Apply the same fix than ones found in :

8d975c15 ("ip6_tunnel: make sure to pull inner header in __ip6_tnl_rcv()")
1ca1ba46 ("geneve: make sure to pull inner header in geneve_rx()")

We have to save skb->network_header in a temporary variable
in order to be able to recompute the network_header pointer
after a pskb_inet_may_pull() call.

pskb_inet_may_pull() makes sure the needed headers are in skb->head.

syzbot reported:
BUG: KMSAN: uninit-value in __INET_ECN_decapsulate include/net/inet_ecn.h:253 [inline]
 BUG: KMSAN: uninit-value in INET_ECN_decapsulate include/net/inet_ecn.h:275 [inline]
 BUG: KMSAN: uninit-value in IP_ECN_decapsulate include/net/inet_ecn.h:302 [inline]
 BUG: KMSAN: uninit-value in ip_tunnel_rcv+0xed9/0x2ed0 net/ipv4/ip_tunnel.c:409
  __INET_ECN_decapsulate include/net/inet_ecn.h:253 [inline]
  INET_ECN_decapsulate include/net/inet_ecn.h:275 [inline]
  IP_ECN_decapsulate include/net/inet_ecn.h:302 [inline]
  ip_tunnel_rcv+0xed9/0x2ed0 net/ipv4/ip_tunnel.c:409
  __ipgre_rcv+0x9bc/0xbc0 net/ipv4/ip_gre.c:389
  ipgre_rcv net/ipv4/ip_gre.c:411 [inline]
  gre_rcv+0x423/0x19f0 net/ipv4/ip_gre.c:447
  gre_rcv+0x2a4/0x390 net/ipv4/gre_demux.c:163
  ip_protocol_deliver_rcu+0x264/0x1300 net/ipv4/ip_input.c:205
  ip_local_deliver_finish+0x2b8/0x440 net/ipv4/ip_input.c:233
  NF_HOOK include/linux/netfilter.h:314 [inline]
  ip_local_deliver+0x21f/0x490 net/ipv4/ip_input.c:254
  dst_input include/net/dst.h:461 [inline]
  ip_rcv_finish net/ipv4/ip_input.c:449 [inline]
  NF_HOOK include/linux/netfilter.h:314 [inline]
  ip_rcv+0x46f/0x760 net/ipv4/ip_input.c:569
  __netif_receive_skb_one_core net/core/dev.c:5534 [inline]
  __netif_receive_skb+0x1a6/0x5a0 net/core/dev.c:5648
  netif_receive_skb_internal net/core/dev.c:5734 [inline]
  netif_receive_skb+0x58/0x660 net/core/dev.c:5793
  tun_rx_batched+0x3ee/0x980 drivers/net/tun.c:1556
  tun_get_user+0x53b9/0x66e0 drivers/net/tun.c:2009
  tun_chr_write_iter+0x3af/0x5d0 drivers/net/tun.c:2055
  call_write_iter include/linux/fs.h:2087 [inline]
  new_sync_write fs/read_write.c:497 [inline]
  vfs_write+0xb6b/0x1520 fs/read_write.c:590
  ksys_write+0x20f/0x4c0 fs/read_write.c:643
  __do_sys_write fs/read_write.c:655 [inline]
  __se_sys_write fs/read_write.c:652 [inline]
  __x64_sys_write+0x93/0xd0 fs/read_write.c:652
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was created at:
  __alloc_pages+0x9a6/0xe00 mm/page_alloc.c:4590
  alloc_pages_mpol+0x62b/0x9d0 mm/mempolicy.c:2133
  alloc_pages+0x1be/0x1e0 mm/mempolicy.c:2204
  skb_page_frag_refill+0x2bf/0x7c0 net/core/sock.c:2909
  tun_build_skb drivers/net/tun.c:1686 [inline]
  tun_get_user+0xe0a/0x66e0 drivers/net/tun.c:1826
  tun_chr_write_iter+0x3af/0x5d0 drivers/net/tun.c:2055
  call_write_iter include/linux/fs.h:2087 [inline]
  new_sync_write fs/read_write.c:497 [inline]
  vfs_write+0xb6b/0x1520 fs/read_write.c:590
  ksys_write+0x20f/0x4c0 fs/read_write.c:643
  __do_sys_write fs/read_write.c:655 [inline]
  __se_sys_write fs/read_write.c:652 [inline]
  __x64_sys_write+0x93/0xd0 fs/read_write.c:652
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Fixes: c5441932 ("GRE: Refactor GRE tunneling code.")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>