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

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

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;

Thanks,
Quang Minh.