gtp: make sure only SOCK_DGRAM UDP sockets are accepted

A malicious user could use RAW sockets and fool
GTP using them as standard SOCK_DGRAM UDP sockets.

BUG: KMSAN: uninit-value in udp_tunnel_encap_enable include/net/udp_tunnel.h:174 [inline]
BUG: KMSAN: uninit-value in setup_udp_tunnel_sock+0x45e/0x6f0 net/ipv4/udp_tunnel.c:85
CPU: 0 PID: 11262 Comm: syz-executor613 Not tainted 5.5.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x220 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 udp_tunnel_encap_enable include/net/udp_tunnel.h:174 [inline]
 setup_udp_tunnel_sock+0x45e/0x6f0 net/ipv4/udp_tunnel.c:85
 gtp_encap_enable_socket+0x37f/0x5a0 drivers/net/gtp.c:827
 gtp_encap_enable drivers/net/gtp.c:844 [inline]
 gtp_newlink+0xfb/0x1e50 drivers/net/gtp.c:666
 __rtnl_newlink net/core/rtnetlink.c:3305 [inline]
 rtnl_newlink+0x2973/0x3920 net/core/rtnetlink.c:3363
 rtnetlink_rcv_msg+0x1153/0x1570 net/core/rtnetlink.c:5424
 netlink_rcv_skb+0x451/0x650 net/netlink/af_netlink.c:2477
 rtnetlink_rcv+0x50/0x60 net/core/rtnetlink.c:5442
 netlink_unicast_kernel net/netlink/af_netlink.c:1302 [inline]
 netlink_unicast+0xf9e/0x1100 net/netlink/af_netlink.c:1328
 netlink_sendmsg+0x1248/0x14d0 net/netlink/af_netlink.c:1917
 sock_sendmsg_nosec net/socket.c:639 [inline]
 sock_sendmsg net/socket.c:659 [inline]
 ____sys_sendmsg+0x12b6/0x1350 net/socket.c:2330
 ___sys_sendmsg net/socket.c:2384 [inline]
 __sys_sendmsg+0x451/0x5f0 net/socket.c:2417
 __do_sys_sendmsg net/socket.c:2426 [inline]
 __se_sys_sendmsg+0x97/0xb0 net/socket.c:2424
 __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2424
 do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x441359
Code: e8 ac e8 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff1cd0ac28 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000441359
RDX: 0000000000000000 RSI: 0000000020000100 RDI: 0000000000000003
RBP: 00000000006cb018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000246 R12: 00000000004020d0
R13: 0000000000402160 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags+0x3c/0x90 mm/kmsan/kmsan.c:144
 kmsan_internal_alloc_meta_for_pages mm/kmsan/kmsan_shadow.c:307 [inline]
 kmsan_alloc_page+0x12a/0x310 mm/kmsan/kmsan_shadow.c:336
 __alloc_pages_nodemask+0x57f2/0x5f60 mm/page_alloc.c:4800
 alloc_pages_current+0x67d/0x990 mm/mempolicy.c:2207
 alloc_pages include/linux/gfp.h:534 [inline]
 alloc_slab_page+0x111/0x12f0 mm/slub.c:1511
 allocate_slab mm/slub.c:1656 [inline]
 new_slab+0x2bc/0x1130 mm/slub.c:1722
 new_slab_objects mm/slub.c:2473 [inline]
 ___slab_alloc+0x1533/0x1f30 mm/slub.c:2624
 __slab_alloc mm/slub.c:2664 [inline]
 slab_alloc_node mm/slub.c:2738 [inline]
 slab_alloc mm/slub.c:2783 [inline]
 kmem_cache_alloc+0xb23/0xd70 mm/slub.c:2788
 sk_prot_alloc+0xf2/0x620 net/core/sock.c:1597
 sk_alloc+0xf0/0xbe0 net/core/sock.c:1657
 inet_create+0x7c7/0x1370 net/ipv4/af_inet.c:321
 __sock_create+0x8eb/0xf00 net/socket.c:1420
 sock_create net/socket.c:1471 [inline]
 __sys_socket+0x1a1/0x600 net/socket.c:1513
 __do_sys_socket net/socket.c:1522 [inline]
 __se_sys_socket+0x8d/0xb0 net/socket.c:1520
 __x64_sys_socket+0x4a/0x70 net/socket.c:1520
 do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pablo Neira <pablo@netfilter.org>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>