Re: [PATCH v9] af_packet: Handle outgoing VLAN packets without hardware offloading

[Willem de Bruijn]

Chengen Du wrote:
> The issue initially stems from libpcap. The ethertype will be overwritten
> as the VLAN TPID if the network interface lacks hardware VLAN offloading.
> In the outbound packet path, if hardware VLAN offloading is unavailable,
> the VLAN tag is inserted into the payload but then cleared from the sk_buff
> struct. Consequently, this can lead to a false negative when checking for
> the presence of a VLAN tag, causing the packet sniffing outcome to lack
> VLAN tag information (i.e., TCI-TPID). As a result, the packet capturing
> tool may be unable to parse packets as expected.
> 
> The TCI-TPID is missing because the prb_fill_vlan_info() function does not
> modify the tp_vlan_tci/tp_vlan_tpid values, as the information is in the
> payload and not in the sk_buff struct. The skb_vlan_tag_present() function
> only checks vlan_all in the sk_buff struct. In cooked mode, the L2 header
> is stripped, preventing the packet capturing tool from determining the
> correct TCI-TPID value. Additionally, the protocol in SLL is incorrect,
> which means the packet capturing tool cannot parse the L3 header correctly.
> 
> Link: https://github.com/the-tcpdump-group/libpcap/issues/1105
> Link: https://lore.kernel.org/netdev/20240520070348.26725-1-chengen.du@xxxxxxxxxxxxx/T/#u
> Fixes: 393e52e33c6c ("packet: deliver VLAN TCI to userspace")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chengen Du <chengen.du@xxxxxxxxxxxxx>

Reviewed-by: Willem de Bruijn <willemb@xxxxxxxxxx>

For next time, please remember to mention the tree: [PATCH net v9].
And give a changelog: Changes v8->v9: no changes, rebased resubmit.

> ---
>  net/packet/af_packet.c | 86 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 84 insertions(+), 2 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index ea3ebc160e25..84e8884a77e3 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -538,6 +538,61 @@ static void *packet_current_frame(struct packet_sock *po,
>  	return packet_lookup_frame(po, rb, rb->head, status);
>  }
>  
> +static u16 vlan_get_tci(struct sk_buff *skb, struct net_device *dev)
> +{
> +	struct vlan_hdr vhdr, *vh;
> +	u8 *skb_orig_data = skb->data;
> +	int skb_orig_len = skb->len;
> +	unsigned int header_len;

