KVM: arm64: Unregister redistributor for failed vCPU creation

Alex reports that syzkaller has managed to trigger a use-after-free when
tearing down a VM:

  BUG: KASAN: slab-use-after-free in kvm_put_kvm+0x300/0xe68 virt/kvm/kvm_main.c:5769
  Read of size 8 at addr ffffff801c6890d0 by task syz.3.2219/10758

  CPU: 3 UID: 0 PID: 10758 Comm: syz.3.2219 Not tainted 6.11.0-rc6-dirty torvalds#64
  Hardware name: linux,dummy-virt (DT)
  Call trace:
   dump_backtrace+0x17c/0x1a8 arch/arm64/kernel/stacktrace.c:317
   show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:324
   __dump_stack lib/dump_stack.c:93 [inline]
   dump_stack_lvl+0x94/0xc0 lib/dump_stack.c:119
   print_report+0x144/0x7a4 mm/kasan/report.c:377
   kasan_report+0xcc/0x128 mm/kasan/report.c:601
   __asan_report_load8_noabort+0x20/0x2c mm/kasan/report_generic.c:381
   kvm_put_kvm+0x300/0xe68 virt/kvm/kvm_main.c:5769
   kvm_vm_release+0x4c/0x60 virt/kvm/kvm_main.c:1409
   __fput+0x198/0x71c fs/file_table.c:422
   ____fput+0x20/0x30 fs/file_table.c:450
   task_work_run+0x1cc/0x23c kernel/task_work.c:228
   do_notify_resume+0x144/0x1a0 include/linux/resume_user_mode.h:50
   el0_svc+0x64/0x68 arch/arm64/kernel/entry-common.c:169
   el0t_64_sync_handler+0x90/0xfc arch/arm64/kernel/entry-common.c:730
   el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:598

Upon closer inspection, it appears that we do not properly tear down the
MMIO registration for a vCPU that fails creation late in the game, e.g.
a vCPU w/ the same ID already exists in the VM.

It is important to consider the context of commit that introduced this bug
by moving the unregistration out of __kvm_vgic_vcpu_destroy(). That
change correctly sought to avoid an srcu v. config_lock inversion by
breaking up the vCPU teardown into two parts, one guarded by the
config_lock.

Fix the use-after-free while avoiding lock inversion by adding a
special-cased unregistration to __kvm_vgic_vcpu_destroy(). This is safe
because failed vCPUs are torn down outside of the config_lock.

Cc: stable@vger.kernel.org
Fixes: f616506 ("KVM: arm64: vgic: Don't hold config_lock while unregistering redistributors")
Reported-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Message-Id: <20241007223909.2157336-1-oliver.upton@linux.dev>

arch/arm64/kvm/vgic/vgic-init.c

@@ -417,8 +417,28 @@ static void __kvm_vgic_vcpu_destroy(struct kvm_vcpu *vcpu)
 	kfree(vgic_cpu->private_irqs);
 	vgic_cpu->private_irqs = NULL;
 
-	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3)
+	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3) {
+		/*
+		 * If this vCPU is being destroyed because of a failed creation
+		 * then unregister the redistributor to avoid leaving behind a
+		 * dangling pointer to the vCPU struct.
+		 *
+		 * vCPUs that have been successfully created (i.e. added to
+		 * kvm->vcpu_array) get unregistered in kvm_vgic_destroy(), as
+		 * this function gets called while holding kvm->arch.config_lock
+		 * in the VM teardown path and would otherwise introduce a lock
+		 * inversion w.r.t. kvm->srcu.
+		 *
+		 * vCPUs that failed creation are torn down outside of the
+		 * kvm->arch.config_lock and do not get unregistered in
+		 * kvm_vgic_destroy(), meaning it is both safe and necessary to
+		 * do so here.
+		 */
+		if (kvm_get_vcpu_by_id(vcpu->kvm, vcpu->vcpu_id) != vcpu)
+			vgic_unregister_redist_iodev(vcpu);
+
 		vgic_cpu->rd_iodev.base_addr = VGIC_ADDR_UNDEF;
+	}
 }
 
 void kvm_vgic_vcpu_destroy(struct kvm_vcpu *vcpu)