Re: [PATCH net] af_packet: Don't send zero-byte data in tpacket_snd().

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

Eric Dumazet wrote:
> syzbot reported a WARNING in __dev_queue_xmit() triggered via tpacket_snd():
> 
> skb_assert_len
> WARNING: at include/linux/skbuff.h:2753 skb_assert_len
> WARNING: at __dev_queue_xmit+0x21bc/0x4970 net/core/dev.c:4781
> 
> Call Trace:
>  <TASK>
>  dev_queue_xmit include/linux/netdevice.h:3448 [inline]
>  packet_xmit+0x243/0x310 net/packet/af_packet.c:276
>  tpacket_snd net/packet/af_packet.c:2907 [inline]
>  packet_sendmsg+0x28d6/0x4eb0 net/packet/af_packet.c:3134
> 
> When sending 0-byte packets via TPACKET ring buffer on devices with no
> hard header (e.g. dev->hard_header_len == 0), tpacket_fill_skb()
> populates an skb with skb->len == 0 and returns 0. tpacket_snd() then
> forwards this empty skb to packet_xmit(), causing __dev_queue_xmit() to
> hit skb_assert_len(skb).
> 
> Similar checks exist in packet_snd() via commit dc633700f00f
> ("net/af_packet: check len when min_header_len equals to 0") and in
> packet_sendmsg_spkt() via commit 6a341729fb31 ("af_packet: Don't send
> zero-byte data in packet_sendmsg_spkt().").
> 
> Return -EINVAL in tpacket_fill_skb() when skb->len is zero to reject
> zero-length packets in tpacket_snd().
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+30b93b6845b19cc38581@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/6a79e807.01d0871a.3a0d52.00ac.GAE@google.com/T/#u
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>