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

[Chengen Du]

Hi Willem,

On Mon, Jun 10, 2024 at 4:21 AM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> 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@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 | 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.

The check will pass even if the offset is negative, but I believe this
may not be the right approach. In my humble opinion, the expected
check should be similar to the skb_push check, which ensures that
after moving forward by the offset bytes, skb->data remains larger
than or equal to skb->head to avoid accessing out-of-bound data. It
might be worth considering adding a check in __skb_header_pointer to
handle negative offsets, as this seems logical. However, this change
could impact a wider range of code. Please correct me if I am
mistaken.

>
> > 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.
>