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Message-ID: <CAPza5qfVzV7NFiVY1jcZR-+0ey-uKgUjV6OcjmDFvKG3T-2SXA@mail.gmail.com>
Date: Mon, 10 Jun 2024 23:06:01 +0800
From: Chengen Du <chengen.du@...onical.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
pabeni@...hat.com, kaber@...sh.net, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v6] af_packet: Handle outgoing VLAN packets without
hardware offloading
Hi Willem,
I'm sorry, but I would like to confirm the issue further.
On Mon, Jun 10, 2024 at 4:19 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> 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@canonical.com/T/#u
> > Fixes: 393e52e33c6c ("packet: deliver VLAN TCI to userspace")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Chengen Du <chengen.du@...onical.com>
>
> Overall, solid.
>
> > ---
> > net/packet/af_packet.c | 57 ++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 55 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index ea3ebc160e25..8cffbe1f912d 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -538,6 +538,43 @@ 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 vlan_hdr vhdr, *vh;
> > + u8 *skb_orig_data = skb->data;
> > + int skb_orig_len = skb->len;
> > +
> > + skb_push(skb, skb->data - skb_mac_header(skb));
> > + vh = skb_header_pointer(skb, ETH_HLEN, sizeof(vhdr), &vhdr);
>
> Don't harcode Ethernet.
>
> According to documentation VLANs are used with other link layers.
>
> More importantly, in practice PF_PACKET allows inserting this
> skb->protocol on any device.
>
> We don't use link layer specific constants anywhere in the packet
> socket code for this reason. But instead dev->hard_header_len.
>
> One caveat there is variable length link layer headers, where
> dev->min_header_len != dev->hard_header_len. Will just have to fail
> on those.
Thank you for pointing out this error. I would like to confirm if I
need to use dev->hard_header_len to get the correct header length and
return zero if dev->min_header_len != dev->hard_header_len to handle
variable-length link layer headers. Is there something I
misunderstand, or are there other aspects I need to consider further?
>
> > + 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);
> > +}
> > +
>
> Only since I had to respond above: this is non-obvious enough to
> deserve a function comment. Something like the following?
>
> /* For SOCK_DGRAM, data starts at the network protocol, after any VLAN
> * headers. sll_protocol must point to the network protocol. The
> * (outer) VLAN TCI is still accessible as auxdata.
> */
>
> > +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 +1044,16 @@ 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);
> > + 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 +2471,10 @@ 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);
> > + 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 +2504,8 @@ 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 +3530,8 @@ 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 +3588,10 @@ 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))) {
> > + aux.tp_vlan_tci = vlan_get_tci(skb);
> > + 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;
> > --
> > 2.43.0
> >
>
>
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