Re: [PATCH v6] af_packet: Handle outgoing VLAN packets without hardware offloading

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Chengen Du wrote:
> Hi,
> 
> I would like to provide some additional explanations about the patch.
> 
> 
> On Sat, Jun 8, 2024 at 10:54 AM Chengen Du <chengen.du@canonical.com> 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@vger.kernel.org
> > Signed-off-by: Chengen Du <chengen.du@canonical.com>
> > ---
> >  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);
> > +       if (skb_orig_data != skb->data) {
> > +               skb->data = skb_orig_data;
> > +               skb->len = skb_orig_len;
> > +       }
> 
> 
> The reason for not directly using skb_header_pointer(skb,
> skb_mac_header(skb) + ETH_HLEN, ...) to get the VLAN header is due to
> the check logic in skb_header_pointer. In the SOCK_DGRAM and
> PACKET_OUTGOING scenarios, the offset can be a negative number, which
> causes the check logic (i.e., likely(hlen - offset >= len)) in
> __skb_header_pointer() to not work as expected.

The calculation is still correct?

I think that this is not the first situation where negative offsets
can be given to skb_header_pointer.
 
> While it is possible to modify __skb_header_pointer() to handle cases
> where the offset is negative, this change could affect a wider range
> of code.