Re: [PATCH 3/7] KVM: Assert that a destroyed/freed vCPU is no longer visible

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 25, 2025, Yan Zhao wrote:
> On Mon, Feb 24, 2025 at 03:55:38PM -0800, Sean Christopherson wrote:
> > After freeing a vCPU, assert that it is no longer reachable, and that
> > kvm_get_vcpu() doesn't return garbage or a pointer to some other vCPU.
> > While KVM obviously shouldn't be attempting to access a freed vCPU, it's
> > all too easy for KVM to make a VM-wide request, e.g. via KVM_BUG_ON() or
> > kvm_flush_remote_tlbs().
> > 
> > Alternatively, KVM could short-circuit problematic paths if the VM's
> > refcount has gone to zero, e.g. in kvm_make_all_cpus_request(), or KVM
> > could try disallow making global requests during teardown.  But given that
> > deleting the vCPU from the array Just Works, adding logic to the requests
> > path is unnecessary, and trying to make requests illegal during teardown
> > would be a fool's errand.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  virt/kvm/kvm_main.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 201c14ff476f..991e8111e88b 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -489,6 +489,14 @@ void kvm_destroy_vcpus(struct kvm *kvm)
> >  	kvm_for_each_vcpu(i, vcpu, kvm) {
> >  		kvm_vcpu_destroy(vcpu);
> >  		xa_erase(&kvm->vcpu_array, i);
> > +
> > +		/*
> > +		 * Assert that the vCPU isn't visible in any way, to ensure KVM
> > +		 * doesn't trigger a use-after-free if destroying vCPUs results
> > +		 * in VM-wide request, e.g. to flush remote TLBs when tearing
> > +		 * down MMUs, or to mark the VM dead if a KVM_BUG_ON() fires.
> > +		 */
> > +		WARN_ON_ONCE(xa_load(&kvm->vcpu_array, i) || kvm_get_vcpu(kvm, i));
> As xa_erase() says "After this function returns, loading from @index will return
> %NULL", is this checking of xa_load() necessary?

None of this is "necessary".  My goal with the assert is to (a) document that KVM
relies the vCPU to be NULL/unreachable and (b) to help ensure that doesn't change
in the future.  Checking xa_load() is mostly about (a).

That said, I agree checking xa_load() is more than a bit gratuitous.  I have no
objection to checking only kvm_get_vcpu().