KVM: x86: Free vCPUs before freeing VM state

Free vCPUs before freeing any VM state, as both SVM and VMX may access
VM state when "freeing" a vCPU that is currently "in" L2, i.e. that needs
to be kicked out of nested guest mode.

Commit 6fcee03 ("KVM: x86: avoid loading a vCPU after .vm_destroy was
called") partially fixed the issue, but for unknown reasons only moved the
MMU unloading before VM destruction.  Complete the change, and free all
vCPU state prior to destroying VM state, as nVMX accesses even more state
than nSVM.

In addition to the AVIC, KVM can hit a use-after-free on MSR filters:

  kvm_msr_allowed+0x4c/0xd0
  __kvm_set_msr+0x12d/0x1e0
  kvm_set_msr+0x19/0x40
  load_vmcs12_host_state+0x2d8/0x6e0 [kvm_intel]
  nested_vmx_vmexit+0x715/0xbd0 [kvm_intel]
  nested_vmx_free_vcpu+0x33/0x50 [kvm_intel]
  vmx_free_vcpu+0x54/0xc0 [kvm_intel]
  kvm_arch_vcpu_destroy+0x28/0xf0
  kvm_vcpu_destroy+0x12/0x50
  kvm_arch_destroy_vm+0x12c/0x1c0
  kvm_put_kvm+0x263/0x3c0
  kvm_vm_release+0x21/0x30

and an upcoming fix to process injectable interrupts on nested VM-Exit
will access the PIC:

  BUG: kernel NULL pointer dereference, address: 0000000000000090
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  CPU: 23 UID: 1000 PID: 2658 Comm: kvm-nx-lpage-re
  RIP: 0010:kvm_cpu_has_extint+0x2f/0x60 [kvm]
  Call Trace:
   <TASK>
   kvm_cpu_has_injectable_intr+0xe/0x60 [kvm]
   nested_vmx_vmexit+0x2d7/0xdf0 [kvm_intel]
   nested_vmx_free_vcpu+0x40/0x50 [kvm_intel]
   vmx_vcpu_free+0x2d/0x80 [kvm_intel]
   kvm_arch_vcpu_destroy+0x2d/0x130 [kvm]
   kvm_destroy_vcpus+0x8a/0x100 [kvm]
   kvm_arch_destroy_vm+0xa7/0x1d0 [kvm]
   kvm_destroy_vm+0x172/0x300 [kvm]
   kvm_vcpu_release+0x31/0x50 [kvm]

Inarguably, both nSVM and nVMX need to be fixed, but punt on those
cleanups for the moment.  Conceptually, vCPUs should be freed before VM
state.  Assets like the I/O APIC and PIC _must_ be allocated before vCPUs
are created, so it stands to reason that they must be freed _after_ vCPUs
are destroyed.

Reported-by: Aaron Lewis <aaronlewis@google.com>
Closes: https://lore.kernel.org/all/20240703175618.2304869-2-aaronlewis@google.com
Cc: Jim Mattson <jmattson@google.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@intel.com>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250224235542.2562848-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -12877,11 +12877,11 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 		mutex_unlock(&kvm->slots_lock);
 	}
 	kvm_unload_vcpu_mmus(kvm);
+	kvm_destroy_vcpus(kvm);
 	kvm_x86_call(vm_destroy)(kvm);
 	kvm_free_msr_filter(srcu_dereference_check(kvm->arch.msr_filter, &kvm->srcu, 1));
 	kvm_pic_destroy(kvm);
 	kvm_ioapic_destroy(kvm);
-	kvm_destroy_vcpus(kvm);
 	kvfree(rcu_dereference_check(kvm->arch.apic_map, 1));
 	kfree(srcu_dereference_check(kvm->arch.pmu_event_filter, &kvm->srcu, 1));
 	kvm_mmu_uninit_vm(kvm);