[PATCH] vhost: fix data-plane access to released vq

[Maxime Coquelin]

Hi,

On 1/27/22 11:30, Wang, YuanX wrote:
> Hi Maxime,
> 
>> -----Original Message-----
>> From: Maxime Coquelin <maxime.coquelin at redhat.com>
>> Sent: Wednesday, January 26, 2022 10:03 PM
>> To: Wang, YuanX <yuanx.wang at intel.com>; Xia, Chenbo
>> <chenbo.xia at intel.com>
>> Cc: dev at dpdk.org; Hu, Jiayu <jiayu.hu at intel.com>; Ding, Xuan
>> <xuan.ding at intel.com>; Ma, WenwuX <wenwux.ma at intel.com>; Ling,
>> WeiX <weix.ling at intel.com>
>> Subject: Re: [PATCH] vhost: fix data-plane access to released vq
>>
>> Hi Yuan,
>>
>> On 12/3/21 17:34, Yuan Wang wrote:
>>> From: yuan wang <yuanx.wang at intel.com>
>>>
>>> When numa reallocation occurs, numa_realoc() on the control plane will
>>> free the old vq. If rte_vhost_dequeue_burst() on the data plane get
>>> the vq just before release, then it will access the released vq. We
>>> need to put the
>>> vq->access_lock into struct virtio_net to ensure that it
>>> can prevents this situation.
>>
>>
>> This patch is a fix, so the Fixes tag would be needed.
>>
>> But are you really facing this issue, or this is just based on code review?
> 
> This issue is run-time checked with AddressSanitizer which can be turned on by:
> meson configure -Db_sanitize=address <build_dir>
> 
>>
>> Currently NUMA reallocation is called whenever
>> translate_ring_addresses() is called.
>>
>> translate_ring_addresses() is primarly called at device initialization, before
>> the .new_device() callback is called. At that stage, there is no risk to
>> performa NUMA reallocation as the application is not expected to use APIs
>> requiring vq->access_lock acquisition.
>>
>> But I agree there are possibilities that numa_realloc() gets called while device
>> is in running state. But even if that happened, I don't think it is possible that
>> numa_realloc() ends-up reallocating the virtqueue on a different NUMA
>> node (the vring should not have moved from a physical memory standpoint).
>> And if even it happened, we should be safe because we ensure the VQ was
>> not ready (so not usable by the
>> application) before proceeding with reallocation:
> 
> Here is a scenario where VQ ready has not been set:
> 1. run the testpmd and then start the data plane process.
> 2. run the front-end.
> 3. new_device() gets called when the first two queues are ready, even if the later queues are not.
> 4. when processing messages from the later queues, it may go to numa_realloc(), the ready flag has not been set and therefore can be reallocated.

I will need a bit more details here.

AFAICT, if the ready flag is not set for a given virtqueue, the
virtqueue is not supposed to be exposed to the application. Is there a
case where it happens? If so, the fix should consist in ensuring the
application cannot use the virtqueue if it is not ready.

Regards,
Maxime

> 
> If all the queues are ready before call new_deivce(), this issue does not occur.
> I think maybe it is another solution.

No, that was the older behaviour but causes issues with vDPA.
We cannot just revert to older behaviour.

Thanks,
Maxime

> Thanks,
> Yuan
> 
>>
>> static struct virtio_net*
>> numa_realloc(struct virtio_net *dev, int index) {
>> 	int node, dev_node;
>> 	struct virtio_net *old_dev;
>> 	struct vhost_virtqueue *vq;
>> 	struct batch_copy_elem *bce;
>> 	struct guest_page *gp;
>> 	struct rte_vhost_memory *mem;
>> 	size_t mem_size;
>> 	int ret;
>>
>> 	old_dev = dev;
>> 	vq = dev->virtqueue[index];
>>
>> 	/*
>> 	 * If VQ is ready, it is too late to reallocate, it certainly already
>> 	 * happened anyway on VHOST_USER_SET_VRING_ADRR.
>> 	 */
>> 	if (vq->ready)
>> 		return dev;
>>
>> So, if this is fixing a real issue, I would need more details on the issue in order
>> to understand why vq->ready was not set when it should have been.
>>
>> On a side note, while trying to understand how you could face an issue, I
>> noticed that translate_ring_addresses() may be called by
>> vhost_user_iotlb_msg(). In that case, vq->access_lock is not held as this is
>> the handler for VHOST_USER_IOTLB_MSG. We may want to protect
>> translate_ring_addresses() calls with locking the VQ locks. I will post a fix for
>> it.
>>
>>> Signed-off-by: Yuan Wang <yuanx.wang at intel.com>
>>> ---
>>>    lib/vhost/vhost.c      | 26 +++++++++++++-------------
>>>    lib/vhost/vhost.h      |  4 +---
>>>    lib/vhost/vhost_user.c |  4 ++--
>>>    lib/vhost/virtio_net.c | 16 ++++++++--------
>>>    4 files changed, 24 insertions(+), 26 deletions(-)
>>>
>>
>> ...
>>
>>> diff --git a/lib/vhost/vhost.h b/lib/vhost/vhost.h index
>>> 7085e0885c..f85ce4fda5 100644
>>> --- a/lib/vhost/vhost.h
>>> +++ b/lib/vhost/vhost.h
>>> @@ -185,9 +185,6 @@ struct vhost_virtqueue {
>>>    	bool			access_ok;
>>>    	bool			ready;
>>>
>>> -	rte_spinlock_t		access_lock;
>>> -
>>> -
>>>    	union {
>>>    		struct vring_used_elem  *shadow_used_split;
>>>    		struct vring_used_elem_packed *shadow_used_packed;
>> @@ -384,6
>>> +381,7 @@ struct virtio_net {
>>>    	int			extbuf;
>>>    	int			linearbuf;
>>>    	struct vhost_virtqueue	*virtqueue[VHOST_MAX_QUEUE_PAIRS * 2];
>>> +	rte_spinlock_t		vq_access_lock[VHOST_MAX_QUEUE_PAIRS
>> * 2];
>>
>> The problem here is that you'll be introducing false sharing, so I expect
>> performance to no more scale with the number of queues.
>>
>> It also consumes unnecessary memory.
>>
>>>    	struct inflight_mem_info *inflight_info;
>>>    #define IF_NAME_SZ (PATH_MAX > IFNAMSIZ ? PATH_MAX : IFNAMSIZ)
>>>    	char			ifname[IF_NAME_SZ];
>>
>> Thanks,
>> Maxime
>