netdev - Re: [PATCH net] virtio-net: fix overflow inside virtnet_rq

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Message-ID: <226a629f-d2fb-2969-b210-072262984c54@oracle.com>
Date: Tue, 20 Aug 2024 16:05:31 -0700
From: Si-Wei Liu <si-wei.liu@...cle.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Jason Wang <jasowang@...hat.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
        virtualization@...ts.linux.dev, Darren Kenny <darren.kenny@...cle.com>,
        netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH net] virtio-net: fix overflow inside virtnet_rq_alloc



On 8/20/2024 1:09 PM, Michael S. Tsirkin wrote:
> On Tue, Aug 20, 2024 at 12:44:46PM -0700, Si-Wei Liu wrote:
>>
>> On 8/20/2024 12:19 AM, Xuan Zhuo wrote:
>>> leads to regression on VM with the sysctl value of:
>>>
>>> - net.core.high_order_alloc_disable=1
>>>
>>> which could see reliable crashes or scp failure (scp a file 100M in size
>>> to VM):
>>>
>>> The issue is that the virtnet_rq_dma takes up 16 bytes at the beginning
>>> of a new frag. When the frag size is larger than PAGE_SIZE,
>>> everything is fine. However, if the frag is only one page and the
>>> total size of the buffer and virtnet_rq_dma is larger than one page, an
>>> overflow may occur. In this case, if an overflow is possible, I adjust
>>> the buffer size. If net.core.high_order_alloc_disable=1, the maximum
>>> buffer size is 4096 - 16. If net.core.high_order_alloc_disable=0, only
>>> the first buffer of the frag is affected.
>>>
>>> Fixes: f9dac92ba908 ("virtio_ring: enable premapped mode whatever use_dma_api")
>>> Reported-by: "Si-Wei Liu" <si-wei.liu@...cle.com>
>>> Closes: http://lore.kernel.org/all/8b20cc28-45a9-4643-8e87-ba164a540c0a@oracle.com
>>> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>>> ---
>>>    drivers/net/virtio_net.c | 12 +++++++++---
>>>    1 file changed, 9 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index c6af18948092..e5286a6da863 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -918,9 +918,6 @@ static void *virtnet_rq_alloc(struct receive_queue *rq, u32 size, gfp_t gfp)
>>>    	void *buf, *head;
>>>    	dma_addr_t addr;
>>> -	if (unlikely(!skb_page_frag_refill(size, alloc_frag, gfp)))
>>> -		return NULL;
>>> -
>>>    	head = page_address(alloc_frag->page);
>>>    	dma = head;
>>> @@ -2421,6 +2418,9 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
>>>    	len = SKB_DATA_ALIGN(len) +
>>>    	      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>>> +	if (unlikely(!skb_page_frag_refill(len, &rq->alloc_frag, gfp)))
>>> +		return -ENOMEM;
>>> +
>> Do you want to document the assumption that small packet case won't end up
>> crossing the page frag boundary unlike the mergeable case? Add a comment
>> block to explain or a WARN_ON() check against potential overflow would work
>> with me.
>>
>>>    	buf = virtnet_rq_alloc(rq, len, gfp);
>>>    	if (unlikely(!buf))
>>>    		return -ENOMEM;
>>> @@ -2521,6 +2521,12 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>>>    	 */
>>>    	len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len, room);
>>> +	if (unlikely(!skb_page_frag_refill(len + room, alloc_frag, gfp)))
>>> +		return -ENOMEM;
>>> +
>>> +	if (!alloc_frag->offset && len + room + sizeof(struct virtnet_rq_dma) > alloc_frag->size)
>>> +		len -= sizeof(struct virtnet_rq_dma);
>>> +
>> This could address my previous concern for possibly regressing every buffer
>> size for the mergeable case, thanks. Though I still don't get why carving up
>> a small chunk from page_frag for storing the virtnet_rq_dma metadata, this
>> would cause perf regression on certain MTU size
> 4Kbyte MTU exactly?
Close to that, excluding all headers upfront (depending on virtio 
features and header layout). The size of struct virtnet_rq_dma is now 16 
bytes, this could lead to performance impact on roughly: 16 / 4096 = 0.4 
% across all MTU sizes, more obviously to be seen with order-0 page 
allocations.

-Siwei

>
>> that happens to end up with
>> one more base page (and an extra descriptor as well) to be allocated
>> compared to the previous code without the extra virtnet_rq_dma content. How
>> hard would it be to allocate a dedicated struct to store the related
>> information without affecting the (size of) datapath pages?
>>
>> FWIW, out of the code review perspective, I've looked up the past
>> conversations but didn't see comprehensive benchmark was done before
>> removing the old code and making premap the sole default mode. Granted this
>> would reduce the footprint of additional code and the associated maintaining
>> cost immediately, but I would assume at least there should have been
>> thorough performance runs upfront to guarantee no regression is seen with
>> every possible use case, or the negative effect is comparatively negligible
>> even though there's slight regression in some limited case. If that kind of
>> perf measurement hadn't been done before getting accepted/merged, I think at
>> least it should allow both modes to coexist for a while such that every user
>> could gauge the performance effect.
>>
>> Thanks,
>> -Siwei
>>
>>>    	buf = virtnet_rq_alloc(rq, len + room, gfp);
>>>    	if (unlikely(!buf))
>>>    		return -ENOMEM;