linux-kernel - RE: [PATCH net v5] virtio-net: fix received length check in big packets

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Message-ID:
 <CY8PR12MB71951A2ADD74508A9FC60956DCFEA@CY8PR12MB7195.namprd12.prod.outlook.com>
Date: Sat, 25 Oct 2025 07:11:30 +0000
From: Parav Pandit <parav@...dia.com>
To: Bui Quang Minh <minhquangbui99@...il.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>
CC: "Michael S. Tsirkin" <mst@...hat.com>, 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>, "Minggang(Gavin) Li"
	<gavinl@...dia.com>, Gavi Teitz <gavi@...dia.com>,
	"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH net v5] virtio-net: fix received length check in big
 packets


> From: Bui Quang Minh <minhquangbui99@...il.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.

> 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 v5:
> - Move the length check to receive_big
> - Link to v4: https://lore.kernel.org/netdev/20251022160623.51191-1-
> minhquangbui99@...il.com/
> Changes in v4:
> - Remove unrelated changes, add more comments
> - Link to v3: https://lore.kernel.org/netdev/20251021154534.53045-1-
> minhquangbui99@...il.com/
> Changes in v3:
> - Convert BUG_ON to WARN_ON_ONCE
> - Link to v2: https://lore.kernel.org/netdev/20250708144206.95091-1-
> minhquangbui99@...il.com/
> Changes in v2:
> - Remove incorrect give_pages call
> - Link to v1: https://lore.kernel.org/netdev/20250706141150.25344-1-
> minhquangbui99@...il.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