Re: [PATCH v2 1/1] vhost: fix a double fetch when dequeue offloading

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Yunjian,

On 2/13/25 1:22 PM, Wangyunjian(wangyunjian,TongTu) wrote:
> 
>> -----Original Message-----
>> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
>> Sent: Saturday, December 21, 2024 1:11 AM
>> To: Wangyunjian(wangyunjian,TongTu) <wangyunjian@huawei.com>
>> Cc: dev@dpdk.org; maxime.coquelin@redhat.com; chenbox@nvidia.com;
>> Lilijun (Jerry) <jerry.lilijun@huawei.com>; xiawei (H) <xiawei40@huawei.com>;
>> wangzengyuan <wangzengyuan@huawei.com>; stable@dpdk.org
>> Subject: Re: [PATCH v2 1/1] vhost: fix a double fetch when dequeue offloading
>>
>> On Fri, 20 Dec 2024 11:49:55 +0800
>> Yunjian Wang <wangyunjian@huawei.com> wrote:
>>
>>> The hdr->csum_start does two successive reads from user space to read a
>>> variable length data structure. The result overflow if the data structure
>>> changes between the two reads.
>>>
>>> To fix this, we can prevent double fetch issue by copying virtio_hdr to
>>> the temporary variable.
>>>
>>> Fixes: 4dc4e33ffa10 ("net/virtio: fix Rx checksum calculation")
>>> Cc: stable@dpdk.org
>>>
>>> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
>>
>>
>> How about something like the following *untested*
> 
> I agree. Can you fix it?

As you were the initial poster, I suggest you implement it and test it
by yourself.

It is not too late for -rc2.

Thanks,
Maxime

> Thanks,
> Yunjian
> 
>>
>> diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
>> index 69901ab3b5..c65cb639b2 100644
>> --- a/lib/vhost/virtio_net.c
>> +++ b/lib/vhost/virtio_net.c
>> @@ -2861,25 +2861,28 @@ vhost_dequeue_offload(struct virtio_net *dev,
>> struct virtio_net_hdr *hdr,
>>   	}
>>   }
>>
>> -static __rte_noinline void
>> +static inline int
>>   copy_vnet_hdr_from_desc(struct virtio_net_hdr *hdr,
>> -		struct buf_vector *buf_vec)
>> +			const struct buf_vector *buf_vec,
>> +			uint16_t nr_vec)
>>   {
>> -	uint64_t len;
>> -	uint64_t remain = sizeof(struct virtio_net_hdr);
>> -	uint64_t src;
>> -	uint64_t dst = (uint64_t)(uintptr_t)hdr;
>> +	size_t remain = sizeof(struct virtio_net_hdr);
>> +	uint8_t *dst = (uint8_t *)hdr;
>>
>> -	while (remain) {
>> -		len = RTE_MIN(remain, buf_vec->buf_len);
>> -		src = buf_vec->buf_addr;
>> -		rte_memcpy((void *)(uintptr_t)dst,
>> -				(void *)(uintptr_t)src, len);
>> +	while (remain > 0) {
>> +		size_t len = RTE_MIN(remain, buf_vec->buf_len);
>> +		const void *src = (const void *)(uintptr_t)buf_vec->buf_addr;
>>
>> +		if (unlikely(nr_vec == 0))
>> +			return -1;
>> +
>> +		memcpy(dst, src, len);
>>   		remain -= len;
>>   		dst += len;
>>   		buf_vec++;
>> +		--nr_vec;
>>   	}
>> +	return 0;
>>   }
>>
>>   static __rte_always_inline int
>> @@ -2908,16 +2911,12 @@ desc_to_mbuf(struct virtio_net *dev, struct
>> vhost_virtqueue *vq,
>>   	 */
>>
>>   	if (virtio_net_with_host_offload(dev)) {
>> -		if (unlikely(buf_vec[0].buf_len < sizeof(struct virtio_net_hdr))) {
>> -			/*
>> -			 * No luck, the virtio-net header doesn't fit
>> -			 * in a contiguous virtual area.
>> -			 */
>> -			copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec);
>> -			hdr = &tmp_hdr;
>> -		} else {
>> -			hdr = (struct virtio_net_hdr *)((uintptr_t)buf_vec[0].buf_addr);
>> -		}
>> +		if (unlikely(copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec, nr_vec) !=
>> 0))
>> +		    return -1;
>> +
>> +		/* ensure that compiler does not delay copy */
>> +		rte_compiler_barrier();
>> +		hdr = &tmp_hdr;
>>   	}
>>
>>   	for (vec_idx = 0; vec_idx < nr_vec; vec_idx++) {
>> @@ -3363,7 +3362,6 @@ virtio_dev_tx_batch_packed(struct virtio_net *dev,
>>   {
>>   	uint16_t avail_idx = vq->last_avail_idx;
>>   	uint32_t buf_offset = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>> -	struct virtio_net_hdr *hdr;
>>   	uintptr_t desc_addrs[PACKED_BATCH_SIZE];
>>   	uint16_t ids[PACKED_BATCH_SIZE];
>>   	uint16_t i;
>> @@ -3382,8 +3380,12 @@ virtio_dev_tx_batch_packed(struct virtio_net *dev,
>>
>>   	if (virtio_net_with_host_offload(dev)) {
>>   		vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
>> -			hdr = (struct virtio_net_hdr *)(desc_addrs[i]);
>> -			vhost_dequeue_offload(dev, hdr, pkts[i], legacy_ol_flags);
>> +			struct virtio_net_hdr hdr;
>> +
>> +			memcpy(&hdr, (void *)desc_addrs[i], sizeof(struct
>> virtio_net_hdr));
>> +			rte_compiler_barrier();
>> +
>> +			vhost_dequeue_offload(dev, &hdr, pkts[i], legacy_ol_flags);
>>   		}
>>   	}
>>
>