When removing a nexthop from a group, remove_nh_grp_entry() publishes the new group via rcu_assign_pointer() then immediately frees the removed entry's percpu stats with free_percpu(). However, the synchronize_net() grace period in the caller remove_nexthop_from_groups() runs after the free. RCU readers that entered before the publish still see the old group and can dereference the freed stats via nh_grp_entry_stats_inc() -> get_cpu_ptr(nhge->stats), causing a use-after-free on percpu memory. Fix by deferring the free_percpu() until after synchronize_net() in the caller. Removed entries are chained via nh_list onto a local deferred free list. After the grace period completes and all RCU readers have finished, the percpu stats are safely freed. Fixes: f4676ea74b85 ("net: nexthop: Add nexthop group entry stats") Cc: stable@vger.kernel.org Signed-off-by: Mehul Rao --- net/ipv4/nexthop.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c index 1aa2b05ee8de..c942f1282236 100644 --- a/net/ipv4/nexthop.c +++ b/net/ipv4/nexthop.c @@ -2002,7 +2002,8 @@ static void nh_hthr_group_rebalance(struct nh_group *nhg) } static void remove_nh_grp_entry(struct net *net, struct nh_grp_entry *nhge, - struct nl_info *nlinfo) + struct nl_info *nlinfo, + struct list_head *deferred_free) { struct nh_grp_entry *nhges, *new_nhges; struct nexthop *nhp = nhge->nh_parent; @@ -2062,8 +2063,8 @@ static void remove_nh_grp_entry(struct net *net, struct nh_grp_entry *nhge, rcu_assign_pointer(nhp->nh_grp, newg); list_del(&nhge->nh_list); - free_percpu(nhge->stats); nexthop_put(nhge->nh); + list_add(&nhge->nh_list, deferred_free); /* Removal of a NH from a resilient group is notified through * bucket notifications. @@ -2083,6 +2084,7 @@ static void remove_nexthop_from_groups(struct net *net, struct nexthop *nh, struct nl_info *nlinfo) { struct nh_grp_entry *nhge, *tmp; + LIST_HEAD(deferred_free); /* If there is nothing to do, let's avoid the costly call to * synchronize_net() @@ -2091,10 +2093,16 @@ static void remove_nexthop_from_groups(struct net *net, struct nexthop *nh, return; list_for_each_entry_safe(nhge, tmp, &nh->grp_list, nh_list) - remove_nh_grp_entry(net, nhge, nlinfo); + remove_nh_grp_entry(net, nhge, nlinfo, &deferred_free); /* make sure all see the newly published array before releasing rtnl */ synchronize_net(); + + /* Now safe to free percpu stats — all RCU readers have finished */ + list_for_each_entry_safe(nhge, tmp, &deferred_free, nh_list) { + list_del(&nhge->nh_list); + free_percpu(nhge->stats); + } } static void remove_nexthop_group(struct nexthop *nh, struct nl_info *nlinfo) -- 2.53.0