vif_dev_read() should be used from RCU protected sections only.

ipmr_rtm_dumplink() is holding RTNL, so the data structures
can not be changed.

syzbot reported:

net/ipv4/ipmr.c:84 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz-executor.4/3068:

stack backtrace:
CPU: 1 PID: 3068 Comm: syz-executor.4 Not tainted 5.19.0-rc3-syzkaller-00565-g5d04b0b6 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
vif_dev_read net/ipv4/ipmr.c:84 [inline]
vif_dev_read net/ipv4/ipmr.c:82 [inline]
ipmr_fill_vif net/ipv4/ipmr.c:2756 [inline]
ipmr_rtm_dumplink+0x1343/0x18c0 net/ipv4/ipmr.c:2866
netlink_dump+0x541/0xc20 net/netlink/af_netlink.c:2275
__netlink_dump_start+0x647/0x900 net/netlink/af_netlink.c:2380
netlink_dump_start include/linux/netlink.h:245 [inline]
rtnetlink_rcv_msg+0x73e/0xc90 net/core/rtnetlink.c:6046
netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2501
netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
netlink_unicast+0x543/0x7f0 net/netlink/af_netlink.c:1345
netlink_sendmsg+0x917/0xe10 net/netlink/af_netlink.c:1921
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg+0xcf/0x120 net/socket.c:734
____sys_sendmsg+0x334/0x810 net/socket.c:2489
___sys_sendmsg+0xf3/0x170 net/socket.c:2543
__sys_sendmmsg+0x195/0x470 net/socket.c:2629
__do_sys_sendmmsg net/socket.c:2658 [inline]
__se_sys_sendmmsg net/socket.c:2655 [inline]
__x64_sys_sendmmsg+0x99/0x100 net/socket.c:2655
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7fefd8a89109
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fefd9ca6168 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007fefd8b9bf60 RCX: 00007fefd8a89109
RDX: 0000000004924b68 RSI: 0000000020000140 RDI: 0000000000000003
RBP: 00007fefd8ae305d R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffc346febaf R14: 00007fefd9ca6300 R15: 0000000000022000
</TASK>

Fixes: ebc31979 ("ipmr: add rcu protection over (struct vif_device)->dev")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>