Commit 81c89507 authored Apr 17, 2018 by Bjørn Mork Committed by David S. Miller Apr 18, 2018

tun: fix vlan packet truncation



Bogus trimming in tun_net_xmit() causes truncated vlan packets.

skb->len is correct whether or not skb_vlan_tag_present() is true. There
is no more reason to adjust the skb length on xmit in this driver than
any other driver. tun_put_user() adds 4 bytes to the total for tagged
packets because it transmits the tag inline to userspace.  This is
similar to a nic transmitting the tag inline on the wire.

Reproducing the bug by sending any tagged packet through back-to-back
connected tap interfaces:

 socat TUN,tun-type=tap,iff-up,tun-name=in TUN,tun-type=tap,iff-up,tun-name=out &
 ip link add link in name in.20 type vlan id 20
 ip addr add 10.9.9.9/24 dev in.20
 ip link set in.20 up
 tshark -nxxi in -f arp -c1 2>/dev/null &
 tshark -nxxi out -f arp -c1 2>/dev/null &
 ping -c 1 10.9.9.5 >/dev/null 2>&1

The output from the 'in' and 'out' interfaces are different when the
bug is present:

 Capturing on 'in'
 0000  ff ff ff ff ff ff 76 cf 76 37 d5 0a 81 00 00 14   ......v.v7......
 0010  08 06 00 01 08 00 06 04 00 01 76 cf 76 37 d5 0a   ..........v.v7..
 0020  0a 09 09 09 00 00 00 00 00 00 0a 09 09 05         ..............

 Capturing on 'out'
 0000  ff ff ff ff ff ff 76 cf 76 37 d5 0a 81 00 00 14   ......v.v7......
 0010  08 06 00 01 08 00 06 04 00 01 76 cf 76 37 d5 0a   ..........v.v7..
 0020  0a 09 09 09 00 00 00 00 00 00                     ..........

Fixes: aff3d70a ("tun: allow to attach ebpf socket filter")
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 36a50a98

drivers/net/tun.c

+1 −6

Original line number	Diff line number	Diff line
		@@ -1102,12 +1102,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff skb, struct net_device dev)
		goto drop;

		len = run_ebpf_filter(tun, skb, len);

		/* Trim extra bytes since we may insert vlan proto & TCI
		* in tun_put_user().
		*/
		len -= skb_vlan_tag_present(skb) ? sizeof(struct veth) : 0;
		if (len <= 0 \|\| pskb_trim(skb, len))
		if (len == 0 \|\| pskb_trim(skb, len))
		goto drop;

		if (unlikely(skb_orphan_frags_rx(skb, GFP_ATOMIC)))

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