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

Message ID 20250224235542.2562848-4-seanjc@google.com (mailing list archive)
State New
Headers show
Series KVM: x86: nVMX IRQ fix and VM teardown cleanups | expand

Commit Message

Sean Christopherson Feb. 24, 2025, 11:55 p.m. UTC 
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(+)
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Patch

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));
 	}
 
 	atomic_set(&kvm->online_vcpus, 0);