KMSAN: uninit-value in dev_mc_add_excl

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    4bb25354f0b0 kmsan: unpoison pt_regs in do_nmi()
git tree:       https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=11e4ae95400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=36c582b1a617b1e6
dashboard link: https://syzkaller.appspot.com/bug?extid=d53ab4e92a1db04110ff
compiler:       clang version 8.0.0 (trunk 339414)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ac7c79400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15dbbf0d400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d53ab4e92a1db04110ff@xxxxxxxxxxxxxxxxxxxxxxxxx

sshd (6129) used greatest stack depth: 53504 bytes left
==================================================================
BUG: KMSAN: uninit-value in memcmp+0x117/0x180 lib/string.c:863
CPU: 1 PID: 6214 Comm: syz-executor693 Not tainted 4.19.0-rc8+ #70
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+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
 dev_mc_add_excl+0x165/0x770 net/core/dev_addr_lists.c:648
 ndo_dflt_fdb_add net/core/rtnetlink.c:3469 [inline]
 rtnl_fdb_add+0xe83/0x12a0 net/core/rtnetlink.c:3562
 rtnetlink_rcv_msg+0xa53/0x1590 net/core/rtnetlink.c:4730
 netlink_rcv_skb+0x394/0x640 net/netlink/af_netlink.c:2454
 rtnetlink_rcv+0x50/0x60 net/core/rtnetlink.c:4748
 netlink_unicast_kernel net/netlink/af_netlink.c:1317 [inline]
 netlink_unicast+0x166d/0x1720 net/netlink/af_netlink.c:1343
 netlink_sendmsg+0x1391/0x1420 net/netlink/af_netlink.c:1908
 sock_sendmsg_nosec net/socket.c:621 [inline]
 sock_sendmsg net/socket.c:631 [inline]
 ___sys_sendmsg+0xe47/0x1200 net/socket.c:2116
 __sys_sendmsg net/socket.c:2154 [inline]
 __do_sys_sendmsg net/socket.c:2163 [inline]
 __se_sys_sendmsg+0x307/0x460 net/socket.c:2161
 __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2161
 do_syscall_64+0xbe/0x100 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x440fd9
Code: e8 cc ab 02 00 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 bb 0a fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffe833197b8 EFLAGS: 00000213 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440fd9
RDX: 0000000000000000 RSI: 0000000020000500 RDI: 0000000000000003
RBP: 0000000000000000 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000213 R12: 0000000000010c7d
R13: 0000000000401fb0 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:255 [inline]
 kmsan_internal_poison_shadow+0xc8/0x1d0 mm/kmsan/kmsan.c:180
 kmsan_kmalloc+0xa4/0x120 mm/kmsan/kmsan_hooks.c:104
 kmsan_slab_alloc+0x10/0x20 mm/kmsan/kmsan_hooks.c:113
 slab_post_alloc_hook mm/slab.h:446 [inline]
 slab_alloc_node mm/slub.c:2727 [inline]
 __kmalloc_node_track_caller+0xb43/0x1400 mm/slub.c:4360
 __kmalloc_reserve net/core/skbuff.c:138 [inline]
 __alloc_skb+0x422/0xe90 net/core/skbuff.c:206
 alloc_skb include/linux/skbuff.h:996 [inline]
 netlink_alloc_large_skb net/netlink/af_netlink.c:1189 [inline]
 netlink_sendmsg+0xcaf/0x1420 net/netlink/af_netlink.c:1883
 sock_sendmsg_nosec net/socket.c:621 [inline]
 sock_sendmsg net/socket.c:631 [inline]
 ___sys_sendmsg+0xe47/0x1200 net/socket.c:2116
 __sys_sendmsg net/socket.c:2154 [inline]
 __do_sys_sendmsg net/socket.c:2163 [inline]
 __se_sys_sendmsg+0x307/0x460 net/socket.c:2161
 __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2161
 do_syscall_64+0xbe/0x100 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================


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