Re: [PATCH 6/6] KVM: Do compatibility checks on hotplugged CPUs

[Chao Gao]

On Tue, Jan 11, 2022 at 12:46:52AM +0000, Sean Christopherson wrote:
>On Mon, Dec 27, 2021, Chao Gao wrote:
>> 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. If any VM is running,
>> KVM hotplug callback returns an error to abort onlining incompatible
>> CPUs.
>> 
>> But if no VM is running, onlining incompatible CPUs is allowed. Instead,
>> KVM is prohibited from creating VMs similar to the policy for init-time
>> compatibility checks.
>
>...
>
>> ---
>>  virt/kvm/kvm_main.c | 36 ++++++++++++++++++++++++++++++++++--
>>  1 file changed, 34 insertions(+), 2 deletions(-)
>> 
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index c1054604d1e8..0ff80076d48d 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -106,6 +106,8 @@ LIST_HEAD(vm_list);
>>  static cpumask_var_t cpus_hardware_enabled;
>>  static int kvm_usage_count;
>>  static atomic_t hardware_enable_failed;
>> +/* Set if hardware becomes incompatible after CPU hotplug */
>> +static bool hardware_incompatible;
>>  
>>  static struct kmem_cache *kvm_vcpu_cache;
>>  
>> @@ -4855,20 +4857,32 @@ 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();
>>  	raw_spin_lock(&kvm_count_lock);
>>  	/*
>>  	 * Abort the CPU online process if hardware virtualization cannot
>>  	 * be enabled. Otherwise running VMs would encounter unrecoverable
>>  	 * errors when scheduled to this CPU.
>>  	 */
>> -	if (kvm_usage_count) {
>> +	if (!ret && kvm_usage_count) {
>>  		hardware_enable_nolock(NULL);
>>  		if (atomic_read(&hardware_enable_failed)) {
>>  			ret = -EIO;
>>  			pr_info("kvm: abort onlining CPU%d", cpu);
>>  		}
>> +	} else if (ret && !kvm_usage_count) {
>> +		/*
>> +		 * Continue onlining an incompatible CPU if no VM is
>> +		 * running. KVM should reject creating any VM after this
>> +		 * point. Then this CPU can be still used to run non-VM
>> +		 * workload.
>> +		 */
>> +		ret = 0;
>> +		hardware_incompatible = true;
>
>This has a fairly big flaw in that it prevents KVM from creating VMs even if the
>offending CPU is offlined.  That seems like a very reasonable thing to do, e.g.
>admin sees that hotplugging a CPU broke KVM and removes the CPU to remedy the
>problem.  And if KVM is built-in, reloading KVM to wipe hardware_incompatible
>after offlining the CPU isn't an option.

Ideally, yes, creation VMs should be allowed after offending CPUs are offlined.
But the problem is kind of foundamental: 

After kernel tries to online a CPU without VMX, boot_cpu_has(X86_FEATURE_VMX)
returns false. So, the current behavior is reloading KVM would fail if
kernel *tried* to bring up a CPU without VMX. So, it looks to me that
boot_cpu_has() doesn't do feature re-evalution either. Given that, I doubt
the value of making KVM able to create VM in this case.

>
>To make this approach work, I think kvm_offline_cpu() would have to reevaluate
>hardware_incompatible if the flag is set.
>
>And should there be a KVM module param to let the admin opt in/out of this
>behavior?  E.g. if the primary use case for a system is to run VMs, disabling
>KVM just to online a CPU isn't very helpful.
>
>That said, I'm not convinced that continuing with the hotplug in this scenario
>is ever the right thing to do.  Either the CPU being hotplugged really is a different
>CPU, or it's literally broken.  In both cases, odds are very, very good that running
>on the dodgy CPU will hose the kernel sooner or later, i.e. KVM's compatibility checks
>are just the canary in the coal mine.

Ok. Then here are two options:
1. KVM always prevents incompatible CPUs from being brought up regardless of running VMs
2. make "disabling KVM on incompatible CPUs" an opt-in feature.

Which one do you think is better?

And as said above, even with option 1, KVM reloading would fail due to
boot_cpu_has(X86_FEATURE_VMX). I suppose it isn't necessary to be fixed in this series.

>
>TDX is a different beast as (a) that's purely a security restriction and (b) anyone
>trying to run TDX guests darn well better know that TDX doesn't allow hotplug.
>In other words, if TDX gets disabled due to hotplug, either someone majorly screwed
>up and is going to be unhappy no matter what, or there's no intention of using TDX
>and it's a complete don't care.
>
>> +		pr_info("kvm: prohibit VM creation due to incompatible CPU%d",
>
>pr_info() is a bit weak, this should be at least pr_warn() and maybe even pr_err().
>
>> +			cpu);
>
>Eh, I'd omit the newline and let that poke out.

Will do.

>
>>  	}
>>  	raw_spin_unlock(&kvm_count_lock);
>>  	return ret;
>> @@ -4913,8 +4927,24 @@ 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);
>>  
>> +	if (hardware_incompatible) {
>
>Another error message would likely be helpful here.  Even better would be if KVM
>could provide some way for userspace to query which CPU(s) is bad.

If option 1 is chosen, this check will be removed.

For option 2, will add an error message. And how about a debugfs tunable to provide
the list of bad CPUs?