Re: [PATCH 07/29] KVM: do not use online_vcpus to test vCPU validity

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 01, 2025, Paolo Bonzini wrote:
> Different planes can initialize their vCPUs separately, therefore there is
> no single online_vcpus value that can be used to test that a vCPU has
> indeed been fully initialized.
> 
> Use the shiny new plane field instead, initializing it to an invalid value
> (-1) while the vCPU is visible in the xarray but may still disappear if
> the creation fails.

Checking vcpu->plane _in addition_ to online_cpus seems way safer than checking
vcpu->plane _instead_ of online_cpus.  Even if we end up checking only vcpu->plane,
I think that should be a separate patch.

> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/i8254.c     |  3 ++-
>  include/linux/kvm_host.h | 23 ++++++-----------------
>  virt/kvm/kvm_main.c      | 20 +++++++++++++-------
>  3 files changed, 21 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index d7ab8780ab9e..e3a3e7b90c26 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -260,9 +260,10 @@ static void pit_do_work(struct kthread_work *work)
>  	 * VCPUs and only when LVT0 is in NMI mode.  The interrupt can
>  	 * also be simultaneously delivered through PIC and IOAPIC.
>  	 */
> -	if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0)
> +	if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0) {

Spurious change (a good change, but noisy for this patch).

>  		kvm_for_each_vcpu(i, vcpu, kvm)
>  			kvm_apic_nmi_wd_deliver(vcpu);
> +	}
>  }
>  
>  static enum hrtimer_restart pit_timer_fn(struct hrtimer *data)
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 4d408d1d5ccc..0db27814294f 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -992,27 +992,16 @@ static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
>  
>  static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
>  {
> -	int num_vcpus = atomic_read(&kvm->online_vcpus);
> -
> -	/*
> -	 * Explicitly verify the target vCPU is online, as the anti-speculation
> -	 * logic only limits the CPU's ability to speculate, e.g. given a "bad"
> -	 * index, clamping the index to 0 would return vCPU0, not NULL.
> -	 */
> -	if (i >= num_vcpus)
> +	struct kvm_vcpu *vcpu = xa_load(&kvm->vcpu_array, i);

newline

> +	if (vcpu && unlikely(vcpu->plane == -1))
>  		return NULL;
>  
> -	i = array_index_nospec(i, num_vcpus);

Don't we still need to prevent speculating into the xarray ?

> -
> -	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu.  */
> -	smp_rmb();
> -	return xa_load(&kvm->vcpu_array, i);
> +	return vcpu;
>  }
>  
> -#define kvm_for_each_vcpu(idx, vcpup, kvm)				\
> -	if (atomic_read(&kvm->online_vcpus))				\
> -		xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0,	\
> -				  (atomic_read(&kvm->online_vcpus) - 1))
> +#define kvm_for_each_vcpu(idx, vcpup, kvm)			\
> +	xa_for_each(&kvm->vcpu_array, idx, vcpup)		\
> +		if ((vcpup)->plane == -1) ; else		\
>  
>  static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
>  {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e343905e46d8..eba02cb7cc57 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4186,6 +4186,11 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  		goto unlock_vcpu_destroy;
>  	}
>  
> +	/*
> +	 * Store an invalid plane number until fully initialized.  xa_insert() has
> +	 * release semantics, which ensures the write is visible to kvm_get_vcpu().
> +	 */
> +	vcpu->plane = -1;
>  	vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
>  	r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
>  	WARN_ON_ONCE(r == -EBUSY);
> @@ -4195,7 +4200,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  	/*
>  	 * Now it's all set up, let userspace reach it.  Grab the vCPU's mutex
>  	 * so that userspace can't invoke vCPU ioctl()s until the vCPU is fully
> -	 * visible (per online_vcpus), e.g. so that KVM doesn't get tricked
> +	 * visible (valid vcpu->plane), e.g. so that KVM doesn't get tricked
>  	 * into a NULL-pointer dereference because KVM thinks the _current_
>  	 * vCPU doesn't exist.  As a bonus, taking vcpu->mutex ensures lockdep
>  	 * knows it's taken *inside* kvm->lock.
> @@ -4206,12 +4211,13 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  	if (r < 0)
>  		goto kvm_put_xa_erase;
>  
> -	/*
> -	 * Pairs with smp_rmb() in kvm_get_vcpu.  Store the vcpu
> -	 * pointer before kvm->online_vcpu's incremented value.

Bad me for not updating this comment, but kvm_vcpu_on_spin() also pairs with this
barrier, and needs to be updated to be planes-aware, e.g. this looks like a NULL
pointer deref waiting to happen:

		vcpu = xa_load(&plane->vcpu_array, idx);
		if (!READ_ONCE(vcpu->ready))
			continue;