linux-kernel - Re: [PATCH net] virtio-net: fix received length check in big packets

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Message-ID: <a9e0138f-3d8a-4178-b765-869de8ab13b6@gmail.com>
Date: Tue, 8 Jul 2025 21:30:58 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: Jason Wang <jasowang@...hat.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Gavin Li <gavinl@...dia.com>, Gavi Teitz <gavi@...dia.com>,
 Parav Pandit <parav@...dia.com>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] virtio-net: fix received length check in big packets

On 7/8/25 21:26, Bui Quang Minh wrote:
> On 7/7/25 10:48, Jason Wang wrote:
>> On Sun, Jul 6, 2025 at 10:12 PM Bui Quang Minh 
>> <minhquangbui99@...il.com> wrote:
>>> Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
>>> for big packets"), the allocated size for big packets is not
>>> MAX_SKB_FRAGS * PAGE_SIZE anymore but depends on negotiated MTU. The
>>> number of allocated frags for big packets is stored in
>>> vi->big_packets_num_skbfrags. This commit fixes the received length
>>> check corresponding to that change. The current incorrect check can 
>>> lead
>>> to NULL page pointer dereference in the below while loop when erroneous
>>> length is received.
>>>
>>> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for 
>>> big packets")
>>> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
>>> ---
>>>   drivers/net/virtio_net.c | 10 +++++++---
>>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 5d674eb9a0f2..ead1cd2fb8af 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -823,7 +823,7 @@ static struct sk_buff *page_to_skb(struct 
>>> virtnet_info *vi,
>>>   {
>>>          struct sk_buff *skb;
>>>          struct virtio_net_common_hdr *hdr;
>>> -       unsigned int copy, hdr_len, hdr_padded_len;
>>> +       unsigned int copy, hdr_len, hdr_padded_len, max_remaining_len;
>>>          struct page *page_to_free = NULL;
>>>          int tailroom, shinfo_size;
>>>          char *p, *hdr_p, *buf;
>>> @@ -887,12 +887,16 @@ static struct sk_buff *page_to_skb(struct 
>>> virtnet_info *vi,
>>>           * tries to receive more than is possible. This is usually
>>>           * the case of a broken device.
>>>           */
>>> -       if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
>>> +       BUG_ON(offset >= PAGE_SIZE);
>>> +       max_remaining_len = (unsigned int)PAGE_SIZE - offset;
>>> +       max_remaining_len += vi->big_packets_num_skbfrags * PAGE_SIZE;
>>> +       if (unlikely(len > max_remaining_len)) {
>>>                  net_dbg_ratelimited("%s: too much data\n", 
>>> skb->dev->name);
>>>                  dev_kfree_skb(skb);
>>> +               give_pages(rq, page);
>> Should this be an independent fix?
>
> I've realized this is not needed. In receive_big(), if page_to_skb() 
> returns NULL then give_pages() is called. I'll remove this in next 
> version.

It's a wrong fix actually. Calling give_pages twice will create bug.

>
>>
>>>                  return NULL;
>>>          }
>>> -       BUG_ON(offset >= PAGE_SIZE);
>>> +
>>>          while (len) {
>>>                  unsigned int frag_size = min((unsigned)PAGE_SIZE - 
>>> offset, len);
>>>                  skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, 
>>> page, offset,
>>> -- 
>>> 2.43.0
>>>
>> Thanks
>>
>