KASAN: use-after-free Read in vlan_dev_get_iflink

Hello,

syzbot found the following issue on:

HEAD commit:    8c26c87b Merge tag 'sound-5.8-rc7' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=128af5ef100000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f87a5e4232fdb267
dashboard link: https://syzkaller.appspot.com/bug?extid=d702fd2351989927037c
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1567cae8900000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=150631b4900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: <redacted>

==================================================================
BUG: KASAN: use-after-free in vlan_dev_get_iflink+0x5f/0x70 net/8021q/vlan_dev.c:767
Read of size 4 at addr ffff888094a76100 by task syz-executor015/9454

CPU: 1 PID: 9454 Comm: syz-executor015 Not tainted 5.8.0-rc6-syzkaller #0
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+0x18f/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x436 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 vlan_dev_get_iflink+0x5f/0x70 net/8021q/vlan_dev.c:767
 dev_get_iflink+0x73/0xe0 net/core/dev.c:814
 rfc2863_policy+0x243/0x2e0 net/core/link_watch.c:41
 linkwatch_do_dev+0x2a/0x180 net/core/link_watch.c:160
 linkwatch_forget_dev+0x16a/0x200 net/core/link_watch.c:237
 netdev_wait_allrefs net/core/dev.c:9678 [inline]
 netdev_run_todo+0x258/0xac0 net/core/dev.c:9774
 rtnl_unlock net/core/rtnetlink.c:112 [inline]
 rtnetlink_rcv_msg+0x45b/0xad0 net/core/rtnetlink.c:5461
 netlink_rcv_skb+0x15a/0x430 net/netlink/af_netlink.c:2469
 netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
 netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1329

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ffff888094a76100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

                   ^
 ffff888094a76180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888094a76200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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#syz test: https://github.com/congwang/linux.git net

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
general protection fault in vlan_get_link_net

general protection fault, probably for non-canonical address 0xdffffc00000000b3: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000598-0x000000000000059f]
CPU: 1 PID: 8207 Comm: syz-executor.3 Not tainted 5.8.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:read_pnet include/net/net_namespace.h:330 [inline]
RIP: 0010:dev_net include/linux/netdevice.h:2261 [inline]
RIP: 0010:vlan_get_link_net+0x46/0x70 net/8021q/vlan_netlink.c:279
Code: fa 48 c1 ea 03 80 3c 02 00 75 2e 48 8b 9b 70 0c 00 00 48 b8 00 00 00 00 00 fc ff df 48 8d bb 98 05 00 00 48 89 fa 48 c1 ea 03 <80> 3c 02 00 75 10 48 8b 83 98 05 00 00 5b c3 e8 86 1f 63 fa eb cb
RSP: 0018:ffffc90004706eb8 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffff888092c85230
RDX: 00000000000000b3 RSI: ffffffff874ffe39 RDI: 0000000000000598
RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffff8a7b8647
R10: 0000000000000000 R11: 0000000000000000 R12: ffffc90004706f80
R13: 0000000000000030 R14: ffff8880930ba900 R15: ffff8880930cc000
FS:  00007f4a9113a700(0000) GS:ffff8880ae700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005643b6ad2140 CR3: 000000009335e000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 rtnl_fill_link_netnsid net/core/rtnetlink.c:1569 [inline]
 rtnl_fill_ifinfo+0x1bc5/0x3c40 net/core/rtnetlink.c:1758
 rtmsg_ifinfo_build_skb+0xcd/0x1a0 net/core/rtnetlink.c:3706
 rollback_registered_many+0xb7d/0xf60 net/core/dev.c:8972
 unregister_netdevice_many.part.0+0x1a/0x2f0 net/core/dev.c:10113
 unregister_netdevice_many+0x36/0x50 net/core/dev.c:10112
 __rtnl_newlink+0x13bd/0x1750 net/core/rtnetlink.c:3381
 rtnl_newlink+0x64/0xa0 net/core/rtnetlink.c:3398
 rtnetlink_rcv_msg+0x44e/0xad0 net/core/rtnetlink.c:5461
 netlink_rcv_skb+0x15a/0x430 net/netlink/af_netlink.c:2469
 netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
 netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1329
 netlink_sendmsg+0x856/0xd90 net/netlink/af_netlink.c:1918
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:672

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