linux-kernel - RE: [PATCH net v2] virtio-net: fix received length check in big packets

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Message-ID:
 <CY8PR12MB7195AC4C23690D748ABAD675DC4EA@CY8PR12MB7195.namprd12.prod.outlook.com>
Date: Tue, 8 Jul 2025 14:52:22 +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>
Subject: RE: [PATCH net v2] virtio-net: fix received length check in big
 packets


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

> 
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big
> packets")
> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.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