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Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net v1] ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe Message-ID: References: <20251111064328.24440-1-nashuiliang@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251111064328.24440-1-nashuiliang@gmail.com> On Tue, Nov 11, 2025 at 02:43:24PM +0800, Chuang Wang wrote: > 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 Reviewed-by: Ido Schimmel