Re: [PATCH net v4] virtio-net: fix received length check in big packets

[Bui Quang Minh <minhquangbui99@gmail.com>]

On 10/23/25 21:39, Bui Quang Minh wrote:
> On 10/23/25 15:05, Xuan Zhuo wrote:
>> On Wed, 22 Oct 2025 23:06:23 +0700, Bui Quang Minh 
>> <minhquangbui99@gmail.com> wrote:
>>> Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
>>> for big packets"), when guest gso is off, 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.
>>>
>>> Because the host announced buffer length can be malicious (e.g. the 
>>> host
>>> vhost_net driver's get_rx_bufs is modified to announce incorrect
>>> length), we need a check in virtio_net receive path. Currently, the
>>> check is not adapted to the new change which can lead to NULL page
>>> pointer dereference in the below while loop when receiving length that
>>> is larger than the allocated one.
>>>
>>> This commit fixes the received length check corresponding to the new
>>> change.
>>>
>>> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for 
>>> big packets")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
>>> ---
>>> Changes in v4:
>>> - Remove unrelated changes, add more comments
>>> Changes in v3:
>>> - Convert BUG_ON to WARN_ON_ONCE
>>> Changes in v2:
>>> - Remove incorrect give_pages call
>>> ---
>>>   drivers/net/virtio_net.c | 16 +++++++++++++---
>>>   1 file changed, 13 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index a757cbcab87f..0ffe78b3fd8d 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -852,7 +852,7 @@ 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;
>>> @@ -915,13 +915,23 @@ static struct sk_buff *page_to_skb(struct 
>>> virtnet_info *vi,
>>>        * This is here to handle cases when the device erroneously
>>>        * tries to receive more than is possible. This is usually
>>>        * the case of a broken device.
>>> +     *
>>> +     * The number of allocated pages for big packet is
>>> +     * vi->big_packets_num_skbfrags + 1, the start of first page is
>>> +     * for virtio header, the remaining is for data. We need to ensure
>>> +     * the remaining len does not go out of the allocated pages.
>>> +     * Please refer to add_recvbuf_big for more details on big packet
>>> +     * buffer allocation.
>>>        */
>>> -    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;
>>
>> Could we perform this check inside `receive_big` to avoid computing
>> `max_remaining_len` altogether? Instead, we could directly compare 
>> `len` against
>> `(vi->big_packets_num_skbfrags + 1) * PAGE_SIZE`.
>
> That looks better, I'll do that in the next version.
>
>> And I’d like to know if this check is necessary for other modes as well.
>
> Other modes have this check as well. check_mergeable_len is used in 
> mergeable mode. In receive_small, there is a check
>
>     if (unlikely(len > GOOD_PACKET_LEN)) {
>         goto err; 

I forgot about XDP zerocopy (XSK) mode. In that mode, there is a check 
in buf_to_xdp.

     if (unlikely(len > bufsize)) {
         return NULL;

Thanks,
Quang Minh.