Re: [PATCH net v5] virtio-net: fix received length check in big packets

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

On 10/25/25 14:11, Parav Pandit wrote:
>> From: Bui Quang Minh <minhquangbui99@gmail.com>
>> Sent: 24 October 2025 08:37 PM
>>
>> 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 looks wrong.
> A device DMAed N bytes, and it reports N + M bytes in the completion?
> Such devices should be fixed.
>
> If driver allocated X bytes, and device copied X + Y bytes on receive packet, it will crash the driver host anyway.
>
> The fixes tag in this patch is incorrect because this is not a driver bug.
> It is just adding resiliency in driver for broken device. So driver cannot have fixes tag here.

Yes, I agree that the check is a protection against broken device.

The check is already there before this commit, but it is not correct 
since the changes in commit 4959aebba8c0 ("virtio-net: use mtu size as 
buffer length for big packets"). So this patch fixes the check 
corresponding to the new change. I think this is a valid use of Fixes tag.

Thanks,
Quang Minh.

>
>> 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 v5:
>> - Move the length check to receive_big
>> - Link to v4: https://lore.kernel.org/netdev/20251022160623.51191-1-
>> minhquangbui99@gmail.com/
>> Changes in v4:
>> - Remove unrelated changes, add more comments
>> - Link to v3: https://lore.kernel.org/netdev/20251021154534.53045-1-
>> minhquangbui99@gmail.com/
>> Changes in v3:
>> - Convert BUG_ON to WARN_ON_ONCE
>> - Link to v2: https://lore.kernel.org/netdev/20250708144206.95091-1-
>> minhquangbui99@gmail.com/
>> Changes in v2:
>> - Remove incorrect give_pages call
>> - Link to v1: https://lore.kernel.org/netdev/20250706141150.25344-1-
>> minhquangbui99@gmail.com/
>> ---
>>   drivers/net/virtio_net.c | 25 ++++++++++++-------------
>>   1 file changed, 12 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index
>> a757cbcab87f..2c3f544add5e 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -910,17 +910,6 @@ static struct sk_buff *page_to_skb(struct
>> virtnet_info *vi,
>>   		goto ok;
>>   	}
>>
>> -	/*
>> -	 * Verify that we can indeed put this data into a skb.
>> -	 * 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.
>> -	 */
>> -	if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
>> -		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); @@ -2107,9 +2096,19 @@ static struct sk_buff *receive_big(struct
>> net_device *dev,
>>   				   struct virtnet_rq_stats *stats)
>>   {
>>   	struct page *page = buf;
>> -	struct sk_buff *skb =
>> -		page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
>> +	struct sk_buff *skb;
>> +
>> +	/* Make sure that len does not exceed the allocated size in
>> +	 * add_recvbuf_big.
>> +	 */
>> +	if (unlikely(len > vi->big_packets_num_skbfrags * PAGE_SIZE)) {
>> +		pr_debug("%s: rx error: len %u exceeds allocate size %lu\n",
>> +			 dev->name, len,
>> +			 vi->big_packets_num_skbfrags * PAGE_SIZE);
>> +		goto err;
>> +	}
>>
>> +	skb = page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
>>   	u64_stats_add(&stats->bytes, len - vi->hdr_len);
>>   	if (unlikely(!skb))
>>   		goto err;
>> --
>> 2.43.0