Re: KMSAN: uninit-value in sit_tunnel_xmit

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    0cc51dc9a291 kmsan: instrument get_user() and put_user() p..
git tree:       https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=14973622400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3618361797c180a3
dashboard link: https://syzkaller.appspot.com/bug?extid=782ee96f9147673d8822
compiler:       clang version 8.0.0 (trunk 339414)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=10fd6e30400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17a2aee2400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+782ee96f9147673d8822@xxxxxxxxxxxxxxxxxxxxxxxxx

IPv6: ADDRCONF(NETDEV_UP): veth1: link is not ready
IPv6: ADDRCONF(NETDEV_CHANGE): veth1: link becomes ready
IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
8021q: adding VLAN 0 to HW filter on device team0
==================================================================
BUG: KMSAN: uninit-value in __ipv6_prefix_equal64_half  
include/net/ipv6.h:515 [inline]
BUG: KMSAN: uninit-value in ipv6_prefix_equal include/net/ipv6.h:532  
[inline]
BUG: KMSAN: uninit-value in check_6rd net/ipv6/sit.c:771 [inline]
BUG: KMSAN: uninit-value in try_6rd net/ipv6/sit.c:805 [inline]
BUG: KMSAN: uninit-value in ipip6_tunnel_xmit net/ipv6/sit.c:865 [inline]
BUG: KMSAN: uninit-value in sit_tunnel_xmit+0x18c0/0x3640  
net/ipv6/sit.c:1029
CPU: 0 PID: 4467 Comm: syz-executor896 Not tainted 4.18.0-rc8+ #33
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+0x17c/0x1c0 lib/dump_stack.c:113
 kmsan_report+0x188/0x2a0 mm/kmsan/kmsan.c:982
 __msan_warning+0x70/0xc0 mm/kmsan/kmsan_instr.c:645
 __ipv6_prefix_equal64_half include/net/ipv6.h:515 [inline]
 ipv6_prefix_equal include/net/ipv6.h:532 [inline]
 check_6rd net/ipv6/sit.c:771 [inline]
 try_6rd net/ipv6/sit.c:805 [inline]
 ipip6_tunnel_xmit net/ipv6/sit.c:865 [inline]
 sit_tunnel_xmit+0x18c0/0x3640 net/ipv6/sit.c:1029
 __netdev_start_xmit include/linux/netdevice.h:4148 [inline]
 netdev_start_xmit include/linux/netdevice.h:4157 [inline]
 xmit_one net/core/dev.c:3034 [inline]
 dev_hard_start_xmit+0x5df/0xc20 net/core/dev.c:3050
 __dev_queue_xmit+0x2eea/0x3a70 net/core/dev.c:3569
 dev_queue_xmit+0x4b/0x60 net/core/dev.c:3602
 packet_snd net/packet/af_packet.c:2921 [inline]
 packet_sendmsg+0x7fb5/0x8ae0 net/packet/af_packet.c:2946
 sock_sendmsg_nosec net/socket.c:642 [inline]
 sock_sendmsg net/socket.c:652 [inline]
 ___sys_sendmsg+0xe32/0x1250 net/socket.c:2126
 __sys_sendmsg net/socket.c:2164 [inline]
 __do_sys_sendmsg net/socket.c:2173 [inline]
 __se_sys_sendmsg net/socket.c:2171 [inline]
 __x64_sys_sendmsg+0x32d/0x460 net/socket.c:2171
 do_syscall_64+0x15b/0x220 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x441149
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 db 0a fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffdb58f66a8 EFLAGS: 00000286 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000441149
RDX: 0000000000000000 RSI: 0000000020000000 RDI: 0000000000000003
RBP: 00000000006cc018 R08: 0000000000000100 R09: 0000000000000100
R10: 0000000000000100 R11: 0000000000000286 R12: 00000000004020b0
R13: 0000000000402140 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:256 [inline]
 kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:181
 kmsan_kmalloc+0x98/0x100 mm/kmsan/kmsan_hooks.c:91
 kmsan_slab_alloc+0x10/0x20 mm/kmsan/kmsan_hooks.c:100
 slab_post_alloc_hook mm/slab.h:446 [inline]
 slab_alloc_node mm/slub.c:2720 [inline]
 __kmalloc_node_track_caller+0xb4c/0x11d0 mm/slub.c:4353
 __kmalloc_reserve net/core/skbuff.c:138 [inline]
 __alloc_skb+0x2ce/0x9b0 net/core/skbuff.c:206
 alloc_skb include/linux/skbuff.h:988 [inline]
 alloc_skb_with_frags+0x1d0/0xac0 net/core/skbuff.c:5272
 sock_alloc_send_pskb+0xb47/0x1120 net/core/sock.c:2095
 packet_alloc_skb net/packet/af_packet.c:2776 [inline]
 packet_snd net/packet/af_packet.c:2867 [inline]
 packet_sendmsg+0x6480/0x8ae0 net/packet/af_packet.c:2946
 sock_sendmsg_nosec net/socket.c:642 [inline]
 sock_sendmsg net/socket.c:652 [inline]
 ___sys_sendmsg+0xe32/0x1250 net/socket.c:2126
 __sys_sendmsg net/socket.c:2164 [inline]
 __do_sys_sendmsg net/socket.c:2173 [inline]
 __se_sys_sendmsg net/socket.c:2171 [inline]
 __x64_sys_sendmsg+0x32d/0x460 net/socket.c:2171
 do_syscall_64+0x15b/0x220 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================