[PATCH net 4/5] ipv6: mcast: use rcu_assign_pointer() for __rcu list updates
From: Eric Dumazet <edumazet@google.com>
Date: 2026-08-26 10:37:19
Subsystem:
networking [general], networking [ipv4/ipv6], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, David Ahern, Ido Schimmel, Linus Torvalds
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