[PATCH 6.18 246/333] net: nexthop: fix percpu use-after-free in remove_nh_grp_entry

Greg Kroah-Hartman
2026-03-17T17:24:35Z

From: Mehul Rao <mehulrao@gmail.com>

commit b2662e7593e94ae09b1cf7ee5f09160a3612bcb2 upstream.

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 <mehulrao@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260306233821.196789-1-mehulrao@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ipv4/nexthop.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -2005,7 +2005,8 @@ static void nh_hthr_group_rebalance(stru
}

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;
@@ -2065,8 +2066,8 @@ static void remove_nh_grp_entry(struct n
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.
@@ -2086,6 +2087,7 @@ static void remove_nexthop_from_groups(s
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()
@@ -2094,10 +2096,16 @@ static void remove_nexthop_from_groups(s
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)