KVM: TDX: Fix list_add corruption during vcpu_load()

During vCPU creation, a vCPU may be destroyed immediately after
kvm_arch_vcpu_create() (e.g., due to vCPU id confiliction). However, the
vcpu_load() inside kvm_arch_vcpu_create() may have associate the vCPU to
pCPU via "list_add(&tdx->cpu_list, &per_cpu(associated_tdvcpus, cpu))"
before invoking tdx_vcpu_free().

Though there's no need to invoke tdh_vp_flush() on the vCPU, failing to
dissociate the vCPU from pCPU (i.e., "list_del(&to_tdx(vcpu)->cpu_list)")
will cause list corruption of the per-pCPU list associated_tdvcpus.

Then, a later list_add() during vcpu_load() would detect list corruption
and print calltrace as shown below.

Dissociate a vCPU from its associated pCPU in tdx_vcpu_free() for the vCPUs
destroyed immediately after creation which must be in
VCPU_TD_STATE_UNINITIALIZED state.

kernel BUG at lib/list_debug.c:29!
Oops: invalid opcode: 0000 [#2] SMP NOPTI
RIP: 0010:__list_add_valid_or_report+0x82/0xd0

Call Trace:
 <TASK>
 tdx_vcpu_load+0xa8/0x120
 vt_vcpu_load+0x25/0x30
 kvm_arch_vcpu_load+0x81/0x300
 vcpu_load+0x55/0x90
 kvm_arch_vcpu_create+0x24f/0x330
 kvm_vm_ioctl_create_vcpu+0x1b1/0x53
 kvm_vm_ioctl+0xc2/0xa60
  __x64_sys_ioctl+0x9a/0xf0
 x64_sys_call+0x10ee/0x20d0
 do_syscall_64+0xc3/0x470
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: d789fa6 ("KVM: TDX: Handle vCPU dissociation")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-29-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: yanzhao56

arch/x86/kvm/vmx/tdx.c

@@ -843,19 +843,52 @@ void tdx_vcpu_put(struct kvm_vcpu *vcpu)
 	tdx_prepare_switch_to_host(vcpu);
 }
 
+/*
+ * Life cycles for a TD and a vCPU:
+ * 1. KVM_CREATE_VM ioctl.
+ *    TD state is TD_STATE_UNINITIALIZED.
+ *    hkid is not assigned at this stage.
+ * 2. KVM_TDX_INIT_VM ioctl.
+ *    TD transitions to TD_STATE_INITIALIZED.
+ *    hkid is assigned after this stage.
+ * 3. KVM_CREATE_VCPU ioctl. (only when TD is TD_STATE_INITIALIZED).
+ *    3.1 tdx_vcpu_create() transitions vCPU state to VCPU_TD_STATE_UNINITIALIZED.
+ *    3.2 vcpu_load() and vcpu_put() in kvm_arch_vcpu_create().
+ *    3.3 (conditional) if any error encountered after kvm_arch_vcpu_create()
+ *        kvm_arch_vcpu_destroy() --> tdx_vcpu_free().
+ * 4. KVM_TDX_INIT_VCPU ioctl.
+ *    tdx_vcpu_init() transitions vCPU state to VCPU_TD_STATE_INITIALIZED.
+ *    vCPU control structures are allocated at this stage.
+ * 5. kvm_destroy_vm().
+ *    5.1 tdx_mmu_release_hkid(): (1) tdh_vp_flush(), disassociates all vCPUs.
+ *                                (2) puts hkid to !assigned state.
+ *    5.2 kvm_destroy_vcpus() --> tdx_vcpu_free():
+ *        transitions vCPU to VCPU_TD_STATE_UNINITIALIZED state.
+ *    5.3 tdx_vm_destroy()
+ *        transitions TD to TD_STATE_UNINITIALIZED state.
+ *
+ * tdx_vcpu_free() can be invoked only at 3.3 or 5.2.
+ * - If at 3.3, hkid is still assigned, but the vCPU must be in
+ *   VCPU_TD_STATE_UNINITIALIZED state.
+ * - if at 5.2, hkid must be !assigned and all vCPUs must be in
+ *   VCPU_TD_STATE_INITIALIZED state and have been dissociated.
+ */
 void tdx_vcpu_free(struct kvm_vcpu *vcpu)
 {
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 	int i;
 
+	if (vcpu->cpu != -1) {
+		KVM_BUG_ON(tdx->state == VCPU_TD_STATE_INITIALIZED, vcpu->kvm);
+		tdx_flush_vp_on_cpu(vcpu);
+		return;
+	}
+
 	/*
 	 * It is not possible to reclaim pages while hkid is assigned. It might
-	 * be assigned if:
-	 * 1. the TD VM is being destroyed but freeing hkid failed, in which
-	 * case the pages are leaked
-	 * 2. TD VCPU creation failed and this on the error path, in which case
-	 * there is nothing to do anyway
+	 * be assigned if the TD VM is being destroyed but freeing hkid failed,
+	 * in which case the pages are leaked.
 	 */
 	if (is_hkid_assigned(kvm_tdx))
 		return;