KVM: Prevent module exit until all VMs are freed

Tie the lifetime the KVM module to the lifetime of each VM via
kvm.users_count. This way anything that grabs a reference to the VM via
kvm_get_kvm() cannot accidentally outlive the KVM module.

Prior to this commit, the lifetime of the KVM module was tied to the
lifetime of /dev/kvm file descriptors, VM file descriptors, and vCPU
file descriptors by their respective file_operations "owner" field.
This approach is insufficient because references grabbed via
kvm_get_kvm() do not prevent closing any of the aforementioned file
descriptors.

This fixes a long standing theoretical bug in KVM that at least affects
async page faults. kvm_setup_async_pf() grabs a reference via
kvm_get_kvm(), and drops it in an asynchronous work callback. Nothing
prevents the VM file descriptor from being closed and the KVM module
from being unloaded before this callback runs.

Fixes: af585b9 ("KVM: Halt vcpu if page it tries to access is swapped out")
Fixes: 3d3aab1 ("KVM: set owner of cpu and vm file operations")
Cc: stable@vger.kernel.org
Suggested-by: Ben Gardon <bgardon@google.com>
[ Based on a patch from Ben implemented for Google's kernel. ]
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220303183328.1499189-2-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -117,6 +117,8 @@ EXPORT_SYMBOL_GPL(kvm_debugfs_dir);
 
 static const struct file_operations stat_fops_per_vm;
 
+static struct file_operations kvm_chardev_ops;
+
 static long kvm_vcpu_ioctl(struct file *file, unsigned int ioctl,
 			   unsigned long arg);
 #ifdef CONFIG_KVM_COMPAT
@@ -1131,6 +1133,16 @@ static struct kvm *kvm_create_vm(unsigned long type)
 	preempt_notifier_inc();
 	kvm_init_pm_notifier(kvm);
 
+	/*
+	 * When the fd passed to this ioctl() is opened it pins the module,
+	 * but try_module_get() also prevents getting a reference if the module
+	 * is in MODULE_STATE_GOING (e.g. if someone ran "rmmod --wait").
+	 */
+	if (!try_module_get(kvm_chardev_ops.owner)) {
+		r = -ENODEV;
+		goto out_err;
+	}
+
 	return kvm;
 
 out_err:
@@ -1220,6 +1232,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
 	preempt_notifier_dec();
 	hardware_disable_all();
 	mmdrop(mm);
+	module_put(kvm_chardev_ops.owner);
 }
 
 void kvm_get_kvm(struct kvm *kvm)