 WARNING: CPU: 2 PID: 1319 at lib/list_debug.c:29 __list_add_valid+0x67/0x70
 Call Trace:
  __register_nmi_handler+0xcf/0x130
  nmi_shootdown_cpus+0x39/0x90
  native_machine_emergency_restart+0x1c9/0x1d0
  panic+0x237/0x29b

Extract the disabling logic to a common helper to deduplicate code, and
to prepare for doing the shootdown in the emergency reboot path if SVM
is supported.

Note, prior to commit ed72736183c4 ("x86/reboot: Force all cpus to exit
VMX root if VMX is supported"), nmi_shootdown_cpus() was subtly protected
against a second invocation by a cpu_vmx_enabled() check as the kdump
handler would disable VMX if it ran first.

Fixes: ed72736183c4 ("x86/reboot: Force all cpus to exit VMX root if VMX is supported")
Cc: stable@vger.kernel.org
Reported-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/all/20220427224924.592546-2-gpiccoli@igalia.com
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221130233650.1404148-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>