linux-kernel - Re: [PATCH net v6] virtio-net: fix received length check in big packets

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Message-ID: <a4081f8d-7d12-46f2-b7d8-4145003a0dac@gmail.com>
Date: Wed, 29 Oct 2025 22:48:53 +0700
From: Bui Quang Minh <minhquangbui99@...il.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, Jason Wang <jasowang@...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, stable@...r.kernel.org
Subject: Re: [PATCH net v6] virtio-net: fix received length check in big
 packets

On 10/28/25 21:41, Michael S. Tsirkin wrote:
> On Tue, Oct 28, 2025 at 09:31:16PM +0700, Bui Quang Minh 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@...r.kernel.org
>> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
>> ---
>> Changes in v6:
>> - Fix the length check
>> - Link to v5: https://lore.kernel.org/netdev/20251024150649.22906-1-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..461ad1019c37 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.
> you mean "the size allocated in add_recvbuf_big"

Sorry, but they feels the same to me. But I'll fix it if it is clearer.

>
>> +	 */
>> +	if (unlikely(len > (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE)) {
>> +		pr_debug("%s: rx error: len %u exceeds allocate size %lu\n",
>
> allocated?

I'll fix it.

Thanks,
Quang Minh.

>
>> +			 dev->name, len,
>> +			 (vi->big_packets_num_skbfrags + 1) * 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