KMSAN: uninit-value in tipc_nl_compat_dumpit

[syzbot <syzbot+6bdb590321a7ae40c1a6@syzkaller.appspotmail.com>]

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=1164f197800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4ca1e57bafa8ab1f
dashboard link: https://syzkaller.appspot.com/bug?extid=6bdb590321a7ae40c1a6
compiler:       clang version 7.0.0 (trunk 329391)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15341157800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17410147800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+6bdb590321a7ae40c1a6@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)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KMSAN: uninit-value in tipc_nl_compat_dumpit+0x490/0x870  
net/tipc/netlink_compat.c:245
CPU: 0 PID: 4507 Comm: syz-executor018 Not tainted 4.17.0-rc3+ #88
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+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
  tipc_nl_compat_dumpit+0x490/0x870 net/tipc/netlink_compat.c:245
  tipc_nl_compat_handle net/tipc/netlink_compat.c:1108 [inline]
  tipc_nl_compat_recv+0x1394/0x2700 net/tipc/netlink_compat.c:1204
  genl_family_rcv_msg net/netlink/genetlink.c:599 [inline]
  genl_rcv_msg+0x1686/0x1810 net/netlink/genetlink.c:624
  netlink_rcv_skb+0x378/0x600 net/netlink/af_netlink.c:2448
  genl_rcv+0x63/0x80 net/netlink/genetlink.c:635
  netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
  netlink_unicast+0x166b/0x1740 net/netlink/af_netlink.c:1336
  netlink_sendmsg+0x1072/0x1370 net/netlink/af_netlink.c:1901
  sock_sendmsg_nosec net/socket.c:629 [inline]
  sock_sendmsg net/socket.c:639 [inline]
  ___sys_sendmsg+0xec0/0x1310 net/socket.c:2117
  __sys_sendmsg net/socket.c:2155 [inline]
  __do_sys_sendmsg net/socket.c:2164 [inline]
  __se_sys_sendmsg net/socket.c:2162 [inline]
  __x64_sys_sendmsg+0x331/0x460 net/socket.c:2162
  do_syscall_64+0x154/0x220 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x445599
RSP: 002b:00007f4c1ecc6db8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000006dac24 RCX: 0000000000445599
RDX: 0000000000000000 RSI: 0000000020023000 RDI: 0000000000000003
RBP: 00000000006dac20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffe5ef31bcf R14: 00007f4c1ecc79c0 R15: 0000000000000001

Uninit was created at:
  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:278 [inline]
  kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:188
  kmsan_kmalloc+0x94/0x100 mm/kmsan/kmsan.c:314
  kmsan_slab_alloc+0x10/0x20 mm/kmsan/kmsan.c:321
  slab_post_alloc_hook mm/slab.h:446 [inline]
  slab_alloc_node mm/slub.c:2753 [inline]
  __kmalloc_node_track_caller+0xb32/0x11b0 mm/slub.c:4395
  __kmalloc_reserve net/core/skbuff.c:138 [inline]
  __alloc_skb+0x2cf/0x9f0 net/core/skbuff.c:206
  alloc_skb include/linux/skbuff.h:988 [inline]
  netlink_alloc_large_skb net/netlink/af_netlink.c:1182 [inline]
  netlink_sendmsg+0x76e/0x1370 net/netlink/af_netlink.c:1876
  sock_sendmsg_nosec net/socket.c:629 [inline]
  sock_sendmsg net/socket.c:639 [inline]
  ___sys_sendmsg+0xec0/0x1310 net/socket.c:2117
  __sys_sendmsg net/socket.c:2155 [inline]
  __do_sys_sendmsg net/socket.c:2164 [inline]
  __se_sys_sendmsg net/socket.c:2162 [inline]
  __x64_sys_sendmsg+0x331/0x460 net/socket.c:2162
  do_syscall_64+0x154/0x220 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
==================================================================


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