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SubjectRe: KMSAN: uninit-value in __dev_mc_add
From
syzbot has found a reproducer for the following crash on:

HEAD commit: 43c85fe5a0ee kmsan: suppress false positives in KVM
git tree: https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=15ffd5b9400000
kernel config: https://syzkaller.appspot.com/x/.config?x=3ff9630e1f32e076
dashboard link: https://syzkaller.appspot.com/bug?extid=001516d86dbe88862cec
compiler: clang version 8.0.0 (trunk 339414)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10adf491400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=100c8159400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+001516d86dbe88862cec@syzkaller.appspotmail.com

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+0x117/0x180 lib/string.c:863
CPU: 1 PID: 18 Comm: kworker/1:0 Not tainted 4.19.0-rc4+ #64
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+0x306/0x460 lib/dump_stack.c:113
kmsan_report+0x1a2/0x2e0 mm/kmsan/kmsan.c:917
__msan_warning+0x7c/0xe0 mm/kmsan/kmsan_instr.c:500
memcmp+0x117/0x180 lib/string.c:863
__hw_addr_add_ex net/core/dev_addr_lists.c:61 [inline]
__dev_mc_add+0x1f9/0x8b0 net/core/dev_addr_lists.c:670
dev_mc_add+0x6d/0x80 net/core/dev_addr_lists.c:687
igmp6_group_added+0x2d7/0xab0 net/ipv6/mcast.c:676
__ipv6_dev_mc_inc+0xeff/0x10f0 net/ipv6/mcast.c:934
ipv6_dev_mc_inc+0x70/0x80 net/ipv6/mcast.c:941
addrconf_join_solict net/ipv6/addrconf.c:2098 [inline]
addrconf_dad_begin net/ipv6/addrconf.c:3879 [inline]
addrconf_dad_work+0x3e7/0x2690 net/ipv6/addrconf.c:4006
process_one_work+0x19c4/0x24f0 kernel/workqueue.c:2153
worker_thread+0x206d/0x2b30 kernel/workqueue.c:2296
kthread+0x59c/0x5d0 kernel/kthread.c:247
ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416

Local variable description: ----buf@igmp6_group_added
Variable was created at:
igmp6_group_added+0x57/0xab0 net/ipv6/mcast.c:664
__ipv6_dev_mc_inc+0xeff/0x10f0 net/ipv6/mcast.c:934
==================================================================

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