[PATCH net] ipv6: mcast: fix data-race in ipv6_mc_down / mld_ifc_work

[Nikita Zhandarovich <n.zhandarovich@fintech.ru>]

idev->mc_ifc_count can be written over without proper locking.

Originally found by syzbot [1], fix this issue by encapsulating calls
to mld_ifc_stop_work() (and mld_gq_stop_work() for good measure) with
mutex_lock() and mutex_unlock() accordingly as these functions
should only be called with mc_lock per their declarations.

[1]
BUG: KCSAN: data-race in ipv6_mc_down / mld_ifc_work

write to 0xffff88813a80c832 of 1 bytes by task 3771 on cpu 0:
 mld_ifc_stop_work net/ipv6/mcast.c:1080 [inline]
 ipv6_mc_down+0x10a/0x280 net/ipv6/mcast.c:2725
 addrconf_ifdown+0xe32/0xf10 net/ipv6/addrconf.c:3949
 addrconf_notify+0x310/0x980
 notifier_call_chain kernel/notifier.c:93 [inline]
 raw_notifier_call_chain+0x6b/0x1c0 kernel/notifier.c:461
 __dev_notify_flags+0x205/0x3d0
 dev_change_flags+0xab/0xd0 net/core/dev.c:8685
 do_setlink+0x9f6/0x2430 net/core/rtnetlink.c:2916
 rtnl_group_changelink net/core/rtnetlink.c:3458 [inline]
 __rtnl_newlink net/core/rtnetlink.c:3717 [inline]
 rtnl_newlink+0xbb3/0x1670 net/core/rtnetlink.c:3754
 rtnetlink_rcv_msg+0x807/0x8c0 net/core/rtnetlink.c:6558
 netlink_rcv_skb+0x126/0x220 net/netlink/af_netlink.c:2545
 rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:6576
 netlink_unicast_kernel net/netlink/af_netlink.c:1342 [inline]
 netlink_unicast+0x589/0x650 net/netlink/af_netlink.c:1368
 netlink_sendmsg+0x66e/0x770 net/netlink/af_netlink.c:1910
 ...

write to 0xffff88813a80c832 of 1 bytes by task 22 on cpu 1:
 mld_ifc_work+0x54c/0x7b0 net/ipv6/mcast.c:2653
 process_one_work kernel/workqueue.c:2627 [inline]
 process_scheduled_works+0x5b8/0xa30 kernel/workqueue.c:2700
 worker_thread+0x525/0x730 kernel/workqueue.c:2781
 ...

Fixes: 2d9a93b4902b ("mld: convert from timer to delayed work")
Reported-by: syzbot+a9400cabb1d784e49abf@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/000000000000994e09060ebcdffb@google.com/
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
---
 net/ipv6/mcast.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index b75d3c9d41bb..bc6e0a0bad3c 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -2722,8 +2722,12 @@ void ipv6_mc_down(struct inet6_dev *idev)
 	synchronize_net();
 	mld_query_stop_work(idev);
 	mld_report_stop_work(idev);
+
+	mutex_lock(&idev->mc_lock);
 	mld_ifc_stop_work(idev);
 	mld_gq_stop_work(idev);
+	mutex_unlock(&idev->mc_lock);
+
 	mld_dad_stop_work(idev);
 }