Re: [PATCH net v2] virtio-net: fix received length check in big packets

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

On 7/8/25 21:52, Parav Pandit wrote:
>> From: Bui Quang Minh <minhquangbui99@gmail.com>
>> Sent: 08 July 2025 08:12 PM
>>
>> 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.
> Do you mean a device has copied X bytes in receive buffer but device reports X+Y bytes in the ring?

Yes, that's what I mean. AFAIK, it's not checked in the ring level.


Thanks,
Quang Minh.

>
>> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big
>> packets")
>> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
>> ---
>> Changes in v2:
>> - Remove incorrect give_pages call
>> ---
>>   drivers/net/virtio_net.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index
>> 5d674eb9a0f2..3a7f435c95ae 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 +887,15 @@ 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);
>>   		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