net: igmp: fix data-race in igmp_ifc_timer_expire()

net: igmp: fix data-race in igmp_ifc_timer_expire()

Fix the data-race reported by syzbot [1]
Issue here is that igmp_ifc_timer_expire() can update in_dev->mr_ifc_count
while another change just occured from another context.

in_dev->mr_ifc_count is only 8bit wide, so the race had little
consequences.

[1]
BUG: KCSAN: data-race in igmp_ifc_event / igmp_ifc_timer_expire

write to 0xffff8881051e3062 of 1 bytes by task 12547 on cpu 0:
 igmp_ifc_event+0x1d5/0x290 net/ipv4/igmp.c:821
 igmp_group_added+0x462/0x490 net/ipv4/igmp.c:1356
 ____ip_mc_inc_group+0x3ff/0x500 net/ipv4/igmp.c:1461
 __ip_mc_join_group+0x24d/0x2c0 net/ipv4/igmp.c:2199
 ip_mc_join_group_ssm+0x20/0x30 net/ipv4/igmp.c:2218
 do_ip_setsockopt net/ipv4/ip_sockglue.c:1285 [inline]
 ip_setsockopt+0x1827/0x2a80 net/ipv4/ip_sockglue.c:1423
 tcp_setsockopt+0x8c/0xa0 net/ipv4/tcp.c:3657
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3362
 __sys_setsockopt+0x18f/0x200 net/socket.c:2159
 __do_sys_setsockopt net/socket.c:2170 [inline]
 __se_sys_setsockopt net/socket.c:2167 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2167
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881051e3062 of 1 bytes by interrupt on cpu 1:
 igmp_ifc_timer_expire+0x706/0xa30 net/ipv4/igmp.c:808
 call_timer_fn+0x2e/0x1d0 kernel/time/timer.c:1419
 expire_timers+0x135/0x250 kernel/time/timer.c:1464
 __run_timers+0x358/0x420 kernel/time/timer.c:1732
 run_timer_softirq+0x19/0x30 kernel/time/timer.c:1745
 __do_softirq+0x12c/0x26e kernel/softirq.c:558
 invoke_softirq kernel/softirq.c:432 [inline]
 __irq_exit_rcu+0x9a/0xb0 kernel/softirq.c:636
 sysvec_apic_timer_interrupt+0x69/0x80 arch/x86/kernel/apic/apic.c:1100
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:638
 console_unlock+0x8e8/0xb30 kernel/printk/printk.c:2646
 vprintk_emit+0x125/0x3d0 kernel/printk/printk.c:2174
 vprintk_default+0x22/0x30 kernel/printk/printk.c:2185
 vprintk+0x15a/0x170 kernel/printk/printk_safe.c:392
 printk+0x62/0x87 kernel/printk/printk.c:2216
 selinux_netlink_send+0x399/0x400 security/selinux/hooks.c:6041
 security_netlink_send+0x42/0x90 security/security.c:2070
 netlink_sendmsg+0x59e/0x7c0 net/netlink/af_netlink.c:1919
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg net/socket.c:723 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2392
 ___sys_sendmsg net/socket.c:2446 [inline]
 __sys_sendmsg+0x1ed/0x270 net/socket.c:2475
 __do_sys_sendmsg net/socket.c:2484 [inline]
 __se_sys_sendmsg net/socket.c:2482 [inline]
 __x64_sys_sendmsg+0x42/0x50 net/socket.c:2482
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x01 -> 0x02

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12539 Comm: syz-executor.1 Not tainted 5.14.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>