[PATCH IPV6 v2 0/4] ipv6: allocate enough headroom in ip6_finish_output2()

Recently Syzkaller found one more issue on RHEL7-based OpenVz kernels.
During its investigation I've found that upstream is affected too. 

TEE target send sbk with small headroom into another interface which requires
an increased headroom.

ipv4 handles this problem in ip_finish_output2() and creates new skb with enough headroom,
though ip6_finish_output2() lacks this logic.

Suzkaller created C reproducer, it can be found in v1 cover-letter.

v2 changes: 
 new helper was created and used in ip6_finish_output2 and in ip6_xmit()
 small refactoring in changed functions: commonly used dereferences was replaced by variables

ToDo:
 clarify proper name for helper,
 move it into proper place,  
 use it in other similar places:
   pptp_xmit
   vrf_finish_output
   ax25_transmit_buffer
   ax25_rt_build_path
   bpf_out_neigh_v6
   bpf_out_neigh_v4
   ip_finish_output2
   ip6_tnl_xmit
   ipip6_tunnel_xmit
   ip_vs_prepare_tunneled_skb

Vasily Averin (4):
  ipv6: allocate enough headroom in ip6_finish_output2()
  ipv6: use new helper skb_expand_head() in ip6_xmit()
  ipv6: ip6_finish_output2 refactoring
  ipv6: ip6_xmit refactoring

 net/ipv6/ip6_output.c | 89 ++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 59 insertions(+), 30 deletions(-)

-- 
1.8.3.1