KMSAN: uninit-value in memcmp (2)

Hello,

syzbot found the following crash on:

HEAD commit:    74ee2200b89f kmsan: bump .config.example to v4.17-rc3
git tree:       https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=11bcd7e7800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4ca1e57bafa8ab1f
dashboard link: https://syzkaller.appspot.com/bug?extid=d3402c47f680ff24b29c
compiler:       clang version 7.0.0 (trunk 329391)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=102533c7800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=156dab5b800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: <redacted>

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
IPVS: ftp: loaded support on port[0] = 21
==================================================================
BUG: KMSAN: uninit-value in memcmp+0x119/0x180 lib/string.c:861
CPU: 0 PID: 38 Comm: kworker/0:1 Not tainted 4.17.0-rc3+ #88
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ipv6_addrconf addrconf_dad_work
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x185/0x1d0 lib/dump_stack.c:113
 kmsan_report+0x142/0x240 mm/kmsan/kmsan.c:1067
 __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:683
 memcmp+0x119/0x180 lib/string.c:861
 __hw_addr_add_ex net/core/dev_addr_lists.c:61 [inline]
 __dev_mc_add+0x1fc/0x900 net/core/dev_addr_lists.c:670
 dev_mc_add+0x6d/0x80 net/core/dev_addr_lists.c:687
 igmp6_group_added+0x2db/0xa00 net/ipv6/mcast.c:662
 ipv6_dev_mc_inc+0xe9e/0x1130 net/ipv6/mcast.c:914
 addrconf_join_solict net/ipv6/addrconf.c:2103 [inline]
 addrconf_dad_begin net/ipv6/addrconf.c:3853 [inline]
 addrconf_dad_work+0x462/0x2a20 net/ipv6/addrconf.c:3979
 process_one_work+0x12c6/0x1f60 kernel/workqueue.c:2145
 worker_thread+0x113c/0x24f0 kernel/workqueue.c:2279
 kthread+0x539/0x720 kernel/kthread.c:239
 ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:412

Local variable description: ----buf@igmp6_group_added
Variable was created at:
 igmp6_group_added+0x4a/0xa00 net/ipv6/mcast.c:650
 ipv6_dev_mc_inc+0xe9e/0x1130 net/ipv6/mcast.c:914
==================================================================


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