KVM: x86: protect KVM_CREATE_PIT/KVM_CREATE_PIT2 with kvm->lock

The syzkaller folks reported a NULL pointer dereference that seems
to be cause by a race between KVM_CREATE_IRQCHIP and KVM_CREATE_PIT2.
The former takes kvm->lock (except when registering the devices,
which needs kvm->slots_lock); the latter takes kvm->slots_lock only.
Change KVM_CREATE_PIT2 to follow the same model as KVM_CREATE_IRQCHIP.

Testcase:

    #include <pthread.h>
    #include <linux/kvm.h>
    #include <fcntl.h>
    #include <sys/ioctl.h>
    #include <stdint.h>
    #include <string.h>
    #include <stdlib.h>
    #include <sys/syscall.h>
    #include <unistd.h>

    long r[23];

    void* thr1(void* arg)
    {
        struct kvm_pit_config pitcfg = { .flags = 4 };
        switch ((long)arg) {
        case 0: r[2]  = open("/dev/kvm", O_RDONLY|O_ASYNC);    break;
        case 1: r[3]  = ioctl(r[2], KVM_CREATE_VM, 0);         break;
        case 2: r[4]  = ioctl(r[3], KVM_CREATE_IRQCHIP, 0);    break;
        case 3: r[22] = ioctl(r[3], KVM_CREATE_PIT2, &pitcfg); break;
        }
        return 0;
    }

    int main(int argc, char **argv)
    {
        long i;
        pthread_t th[4];

        memset(r, -1, sizeof(r));
        for (i = 0; i < 4; i++) {
            pthread_create(&th[i], 0, thr, (void*)i);
            if (argc > 1 && rand()%2) usleep(rand()%1000);
        }
        usleep(20000);
        return 0;
    }

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/i8254.c

@@ -645,7 +645,6 @@ static const struct kvm_io_device_ops speaker_dev_ops = {
 	.write    = speaker_ioport_write,
 };
 
-/* Caller must hold slots_lock */
 struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 {
 	struct kvm_pit *pit;
@@ -690,6 +689,7 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 
 	kvm_pit_set_reinject(pit, true);
 
+	mutex_lock(&kvm->slots_lock);
 	kvm_iodevice_init(&pit->dev, &pit_dev_ops);
 	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, KVM_PIT_BASE_ADDRESS,
 				      KVM_PIT_MEM_LENGTH, &pit->dev);
@@ -704,12 +704,14 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 		if (ret < 0)
 			goto fail_register_speaker;
 	}
+	mutex_unlock(&kvm->slots_lock);
 
 	return pit;
 
 fail_register_speaker:
 	kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &pit->dev);
 fail_register_pit:
+	mutex_unlock(&kvm->slots_lock);
 	kvm_pit_set_reinject(pit, false);
 	kthread_stop(pit->worker_task);
 fail_kthread:

arch/x86/kvm/x86.c

@@ -3879,7 +3879,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
 				   sizeof(struct kvm_pit_config)))
 			goto out;
 	create_pit:
-		mutex_lock(&kvm->slots_lock);
+		mutex_lock(&kvm->lock);
 		r = -EEXIST;
 		if (kvm->arch.vpit)
 			goto create_pit_unlock;
@@ -3888,7 +3888,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
 		if (kvm->arch.vpit)
 			r = 0;
 	create_pit_unlock:
-		mutex_unlock(&kvm->slots_lock);
+		mutex_unlock(&kvm->lock);
 		break;
 	case KVM_GET_IRQCHIP: {
 		/* 0: PIC master, 1: PIC slave, 2: IOAPIC */