Re: [PATCH v5] af_packet: Handle outgoing VLAN packets without hardware offloading

[Willem de Bruijn]

Chengen Du wrote:
> Hi Willem,
> 
> Thank you for your comments on patch v4.
> I have made some modifications and would like to discuss some of your
> suggestions in detail.

Thanks for the revision. It addresses many of my comments. 
> 
> 
> On Tue, Jun 4, 2024 at 1:50 PM Chengen Du <chengen.du@xxxxxxxxxxxxx> 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>
> > ---
> >  net/packet/af_packet.c | 64 ++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 62 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index ea3ebc160e25..53d51ac87ac6 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -538,6 +538,52 @@ 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)
> > +{
> > +       unsigned int vlan_depth = skb->mac_len;
> > +       struct vlan_hdr vhdr, *vh;
> > +       u8 *skb_head = skb->data;
> > +       int skb_len = skb->len;
> > +
> > +       if (vlan_depth) {
> > +               if (WARN_ON(vlan_depth < VLAN_HLEN))
> > +                       return 0;
> > +               vlan_depth -= VLAN_HLEN;
> > +       } else {
> > +               vlan_depth = ETH_HLEN;
> > +       }
> > +
> > +       skb_push(skb, skb->data - skb_mac_header(skb));
> > +       vh = skb_header_pointer(skb, vlan_depth, sizeof(vhdr), &vhdr);
> > +       if (skb_head != skb->data) {
> > +               skb->data = skb_head;
> > +               skb->len = skb_len;
> > +       }
> > +       if (unlikely(!vh))
> > +               return 0;
> > +
> > +       return ntohs(vh->h_vlan_TCI);
> > +}
> 
> As you mentioned, we need the outermost VLAN tag to fit the protocol
> we referenced in tp_vlan_tpid.
> In if_vlan.h, __vlan_get_protocol() only provides the protocol and may
> affect other usage scenarios if we modify it to also provide TCI.
> 
> There is a similar function called __vlan_get_tag(), which also aims
> to extract TCI from the L2 header, but it doesn't check the header
> size as __vlan_get_protocol() does.
> To prevent affecting other usage scenarios, I introduced a new
> function to achieve our purpose.

__vlan_get_protocol exists to parse through possibly multiple VLAG
tags to the inner non-VLAN protocol.

Since you only want the outer VLAN tag, you can use skb_header_pointer
directly. No need for vlan_depth.

Similar to what __vlan_get_tag does, but with safe access as it is not
guaranteed that the data beyond the link layer lies in skb linear.

> Additionally, the starting point of skb->data differs in SOCK_RAW and
> SOCK_DGRAM cases.
> I accounted for this by using skb_push to ensure that skb->data starts
> from the L2 header.
> 
> Would you recommend modifying __vlan_get_tag() to add the size check
> logic instead?
> If so, we would also need to consider how to integrate the size check
> logic into both __vlan_get_protocol() and __vlan_get_tag().

No, let's not touch those.
> 
> > +
> > +static __be16 vlan_get_protocol_dgram(struct sk_buff *skb)
> > +{
> > +       __be16 proto = skb->protocol;
> > +
> > +       if (unlikely(eth_type_vlan(proto))) {
> > +               u8 *skb_head = skb->data;
> > +               int skb_len = skb->len;
> > +
> > +               skb_push(skb, skb->data - skb_mac_header(skb));
> > +               proto = __vlan_get_protocol(skb, proto, NULL);
> > +               if (skb_head != skb->data) {
> > +                       skb->data = skb_head;
> > +                       skb->len = skb_len;
> > +               }
> > +       }
> > +
> > +       return proto;
> > +}
> > +
> >  static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc)
> >  {
> >         del_timer_sync(&pkc->retire_blk_timer);
> > @@ -1011,6 +1057,10 @@ static void prb_fill_vlan_info(struct tpacket_kbdq_core *pkc,
> >                 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(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 +2478,10 @@ 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(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 +2511,8 @@ 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 +3537,8 @@ 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 +3595,10 @@ 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(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
> >