KMSAN: uninit-value in nh_valid_get_del_req

[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=14c120e2600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=27abc558ecb16a3b
dashboard link: https://syzkaller.appspot.com/bug?extid=86ec9d8c02c07571873c
compiler:       clang version 9.0.0 (/home/glider/llvm/clang  
80fee25776c2fb61e74c1ecb1a523375c2500b69)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ed6c4a600000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11de024a600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+86ec9d8c02c07571873c@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KMSAN: uninit-value in nh_valid_get_del_req+0x6f1/0x8c0  
net/ipv4/nexthop.c:1510
CPU: 0 PID: 11812 Comm: syz-executor444 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
 nh_valid_get_del_req+0x6f1/0x8c0 net/ipv4/nexthop.c:1510
 rtm_del_nexthop+0x1b1/0x610 net/ipv4/nexthop.c:1543
 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_sendmmsg+0x53a/0xae0 net/socket.c:2413
 __do_sys_sendmmsg net/socket.c:2442 [inline]
 __se_sys_sendmmsg+0xbd/0xe0 net/socket.c:2439
 __x64_sys_sendmmsg+0x56/0x70 net/socket.c:2439
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x440259
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:00007fff15f10d08 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440259
RDX: 0492492492492805 RSI: 0000000020000140 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 0000000000000000 R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000401ae0
R13: 0000000000401b70 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_sendmmsg+0x53a/0xae0 net/socket.c:2413
 __do_sys_sendmmsg net/socket.c:2442 [inline]
 __se_sys_sendmmsg+0xbd/0xe0 net/socket.c:2439
 __x64_sys_sendmmsg+0x56/0x70 net/socket.c:2439
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================


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