Subject: Re: [PATCH 10/44] KVM: VMX: Clean up eVMCS enabling if KVM initialization fails

From: Vitaly Kuznetsov <[email-masked]>
Date: Tue, 15 Nov 2022 10:30:14 +0100

Sean Christopherson <[email-masked]> writes:

> On Thu, Nov 03, 2022, Vitaly Kuznetsov wrote:
>> Sean Christopherson <[email-masked]> writes:
>> > +	/*
>> > +	 * Reset everything to support using non-enlightened VMCS access later
>> > +	 * (e.g. when we reload the module with enlightened_vmcs=0)
>> > +	 */
>> > +	for_each_online_cpu(cpu) {
>> > +		vp_ap =	hv_get_vp_assist_page(cpu);
>> > +
>> > +		if (!vp_ap)
>> > +			continue;
>> > +
>> > +		vp_ap->nested_control.features.directhypercall = 0;
>> > +		vp_ap->current_nested_vmcs = 0;
>> > +		vp_ap->enlighten_vmentry = 0;
>> > +	}
>> 
>> Unrelated to your patch but while looking at this code I got curious
>> about why don't we need a protection against CPU offlining here. Turns
>> out that even when we offline a CPU, its VP assist page remains
>> allocated (see hv_cpu_die()), we just write '0' to the MSR and thus
>
> Heh, "die".  Hyper-V is quite dramatic.
>
>> accessing the page is safe. The consequent hv_cpu_init(), however, does
>> not restore VP assist page when it's already allocated:
>> 
>> # rdmsr -p 24 0x40000073
>> 10212f001
>> # echo 0 > /sys/devices/system/cpu/cpu24/online 
>> # echo 1 > /sys/devices/system/cpu/cpu24/online 
>> # rdmsr -p 24 0x40000073
>> 0
>> 
>> The culprit is commit e5d9b714fe402 ("x86/hyperv: fix root partition
>> faults when writing to VP assist page MSR"). A patch is inbound.
>> 
>> 'hv_root_partition' case is different though. We do memunmap() and reset
>> VP assist page to zero so it is theoretically possible we're going to
>> clash. Unless I'm missing some obvious reason why module unload can't
>> coincide with CPU offlining, we may be better off surrounding this with
>> cpus_read_lock()/cpus_read_unlock(). 
>
> I finally see what you're concerned about.  If a CPU goes offline and its assist
> page is unmapped, zeroing out the nested/eVMCS stuff will fault.
>
> I think the real problem is that the purging of the eVMCS is in the wrong place.
> Move the clearing to vmx_hardware_disable() and then the CPU hotplug bug goes
> away once KVM disables hotplug during hardware enabling/disable later in the series.
> There's no need to wait until module exit, e.g. it's not like it costs much to
> clear a few variables, and IIUC the state is used only when KVM is actively using
> VMX/eVMCS.
>
> However, I believe there's a second bug.  KVM's CPU online hook is called before
> Hyper-V's online hook (CPUHP_AP_ONLINE_DYN).  Before this series, which moves KVM's
> hook from STARTING to ONLINE, KVM's hook is waaaay before Hyper-V's.  That means
> that hv_cpu_init()'s allocation of the VP assist page will come _after_ KVM's
> check in vmx_hardware_enable()
>
> 	/*
> 	 * This can happen if we hot-added a CPU but failed to allocate
> 	 * VP assist page for it.
> 	 */
> 	if (static_branch_unlikely(&enable_evmcs) &&
> 	    !hv_get_vp_assist_page(cpu))
> 		return -EFAULT;
>
> I.e. CPU hotplug will never work if KVM is running VMs as a Hyper-V guest.  I bet
> you can repro by doing a SUSPEND+RESUME.
>
> Can you try to see if that's actually a bug?  If so, the only sane fix seems to
> be to add a dedicated ONLINE action for Hyper-V.  

It seems we can't get away without a dedicated stage for Hyper-V anyway,
e.g. see our discussion with Michael:

https://lore.kernel.org/linux-hyperv/[email-masked]/

All these issues are more or less "theoretical" as there's no real CPU
hotplug on Hyper-V/Azure. Yes, it is possible to trigger problems by
doing CPU offline/online but I don't see how this may come handy outside
of testing envs.

