[PATCH net 4/5] ipv6: mcast: use rcu_assign_pointer() for __rcu list updates

[Eric Dumazet <edumazet@google.com>]

Several places in net/ipv6/mcast.c update RCU-protected lists
(np->ipv6_mc_list, idev->mc_list, idev->mc_tomb) using direct pointer
assignments instead of rcu_assign_pointer():

1. In __ipv6_dev_mc_dec(), unlinking a group from idev->mc_list did:
       *map = ma->next;
   without rcu_assign_pointer() while concurrent readers traverse
   idev->mc_list locklessly under rcu_read_lock().
2. In ipv6_sock_mc_drop() and __ipv6_sock_mc_close(), unlinking a group
   from np->ipv6_mc_list directly assigned *lnk = mc_lst->next and
   np->ipv6_mc_list = mc_lst->next without rcu_assign_pointer(), racing
   with lockless readers in inet6_mc_check().
3. In __ipv6_sock_mc_join(), mc_lst->next was initialized to
   np->ipv6_mc_list via raw assignment before publishing mc_lst.
4. In mld_del_delrec() and __ipv6_dev_mc_inc(), __rcu source pointers
   passed into rcu_assign_pointer() lacked explicit dereference helpers.

Fix these by consistently using rcu_assign_pointer() along with
mc_dereference() / sock_dereference().

Fixes: 456b61bca8ee ("ipv6: mcast: RCU conversion")
Fixes: 88e2ca308094 ("mld: convert ifmcaddr6 to RCU")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Taehee Yoo <ap420073@gmail.com>
---
 net/ipv6/mcast.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 44fdde940dcdcd16a7642b2be60c6866c574ce92..a90dddba719726f36e0d5176c5e0574d1a488d5e 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -240,7 +240,8 @@ static int __ipv6_sock_mc_join(struct sock *sk, int ifindex,
 		return err;
 	}
 
-	mc_lst->next = np->ipv6_mc_list;
+	rcu_assign_pointer(mc_lst->next,
+			   sock_dereference(np->ipv6_mc_list, sk));
 	rcu_assign_pointer(np->ipv6_mc_list, mc_lst);
 
 	return 0;
@@ -300,7 +301,8 @@ int ipv6_sock_mc_drop(struct sock *sk, int ifindex, const struct in6_addr *addr)
 	      lnk = &mc_lst->next) {
 		if ((ifindex == 0 || mc_lst->ifindex == ifindex) &&
 		    ipv6_addr_equal(&mc_lst->addr, addr)) {
-			*lnk = mc_lst->next;
+			rcu_assign_pointer(*lnk,
+					   sock_dereference(mc_lst->next, sk));
 			__ipv6_sock_mc_drop(sk, mc_lst);
 			return 0;
 		}
@@ -333,7 +335,8 @@ void __ipv6_sock_mc_close(struct sock *sk)
 	struct ipv6_mc_socklist *mc_lst;
 
 	while ((mc_lst = sock_dereference(np->ipv6_mc_list, sk)) != NULL) {
-		np->ipv6_mc_list = mc_lst->next;
+		rcu_assign_pointer(np->ipv6_mc_list,
+				   sock_dereference(mc_lst->next, sk));
 		__ipv6_sock_mc_drop(sk, mc_lst);
 	}
 }
@@ -799,9 +802,11 @@ static void mld_del_delrec(struct inet6_dev *idev, struct ifmcaddr6 *im)
 	if (!pmc)
 		return;
 	if (pmc_prev)
-		rcu_assign_pointer(pmc_prev->next, pmc->next);
+		rcu_assign_pointer(pmc_prev->next,
+				   mc_dereference(pmc->next, idev));
 	else
-		rcu_assign_pointer(idev->mc_tomb, pmc->next);
+		rcu_assign_pointer(idev->mc_tomb,
+				   mc_dereference(pmc->next, idev));
 
 	im->idev = pmc->idev;
 	if (im->mca_sfmode == MCAST_INCLUDE) {
@@ -981,7 +986,7 @@ static int __ipv6_dev_mc_inc(struct net_device *dev,
 		return -ENOMEM;
 	}
 
-	rcu_assign_pointer(mc->next, idev->mc_list);
+	rcu_assign_pointer(mc->next, mc_dereference(idev->mc_list, idev));
 	rcu_assign_pointer(idev->mc_list, mc);
 
 	mld_del_delrec(idev, mc);
@@ -1015,7 +1020,8 @@ int __ipv6_dev_mc_dec(struct inet6_dev *idev, const struct in6_addr *addr)
 
 			WRITE_ONCE(ma->mca_users, new_users);
 			if (new_users == 0) {
-				*map = ma->next;
+				rcu_assign_pointer(*map,
+						   mc_dereference(ma->next, idev));
 
 				igmp6_group_dropped(ma);
 				inet6_ifmcaddr_notify(idev->dev, ma,
-- 
2.55.0.860.g4b6b3295ed-goog