6.12-stable review patch. If anyone has any objections, please let me know.
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From: Chuang Wang nashuiliang@gmail.com
commit ac1499fcd40fe06479e9b933347b837ccabc2a40 upstream.
The sit driver's packet transmission path calls: sit_tunnel_xmit() -> update_or_create_fnhe(), which lead to fnhe_remove_oldest() being called to delete entries exceeding FNHE_RECLAIM_DEPTH+random.
The race window is between fnhe_remove_oldest() selecting fnheX for deletion and the subsequent kfree_rcu(). During this time, the concurrent path's __mkroute_output() -> find_exception() can fetch the soon-to-be-deleted fnheX, and rt_bind_exception() then binds it with a new dst using a dst_hold(). When the original fnheX is freed via RCU, the dst reference remains permanently leaked.
CPU 0 CPU 1 __mkroute_output() find_exception() [fnheX] update_or_create_fnhe() fnhe_remove_oldest() [fnheX] rt_bind_exception() [bind dst] RCU callback [fnheX freed, dst leak]
This issue manifests as a device reference count leak and a warning in dmesg when unregistering the net device:
unregister_netdevice: waiting for sitX to become free. Usage count = N
Ido Schimmel provided the simple test validation method [1].
The fix clears 'oldest->fnhe_daddr' before calling fnhe_flush_routes(). Since rt_bind_exception() checks this field, setting it to zero prevents the stale fnhe from being reused and bound to a new dst just before it is freed.
[1] ip netns add ns1 ip -n ns1 link set dev lo up ip -n ns1 address add 192.0.2.1/32 dev lo ip -n ns1 link add name dummy1 up type dummy ip -n ns1 route add 192.0.2.2/32 dev dummy1 ip -n ns1 link add name gretap1 up arp off type gretap \ local 192.0.2.1 remote 192.0.2.2 ip -n ns1 route add 198.51.0.0/16 dev gretap1 taskset -c 0 ip netns exec ns1 mausezahn gretap1 \ -A 198.51.100.1 -B 198.51.0.0/16 -t udp -p 1000 -c 0 -q & taskset -c 2 ip netns exec ns1 mausezahn gretap1 \ -A 198.51.100.1 -B 198.51.0.0/16 -t udp -p 1000 -c 0 -q & sleep 10 ip netns pids ns1 | xargs kill ip netns del ns1
Cc: stable@vger.kernel.org Fixes: 67d6d681e15b ("ipv4: make exception cache less predictible") Signed-off-by: Chuang Wang nashuiliang@gmail.com Reviewed-by: Ido Schimmel idosch@nvidia.com Reviewed-by: Eric Dumazet edumazet@google.com Link: https://patch.msgid.link/20251111064328.24440-1-nashuiliang@gmail.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/route.c | 5 +++++ 1 file changed, 5 insertions(+)
--- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -605,6 +605,11 @@ static void fnhe_remove_oldest(struct fn oldest_p = fnhe_p; } } + + /* Clear oldest->fnhe_daddr to prevent this fnhe from being + * rebound with new dsts in rt_bind_exception(). + */ + oldest->fnhe_daddr = 0; fnhe_flush_routes(oldest); *oldest_p = oldest->fnhe_next; kfree_rcu(oldest, rcu);