KMSAN: uninit-value in rtm_new_nexthop

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    61ccdad1 Revert "drm/bochs: Use shadow buffer for bochs fr..
git tree:       https://github.com/google/kmsan.git master
console output: https://syzkaller.appspot.com/x/log.txt?x=1596d33c600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=27abc558ecb16a3b
dashboard link: https://syzkaller.appspot.com/bug?extid=4f3abbb335d1bed2287c
compiler:       clang version 9.0.0 (/home/glider/llvm/clang  
80fee25776c2fb61e74c1ecb1a523375c2500b69)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15733302600000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1028e9e2600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+4f3abbb335d1bed2287c@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KMSAN: uninit-value in rtm_to_nh_config net/ipv4/nexthop.c:1317  
[inline]
BUG: KMSAN: uninit-value in rtm_new_nexthop+0x447/0x98e0  
net/ipv4/nexthop.c:1474
CPU: 0 PID: 11026 Comm: syz-executor768 Not tainted 5.3.0-rc3+ #17
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+0x191/0x1f0 lib/dump_stack.c:113
 kmsan_report+0x162/0x2d0 mm/kmsan/kmsan_report.c:109
 __msan_warning+0x75/0xe0 mm/kmsan/kmsan_instr.c:294
 rtm_to_nh_config net/ipv4/nexthop.c:1317 [inline]
 rtm_new_nexthop+0x447/0x98e0 net/ipv4/nexthop.c:1474
 rtnetlink_rcv_msg+0x115a/0x1580 net/core/rtnetlink.c:5223
 netlink_rcv_skb+0x431/0x620 net/netlink/af_netlink.c:2477
 rtnetlink_rcv+0x50/0x60 net/core/rtnetlink.c:5241
 netlink_unicast_kernel net/netlink/af_netlink.c:1302 [inline]
 netlink_unicast+0xf6c/0x1050 net/netlink/af_netlink.c:1328
 netlink_sendmsg+0x110f/0x1330 net/netlink/af_netlink.c:1917
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg net/socket.c:657 [inline]
 ___sys_sendmsg+0x14ff/0x1590 net/socket.c:2311
 __sys_sendmsg net/socket.c:2356 [inline]
 __do_sys_sendmsg net/socket.c:2365 [inline]
 __se_sys_sendmsg+0x305/0x460 net/socket.c:2363
 __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2363
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x4401e9
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 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffef5128b58 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 00000000004401e9
RDX: 0000000000000000 RSI: 0000000020000400 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 0000000000000000 R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000401a70
R13: 0000000000401b00 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:187 [inline]
 kmsan_internal_poison_shadow+0x53/0xa0 mm/kmsan/kmsan.c:146
 kmsan_slab_alloc+0xaa/0x120 mm/kmsan/kmsan_hooks.c:175
 slab_alloc_node mm/slub.c:2790 [inline]
 __kmalloc_node_track_caller+0xb55/0x1320 mm/slub.c:4388
 __kmalloc_reserve net/core/skbuff.c:141 [inline]
 __alloc_skb+0x306/0xa10 net/core/skbuff.c:209
 alloc_skb include/linux/skbuff.h:1056 [inline]
 netlink_alloc_large_skb net/netlink/af_netlink.c:1174 [inline]
 netlink_sendmsg+0x783/0x1330 net/netlink/af_netlink.c:1892
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg net/socket.c:657 [inline]
 ___sys_sendmsg+0x14ff/0x1590 net/socket.c:2311
 __sys_sendmsg net/socket.c:2356 [inline]
 __do_sys_sendmsg net/socket.c:2365 [inline]
 __se_sys_sendmsg+0x305/0x460 net/socket.c:2363
 __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2363
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================


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