From: Sean Christopherson Date: Mon, 24 Feb 2025 23:55:38 +0000 (-0800) Subject: KVM: Assert that a destroyed/freed vCPU is no longer visible X-Git-Tag: v6.15-rc1~195^2~1^2~3 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ed8f966331d618a9577eb79068706217a472be78;p=thirdparty%2Fkernel%2Flinux.git KVM: Assert that a destroyed/freed vCPU is no longer visible 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 Message-ID: <20250224235542.2562848-4-seanjc@google.com> Signed-off-by: Paolo Bonzini --- diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index ba0327e2d0d33..28ba54b874255 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)); } atomic_set(&kvm->online_vcpus, 0);