syzbot/KMSAN reported use of uninit-value in get_dev_xmit() [1]

We must make sure the IPv4 or Ipv6 header is pulled in skb->head
before accessing fields in them.

Use pskb_inet_may_pull() to fix this issue.

[1]
BUG: KMSAN: uninit-value in ipv6_pdp_find drivers/net/gtp.c:220 [inline]
 BUG: KMSAN: uninit-value in gtp_build_skb_ip6 drivers/net/gtp.c:1229 [inline]
 BUG: KMSAN: uninit-value in gtp_dev_xmit+0x1424/0x2540 drivers/net/gtp.c:1281
  ipv6_pdp_find drivers/net/gtp.c:220 [inline]
  gtp_build_skb_ip6 drivers/net/gtp.c:1229 [inline]
  gtp_dev_xmit+0x1424/0x2540 drivers/net/gtp.c:1281
  __netdev_start_xmit include/linux/netdevice.h:4913 [inline]
  netdev_start_xmit include/linux/netdevice.h:4922 [inline]
  xmit_one net/core/dev.c:3580 [inline]
  dev_hard_start_xmit+0x247/0xa20 net/core/dev.c:3596
  __dev_queue_xmit+0x358c/0x5610 net/core/dev.c:4423
  dev_queue_xmit include/linux/netdevice.h:3105 [inline]
  packet_xmit+0x9c/0x6c0 net/packet/af_packet.c:276
  packet_snd net/packet/af_packet.c:3145 [inline]
  packet_sendmsg+0x90e3/0xa3a0 net/packet/af_packet.c:3177
  sock_sendmsg_nosec net/socket.c:730 [inline]
  __sock_sendmsg+0x30f/0x380 net/socket.c:745
  __sys_sendto+0x685/0x830 net/socket.c:2204
  __do_sys_sendto net/socket.c:2216 [inline]
  __se_sys_sendto net/socket.c:2212 [inline]
  __x64_sys_sendto+0x125/0x1d0 net/socket.c:2212
  x64_sys_call+0x3799/0x3c10 arch/x86/include/generated/asm/syscalls_64.h:45
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xcd/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
  slab_post_alloc_hook mm/slub.c:3994 [inline]
  slab_alloc_node mm/slub.c:4037 [inline]
  kmem_cache_alloc_node_noprof+0x6bf/0xb80 mm/slub.c:4080
  kmalloc_reserve+0x13d/0x4a0 net/core/skbuff.c:583
  __alloc_skb+0x363/0x7b0 net/core/skbuff.c:674
  alloc_skb include/linux/skbuff.h:1320 [inline]
  alloc_skb_with_frags+0xc8/0xbf0 net/core/skbuff.c:6526
  sock_alloc_send_pskb+0xa81/0xbf0 net/core/sock.c:2815
  packet_alloc_skb net/packet/af_packet.c:2994 [inline]
  packet_snd net/packet/af_packet.c:3088 [inline]
  packet_sendmsg+0x749c/0xa3a0 net/packet/af_packet.c:3177
  sock_sendmsg_nosec net/socket.c:730 [inline]
  __sock_sendmsg+0x30f/0x380 net/socket.c:745
  __sys_sendto+0x685/0x830 net/socket.c:2204
  __do_sys_sendto net/socket.c:2216 [inline]
  __se_sys_sendto net/socket.c:2212 [inline]
  __x64_sys_sendto+0x125/0x1d0 net/socket.c:2212
  x64_sys_call+0x3799/0x3c10 arch/x86/include/generated/asm/syscalls_64.h:45
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xcd/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 UID: 0 PID: 7115 Comm: syz.1.515 Not tainted 6.11.0-rc1-syzkaller-00043-g94ede2a3 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/27/2024

Fixes: 999cb275 ("gtp: add IPv6 support")
Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Harald Welte <laforge@gnumonks.org>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://patch.msgid.link/20240808132455.3413916-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>