packet: allow to transmit +4 byte in TX_RING slot for VLAN case

Commit 57f89bfa2140 ("network: Allow af_packet to transmit +4 bytes
for VLAN packets.") added the possibility for non-mmaped frames to
send extra 4 byte for VLAN header so the MTU increases from 1500 to
1504 byte, for example.

Commit cbd89acb9eb2 ("af_packet: fix for sending VLAN frames via
packet_mmap") attempted to fix that for the mmap part but was
reverted as it caused regressions while using eth_type_trans()
on output path.

Lets just act analogous to 57f89bfa2140 and add a similar logic
to TX_RING. We presume size_max as overcharged with +4 bytes and
later on after skb has been built by tpacket_fill_skb() check
for ETH_P_8021Q header on packets larger than normal MTU. Can
be easily reproduced with a slightly modified trafgen in mmap(2)
mode, test cases:

 { fill(0xff, 12) const16(0x8100) fill(0xff, <1504|1505>) }
 { fill(0xff, 12) const16(0x0806) fill(0xff, <1500|1501>) }

Note that we need to do the test right after tpacket_fill_skb()
as sockets can have PACKET_LOSS set where we would not fail but
instead just continue to traverse the ring.

Reported-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Ben Greear <greearb@candelatech.com>
Cc: Phil Sutter <phil@nwl.cc>
Tested-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 48a6a93db29602c032fe254d4a1d41cbc7b3f051..292304404fda1696f6cc8c4165a6ff183255fc80 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2257,8 +2257,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
        if (unlikely(!(dev->flags & IFF_UP)))
                goto out_put;
 
-       reserve = dev->hard_header_len;
-
+       reserve = dev->hard_header_len + VLAN_HLEN;
        size_max = po->tx_ring.frame_size
                - (po->tp_hdrlen - sizeof(struct sockaddr_ll));
 
@@ -2285,8 +2284,19 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
                        goto out_status;
 
                tp_len = tpacket_fill_skb(po, skb, ph, dev, size_max, proto,
-                               addr, hlen);
+                                         addr, hlen);
+               if (tp_len > dev->mtu + dev->hard_header_len) {
+                       struct ethhdr *ehdr;
+                       /* Earlier code assumed this would be a VLAN pkt,
+                        * double-check this now that we have the actual
+                        * packet in hand.
+                        */
 
+                       skb_reset_mac_header(skb);
+                       ehdr = eth_hdr(skb);
+                       if (ehdr->h_proto != htons(ETH_P_8021Q))
+                               tp_len = -EMSGSIZE;
+               }
                if (unlikely(tp_len < 0)) {
                        if (po->tp_loss) {
                                __packet_set_status(po, ph,