geneve: fix header validation in geneve[6]_xmit_skb

syzbot is able to trigger an uninit-value in geneve_xmit() [1]

Problem : While most ip tunnel helpers (like ip_tunnel_get_dsfield())
uses skb_protocol(skb, true), pskb_inet_may_pull() is only using
skb->protocol.

If anything else than ETH_P_IPV6 or ETH_P_IP is found in skb->protocol,
pskb_inet_may_pull() does nothing at all.

If a vlan tag was provided by the caller (af_packet in the syzbot case),
the network header might not point to the correct location, and skb
linear part could be smaller than expected.

Add skb_vlan_inet_prepare() to perform a complete mac validation.

Use this in geneve for the moment, I suspect we need to adopt this
more broadly.

v4 - Jakub reported v3 broke l2_tos_ttl_inherit.sh selftest
   - Only call __vlan_get_protocol() for vlan types.
Link: https://lore.kernel.org/netdev/20240404100035.3270a7d5@kernel.org/

v2,v3 - Addressed Sabrina comments on v1 and v2
Link: https://lore.kernel.org/netdev/Zg1l9L2BNoZWZDZG@hog/

[1]

BUG: KMSAN: uninit-value in geneve_xmit_skb drivers/net/geneve.c:910 [inline]
 BUG: KMSAN: uninit-value in geneve_xmit+0x302d/0x5420 drivers/net/geneve.c:1030
  geneve_xmit_skb drivers/net/geneve.c:910 [inline]
  geneve_xmit+0x302d/0x5420 drivers/net/geneve.c:1030
  __netdev_start_xmit include/linux/netdevice.h:4903 [inline]
  netdev_start_xmit include/linux/netdevice.h:4917 [inline]
  xmit_one net/core/dev.c:3531 [inline]
  dev_hard_start_xmit+0x247/0xa20 net/core/dev.c:3547
  __dev_queue_xmit+0x348d/0x52c0 net/core/dev.c:4335
  dev_queue_xmit include/linux/netdevice.h:3091 [inline]
  packet_xmit+0x9c/0x6c0 net/packet/af_packet.c:276
  packet_snd net/packet/af_packet.c:3081 [inline]
  packet_sendmsg+0x8bb0/0x9ef0 net/packet/af_packet.c:3113
  sock_sendmsg_nosec net/socket.c:730 [inline]
  __sock_sendmsg+0x30f/0x380 net/socket.c:745
  __sys_sendto+0x685/0x830 net/socket.c:2191
  __do_sys_sendto net/socket.c:2203 [inline]
  __se_sys_sendto net/socket.c:2199 [inline]
  __x64_sys_sendto+0x125/0x1d0 net/socket.c:2199
 do_syscall_64+0xd5/0x1f0
 entry_SYSCALL_64_after_hwframe+0x6d/0x75

Uninit was created at:
  slab_post_alloc_hook mm/slub.c:3804 [inline]
  slab_alloc_node mm/slub.c:3845 [inline]
  kmem_cache_alloc_node+0x613/0xc50 mm/slub.c:3888
  kmalloc_reserve+0x13d/0x4a0 net/core/skbuff.c:577
  __alloc_skb+0x35b/0x7a0 net/core/skbuff.c:668
  alloc_skb include/linux/skbuff.h:1318 [inline]
  alloc_skb_with_frags+0xc8/0xbf0 net/core/skbuff.c:6504
  sock_alloc_send_pskb+0xa81/0xbf0 net/core/sock.c:2795
  packet_alloc_skb net/packet/af_packet.c:2930 [inline]
  packet_snd net/packet/af_packet.c:3024 [inline]
  packet_sendmsg+0x722d/0x9ef0 net/packet/af_packet.c:3113
  sock_sendmsg_nosec net/socket.c:730 [inline]
  __sock_sendmsg+0x30f/0x380 net/socket.c:745
  __sys_sendto+0x685/0x830 net/socket.c:2191
  __do_sys_sendto net/socket.c:2203 [inline]
  __se_sys_sendto net/socket.c:2199 [inline]
  __x64_sys_sendto+0x125/0x1d0 net/socket.c:2199
 do_syscall_64+0xd5/0x1f0
 entry_SYSCALL_64_after_hwframe+0x6d/0x75

CPU: 0 PID: 5033 Comm: syz-executor346 Not tainted 6.9.0-rc1-syzkaller-00005-g928a87efa423 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024

Fixes: d13f048d

 ("net: geneve: modify IP header check in geneve6_xmit_skb and geneve_xmit_skb")
Reported-by:  <syzbot+9ee20ec1de7b3168db09@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/netdev/000000000000d19c3a06152f9ee4@google.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Phillip Potter <phil@philpotter.co.uk>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>