nit: reverse christmas tree violation

> +
> +	if (!dev)
> +		return 0;
> +
> +	/* In the SOCK_DGRAM scenario, skb data starts at the network
> +	 * protocol, which is after the VLAN headers. The outer VLAN
> +	 * header is at the hard_header_len offset in non-variable
> +	 * length link layer headers. If it's a VLAN device, the
> +	 * min_header_len should be used to exclude the VLAN header
> +	 * size.
> +	 */
> +	if (dev->min_header_len == dev->hard_header_len)
> +		header_len = dev->hard_header_len;
> +	else if (is_vlan_dev(dev))
> +		header_len = dev->min_header_len;
> +	else
> +		return 0;
> +
> +	skb_push(skb, skb->data - skb_mac_header(skb));
> +	vh = skb_header_pointer(skb, header_len, sizeof(vhdr), &vhdr);
> +	if (skb_orig_data != skb->data) {
> +		skb->data = skb_orig_data;
> +		skb->len = skb_orig_len;
> +	}
> +	if (unlikely(!vh))
> +		return 0;
> +
> +	return ntohs(vh->h_vlan_TCI);
> +}
> +
> +static __be16 vlan_get_protocol_dgram(struct sk_buff *skb)
> +{
> +	__be16 proto = skb->protocol;
> +
> +	if (unlikely(eth_type_vlan(proto))) {
> +		u8 *skb_orig_data = skb->data;
> +		int skb_orig_len = skb->len;
> +
> +		skb_push(skb, skb->data - skb_mac_header(skb));
> +		proto = __vlan_get_protocol(skb, proto, NULL);
> +		if (skb_orig_data != skb->data) {
> +			skb->data = skb_orig_data;
> +			skb->len = skb_orig_len;
> +		}
> +	}
> +
> +	return proto;
> +}
> +
>  static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc)
>  {
>  	del_timer_sync(&pkc->retire_blk_timer);
> @@ -1007,10 +1062,16 @@ static void prb_clear_rxhash(struct tpacket_kbdq_core *pkc,
>  static void prb_fill_vlan_info(struct tpacket_kbdq_core *pkc,
>  			struct tpacket3_hdr *ppd)
>  {
> +	struct packet_sock *po = container_of(pkc, struct packet_sock, rx_ring.prb_bdqc);
> +
>  	if (skb_vlan_tag_present(pkc->skb)) {
>  		ppd->hv1.tp_vlan_tci = skb_vlan_tag_get(pkc->skb);
>  		ppd->hv1.tp_vlan_tpid = ntohs(pkc->skb->vlan_proto);
>  		ppd->tp_status = TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID;
> +	} else if (unlikely(po->sk.sk_type == SOCK_DGRAM && eth_type_vlan(pkc->skb->protocol))) {
> +		ppd->hv1.tp_vlan_tci = vlan_get_tci(pkc->skb, pkc->skb->dev);
> +		ppd->hv1.tp_vlan_tpid = ntohs(pkc->skb->protocol);
> +		ppd->tp_status = TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID;
>  	} else {
>  		ppd->hv1.tp_vlan_tci = 0;
>  		ppd->hv1.tp_vlan_tpid = 0;
> @@ -2428,6 +2489,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
>  			h.h2->tp_vlan_tci = skb_vlan_tag_get(skb);
>  			h.h2->tp_vlan_tpid = ntohs(skb->vlan_proto);
>  			status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID;
> +		} else if (unlikely(sk->sk_type == SOCK_DGRAM && eth_type_vlan(skb->protocol))) {
> +			h.h2->tp_vlan_tci = vlan_get_tci(skb, skb->dev);
> +			h.h2->tp_vlan_tpid = ntohs(skb->protocol);
> +			status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID;
>  		} else {
>  			h.h2->tp_vlan_tci = 0;
>  			h.h2->tp_vlan_tpid = 0;
> @@ -2457,7 +2522,8 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
>  	sll->sll_halen = dev_parse_header(skb, sll->sll_addr);
>  	sll->sll_family = AF_PACKET;
>  	sll->sll_hatype = dev->type;
> -	sll->sll_protocol = skb->protocol;
> +	sll->sll_protocol = (sk->sk_type == SOCK_DGRAM) ?
> +		vlan_get_protocol_dgram(skb) : skb->protocol;
>  	sll->sll_pkttype = skb->pkt_type;
>  	if (unlikely(packet_sock_flag(po, PACKET_SOCK_ORIGDEV)))
>  		sll->sll_ifindex = orig_dev->ifindex;
> @@ -3482,7 +3548,8 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>  		/* Original length was stored in sockaddr_ll fields */
>  		origlen = PACKET_SKB_CB(skb)->sa.origlen;
>  		sll->sll_family = AF_PACKET;
> -		sll->sll_protocol = skb->protocol;
> +		sll->sll_protocol = (sock->type == SOCK_DGRAM) ?
> +			vlan_get_protocol_dgram(skb) : skb->protocol;
>  	}
>  
>  	sock_recv_cmsgs(msg, sk, skb);
> @@ -3539,6 +3606,21 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>  			aux.tp_vlan_tci = skb_vlan_tag_get(skb);
>  			aux.tp_vlan_tpid = ntohs(skb->vlan_proto);
>  			aux.tp_status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID;
> +		} else if (unlikely(sock->type == SOCK_DGRAM && eth_type_vlan(skb->protocol))) {
> +			struct sockaddr_ll *sll = &PACKET_SKB_CB(skb)->sa.ll;
> +			struct net_device *dev;
> +
> +			rcu_read_lock();
> +			dev = dev_get_by_index_rcu(sock_net(sk), sll->sll_ifindex);
> +			if (dev) {
> +				aux.tp_vlan_tci = vlan_get_tci(skb, dev);
> +				aux.tp_vlan_tpid = ntohs(skb->protocol);
> +				aux.tp_status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID;
> +			} else {
> +				aux.tp_vlan_tci = 0;
> +				aux.tp_vlan_tpid = 0;
> +			}
> +			rcu_read_unlock();
>  		} else {
>  			aux.tp_vlan_tci = 0;
>  			aux.tp_vlan_tpid = 0;
> -- 
> 2.43.0
>