linux-kernel - [PATCH 2/2] x86/reboot: Disable virtualization in an emergency if SVM is supported

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Date:   Wed, 11 May 2022 23:43:32 +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 2/2] x86/reboot: Disable virtualization in an emergency if SVM
 is supported

Disable SVM on all CPUs via NMI shootdown during an emergency reboot.
Like VMX, SVM can block INIT and thus prevent bringing up other CPUs via
INIT-SIPI-SIPI.

Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kernel/reboot.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index f9543a4e9b09..33c1f4883b27 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -533,27 +533,29 @@ static void nmi_shootdown_nop(int cpu, struct pt_regs *regs)
 	/* Nothing to do, the NMI shootdown handler disables virtualization. */
 }
 
-/* Use NMIs as IPIs to tell all CPUs to disable virtualization */
-static void emergency_vmx_disable_all(void)
+static void emergency_reboot_disable_virtualization(void)
 {
 	/* Just make sure we won't change CPUs while doing this */
 	local_irq_disable();
 
 	/*
-	 * Disable VMX on all CPUs before rebooting, otherwise we risk hanging
-	 * the machine, because the CPU blocks INIT when it's in VMX root.
+	 * Disable virtualization on all CPUs before rebooting to avoid hanging
+	 * the system, as VMX and SVM block INIT when running in the host
 	 *
 	 * We can't take any locks and we may be on an inconsistent state, so
-	 * use NMIs as IPIs to tell the other CPUs to exit VMX root and halt.
+	 * use NMIs as IPIs to tell the other CPUs to disable VMX/SVM and halt.
 	 *
-	 * Do the NMI shootdown even if VMX if off on _this_ CPU, as that
-	 * doesn't prevent a different CPU from being in VMX root operation.
+	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
+	 * other CPUs may have virtualization enabled.
 	 */
-	if (cpu_has_vmx()) {
-		/* Safely force _this_ CPU out of VMX root operation. */
-		__cpu_emergency_vmxoff();
+	if (cpu_has_vmx() || cpu_has_svm(NULL)) {
+		/* Safely force _this_ CPU out of VMX/SVM operation. */
+		if (cpu_has_vmx())
+			__cpu_emergency_vmxoff();
+		else
+			cpu_emergency_svm_disable();
 
-		/* Halt and exit VMX root operation on the other CPUs. */
+		/* Disable VMX/SVM and halt on other CPUs. */
 		nmi_shootdown_cpus(nmi_shootdown_nop);
 	}
 }
@@ -590,7 +592,7 @@ static void native_machine_emergency_restart(void)
 	unsigned short mode;
 
 	if (reboot_emergency)
-		emergency_vmx_disable_all();
+		emergency_reboot_disable_virtualization();
 
 	tboot_shutdown(TB_SHUTDOWN_REBOOT);
 
-- 
2.36.0.512.ge40c2bad7a-goog