Re: [PATCH net] packet: in packet_snd start writing at link layer allocation

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Fri, 11 May 2018 13:24:25 -0400

> From: Willem de Bruijn <willemb@google.com>
> 
> Packet sockets allow construction of packets shorter than
> dev->hard_header_len to accommodate protocols with variable length
> link layer headers. These packets are padded to dev->hard_header_len,
> because some device drivers interpret that as a minimum packet size.
> 
> packet_snd reserves dev->hard_header_len bytes on allocation.
> SOCK_DGRAM sockets call skb_push in dev_hard_header() to ensure that
> link layer headers are stored in the reserved range. SOCK_RAW sockets
> do the same in tpacket_snd, but not in packet_snd.
> 
> Syzbot was able to send a zero byte packet to a device with massive
> 116B link layer header, causing padding to cross over into skb_shinfo.
> Fix this by writing from the start of the llheader reserved range also
> in the case of packet_snd/SOCK_RAW.
> 
> Update skb_set_network_header to the new offset. This also corrects
> it for SOCK_DGRAM, where it incorrectly double counted reserve due to
> the skb_push in dev_hard_header.
> 
> Fixes: 9ed988cd5915 ("packet: validate variable length ll headers")
> Reported-by: syzbot+71d74a5406d02057d559@syzkaller.appspotmail.com
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied and queued up for -stable, thanks Willem.