Re: [PATCH 2/6] vhost: validate MMU notifier registration

[Jason Wang <jasowang@redhat.com>]

On 2019/7/23 下午5:17, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:14AM -0400, Jason Wang wrote:
>> The return value of mmu_notifier_register() is not checked in
>> vhost_vring_set_num_addr(). This will cause an out of sync between mm
>> and MMU notifier thus a double free. To solve this, introduce a
>> boolean flag to track whether MMU notifier is registered and only do
>> unregistering when it was true.
>>
>> Reported-and-tested-by:
>> syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com
>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> Right. This fixes the bug.
> But it's not great that simple things like
> setting vq address put pressure on memory allocator.
> Also, if we get a single during processing
> notifier register will fail, disabling optimization permanently.


Yes, but do we really care about this case. E.g we even fail for -ENOMEM 
for set owner.


>
> In fact, see below:
>
>
>> ---
>>   drivers/vhost/vhost.c | 19 +++++++++++++++----
>>   drivers/vhost/vhost.h |  1 +
>>   2 files changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 34c0d970bcbc..058191d5efad 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -630,6 +630,7 @@ void vhost_dev_init(struct vhost_dev *dev,
>>   	dev->iov_limit = iov_limit;
>>   	dev->weight = weight;
>>   	dev->byte_weight = byte_weight;
>> +	dev->has_notifier = false;
>>   	init_llist_head(&dev->work_list);
>>   	init_waitqueue_head(&dev->wait);
>>   	INIT_LIST_HEAD(&dev->read_list);
>> @@ -731,6 +732,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
>>   	if (err)
>>   		goto err_mmu_notifier;
>>   #endif
>> +	dev->has_notifier = true;
>>   
>>   	return 0;
>>   
> I just noticed that set owner now fails if we get a signal.
> Userspace could retry in theory but it does not:
> this is userspace abi breakage since it used to only
> fail on invalid input.


Well, at least kthread_create() and vhost_dev_alloc_iovecs() will 
allocate memory.

Thanks


>
>> @@ -960,7 +962,11 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>>   	}
>>   	if (dev->mm) {
>>   #if VHOST_ARCH_CAN_ACCEL_UACCESS
>> -		mmu_notifier_unregister(&dev->mmu_notifier, dev->mm);
>> +		if (dev->has_notifier) {
>> +			mmu_notifier_unregister(&dev->mmu_notifier,
>> +						dev->mm);
>> +			dev->has_notifier = false;
>> +		}
>>   #endif
>>   		mmput(dev->mm);
>>   	}
>> @@ -2065,8 +2071,10 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>>   	/* Unregister MMU notifer to allow invalidation callback
>>   	 * can access vq->uaddrs[] without holding a lock.
>>   	 */
>> -	if (d->mm)
>> +	if (d->has_notifier) {
>>   		mmu_notifier_unregister(&d->mmu_notifier, d->mm);
>> +		d->has_notifier = false;
>> +	}
>>   
>>   	vhost_uninit_vq_maps(vq);
>>   #endif
>> @@ -2086,8 +2094,11 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>>   	if (r == 0)
>>   		vhost_setup_vq_uaddr(vq);
>>   
>> -	if (d->mm)
>> -		mmu_notifier_register(&d->mmu_notifier, d->mm);
>> +	if (d->mm) {
>> +		r = mmu_notifier_register(&d->mmu_notifier, d->mm);
>> +		if (!r)
>> +			d->has_notifier = true;
>> +	}
>>   #endif
>>   
>>   	mutex_unlock(&vq->mutex);
>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>> index 819296332913..a62f56a4cf72 100644
>> --- a/drivers/vhost/vhost.h
>> +++ b/drivers/vhost/vhost.h
>> @@ -214,6 +214,7 @@ struct vhost_dev {
>>   	int iov_limit;
>>   	int weight;
>>   	int byte_weight;
>> +	bool has_notifier;
>>   };
>>   
>>   bool vhost_exceeds_weight(struct vhost_virtqueue *vq, int pkts, int total_len);
>> -- 
>> 2.18.1