[PATCH v2 4/4] KVM: Do compatibility checks on hotplugged CPUs

[Chao Gao <chao.gao@xxxxxxxxx>]

At init time, KVM does compatibility checks to ensure that all online
CPUs support hardware virtualization and a common set of features. But
KVM uses hotplugged CPUs without such compatibility checks. On Intel
CPUs, this leads to #GP if the hotplugged CPU doesn't support VMX or
vmentry failure if the hotplugged CPU doesn't meet minimal feature
requirements.

Do compatibility checks when onlining a CPU and abort the online process
if the hotplugged CPU is incompatible with online CPUs.

CPU hotplug is disabled during hardware_enable_all() to prevent the corner
case as shown below. A hotplugged CPU marks itself online in
cpu_online_mask (1) and enables interrupt (2) before invoking callbacks
registered in ONLINE section (3). So, if hardware_enable_all() is invoked
on another CPU right after (2), then on_each_cpu() in hardware_enable_all()
invokes hardware_enable_nolock() on the hotplugged CPU before
kvm_online_cpu() is called. This makes the CPU escape from compatibility
checks, which is risky.

	start_secondary { ...
		set_cpu_online(smp_processor_id(), true); <- 1
		...
		local_irq_enable();  <- 2
		...
		cpu_startup_entry(CPUHP_AP_ONLINE_IDLE); <- 3
	}

Keep compatibility checks at KVM init time. It can help to find
incompatibility issues earlier and refuse to load arch KVM module
(e.g., kvm-intel).

Loosen the WARN_ON in kvm_arch_check_processor_compat so that it
can be invoked from KVM's CPU hotplug callback (i.e., kvm_online_cpu).

Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
 arch/x86/kvm/x86.c  | 11 +++++++++--
 virt/kvm/kvm_main.c | 20 +++++++++++++++++++-
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6f3bf78afb29..21bdb5783f71 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11472,9 +11472,16 @@ void kvm_arch_hardware_unsetup(void)
 
 int kvm_arch_check_processor_compat(void)
 {
-	struct cpuinfo_x86 *c = &cpu_data(smp_processor_id());
+	int cpu = smp_processor_id();
+	struct cpuinfo_x86 *c = &cpu_data(cpu);
 
-	WARN_ON(!irqs_disabled());
+	/*
+	 * Compatibility checks are done when loading KVM or in KVM's CPU
+	 * hotplug callback. It ensures all online CPUs are compatible to run
+	 * vCPUs. For other cases, compatibility checks are unnecessary or
+	 * even problematic. Try to detect improper usages here.
+	 */
+	WARN_ON(!irqs_disabled() && cpu_active(cpu));
 
 	if (__cr4_reserved_bits(cpu_has, c) !=
 	    __cr4_reserved_bits(cpu_has, &boot_cpu_data))
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 528741601122..83f87fb1fa0a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4858,7 +4858,13 @@ static void hardware_enable_nolock(void *junk)
 
 static int kvm_online_cpu(unsigned int cpu)
 {
-	int ret = 0;
+	int ret;
+
+	ret = kvm_arch_check_processor_compat();
+	if (ret) {
+		pr_warn("kvm: CPU%d is incompatible with online CPUs", cpu);
+		return ret;
+	}
 
 	raw_spin_lock(&kvm_count_lock);
 	/*
@@ -4916,6 +4922,17 @@ static int hardware_enable_all(void)
 {
 	int r = 0;
 
+	/*
+	 * During onlining a CPU, cpu_online_mask is set before kvm_online_cpu()
+	 * is called. on_each_cpu() between them includes the CPU. As a result,
+	 * hardware_enable_nolock() may get invoked before kvm_online_cpu().
+	 * This would enable hardware virtualization on that cpu without
+	 * compatibility checks, which can potentially crash system or break
+	 * running VMs.
+	 *
+	 * Disable CPU hotplug to prevent this case from happening.
+	 */
+	cpus_read_lock();
 	raw_spin_lock(&kvm_count_lock);
 
 	kvm_usage_count++;
@@ -4930,6 +4947,7 @@ static int hardware_enable_all(void)
 	}
 
 	raw_spin_unlock(&kvm_count_lock);
+	cpus_read_unlock();
 
 	return r;
 }
-- 
2.25.1