> Per patch
>
>   KVM: Rename and move CPUHP_AP_KVM_STARTING to ONLINE section
>
> from this series, CPUHP_AP_KVM_ONLINE needs to be before CPUHP_AP_SCHED_WAIT_EMPTY
> to ensure there are no tasks, i.e. no vCPUs, running on the to-be-unplugged CPU.
>
> Back to the original bug, proposed fix is below.  The other advantage of moving
> the reset to hardware disabling is that the "cleanup" is just disabling the static
> key, and at that point can simply be deleted as there's no need to disable the
> static key when kvm-intel is unloaded since kvm-intel owns the key.  I.e. this
> patch (that we're replying to) would get replaced with a patch to delete the
> disabling of the static key.
>

>From a quick glance looks good to me, I'll try to find some time to work
on this issue. I will likely end up proposing a dedicated CPU hotplug
stage for Hyper-V (which needs to happen before KVM's
CPUHP_AP_KVM_ONLINE on CPU hotplug and after on unplug) anyway.

Thanks for looking into this!

> --
> From: Sean Christopherson <[email-masked]>
> Date: Thu, 10 Nov 2022 17:28:08 -0800
> Subject: [PATCH] KVM: VMX: Reset eVMCS controls in VP assist page during
>  hardware disabling
>
> Reset the eVMCS controls in the per-CPU VP assist page during hardware
> disabling instead of waiting until kvm-intel's module exit.  The controls
> are activated if and only if KVM creates a VM, i.e. don't need to be
> reset if hardware is never enabled.
>
> Doing the reset during hardware disabling will naturally fix a potential
> NULL pointer deref bug once KVM disables CPU hotplug while enabling and
> disabling hardware (which is necessary to fix a variety of bugs).  If the
> kernel is running as the root partition, the VP assist page is unmapped
> during CPU hot unplug, and so KVM's clearing of the eVMCS controls needs
> to occur with CPU hot(un)plug disabled, otherwise KVM could attempt to
> write to a CPU's VP assist page after it's unmapped.
>
> Reported-by: Vitaly Kuznetsov <[email-masked]>
> Signed-off-by: Sean Christopherson <[email-masked]>
> ---
>  arch/x86/kvm/vmx/vmx.c | 50 +++++++++++++++++++++++++-----------------
>  1 file changed, 30 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index aca88524fd1e..ae13aa3e8a1d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -552,6 +552,33 @@ static int hv_enable_direct_tlbflush(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static void hv_reset_evmcs(void)
> +{
> +	struct hv_vp_assist_page *vp_ap;
> +
> +	if (!static_branch_unlikely(&enable_evmcs))
> +		return;
> +
> +	/*
> +	 * KVM should enable eVMCS if and only if all CPUs have a VP assist
> +	 * page, and should reject CPU onlining if eVMCS is enabled the CPU
> +	 * doesn't have a VP assist page allocated.
> +	 */
> +	vp_ap = hv_get_vp_assist_page(smp_processor_id());
> +	if (WARN_ON_ONCE(!vp_ap))
> +		return;
> +
> +	/*
> +	 * Reset everything to support using non-enlightened VMCS access later
> +	 * (e.g. when we reload the module with enlightened_vmcs=0)
> +	 */
> +	vp_ap->nested_control.features.directhypercall = 0;
> +	vp_ap->current_nested_vmcs = 0;
> +	vp_ap->enlighten_vmentry = 0;
> +}
> +
> +#else /* IS_ENABLED(CONFIG_HYPERV) */
> +static void hv_reset_evmcs(void) {}
>  #endif /* IS_ENABLED(CONFIG_HYPERV) */
>  
>  /*
> @@ -2497,6 +2524,8 @@ static void vmx_hardware_disable(void)
>  	if (cpu_vmxoff())
>  		kvm_spurious_fault();
>  
> +	hv_reset_evmcs();
> +
>  	intel_pt_handle_vmx(0);
>  }
>  
> @@ -8463,27 +8492,8 @@ static void vmx_exit(void)
>  	kvm_exit();
>  
>  #if IS_ENABLED(CONFIG_HYPERV)
> -	if (static_branch_unlikely(&enable_evmcs)) {
> -		int cpu;
> -		struct hv_vp_assist_page *vp_ap;
> -		/*
> -		 * Reset everything to support using non-enlightened VMCS
> -		 * access later (e.g. when we reload the module with
> -		 * enlightened_vmcs=0)
> -		 */
> -		for_each_online_cpu(cpu) {
> -			vp_ap =	hv_get_vp_assist_page(cpu);
> -
> -			if (!vp_ap)
> -				continue;
> -
> -			vp_ap->nested_control.features.directhypercall = 0;
> -			vp_ap->current_nested_vmcs = 0;
> -			vp_ap->enlighten_vmentry = 0;
> -		}
> -
> +	if (static_branch_unlikely(&enable_evmcs))
>  		static_branch_disable(&enable_evmcs);
> -	}
>  #endif
>  	vmx_cleanup_l1d_flush();
>  
>
> base-commit: 5f47ba6894477dfbdc5416467a25fb7acb47d404

-- 
Vitaly