Re: [PATCH net] ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe

[Ido Schimmel <idosch@idosch.org>]

On Tue, Nov 04, 2025 at 11:09:08AM +0800, chuang wrote:
> From 35dbc9abd8da820007391b707bd2c1a9c99ee67d Mon Sep 17 00:00:00 2001
> From: Chuang Wang <nashuiliang@gmail.com>
> Date: Tue, 4 Nov 2025 02:52:11 +0000
> Subject: [PATCH net] ipv4: route: Prevent rt_bind_exception() from rebinding
>  stale fnhe

I'm unable to apply the patch:

$ b4 am -o - CACueBy7yNo4jq4HbiLXn0ez14w8CUTtTpPHmpSB-Ou6jhhNypA@mail.gmail.com | git am
[...]
Applying: ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe
error: corrupt patch at line 10
Patch failed at 0001 ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe

Did you use git send-email?

> 
> A race condition exists between fnhe_remove_oldest() and
> rt_bind_exception() where a fnhe that is scheduled for removal can be
> rebound to a new dst.
> 
> The issue occurs when fnhe_remove_oldest() selects an fnhe (fnheX)
> for deletion, but before it can be flushed and freed via RCU,
> CPU 0 enters rt_bind_exception() and attempts to reuse the entry.
> 
> 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]
> 
> If rt_bind_exception() successfully binds fnheX to a new dst, the
> newly bound dst will never be properly freed because fnheX will
> soon be released by the RCU callback, leading to a permanent
> reference count leak on the old dst and the device.
> 
> This issue manifests as a device reference count leak and a
> warning in dmesg when unregistering the net device:
> 
>   unregister_netdevice: waiting for ethX to become free. Usage count = N

Can you say more about how you debugged this? It seems like a very rare
race condition. I expect netdevice_tracker to only show that a dst entry
took a reference on the net device, but it wouldn't show who took a
reference on the dst entry.

> 
> Fix this race by clearing '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.

Seems safe given that both fnhe_remove_oldest() and rt_bind_exception()
access 'fnhe_daddr' while holding 'fnhe_lock'. Same trick was used in
commit ee60ad219f5c ("route: set the deleted fnhe fnhe_daddr to 0 in
ip_del_fnhe to fix a race").

> 
> Cc: stable@vger.kernel.org
> Fixes: 67d6d681e15b ("ipv4: make exception cache less predictible")
> Signed-off-by: Chuang Wang <nashuiliang@gmail.com>
> ---
>  net/ipv4/route.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 6d27d3610c1c..b549d6a57307 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -607,6 +607,11 @@ static void fnhe_remove_oldest(struct
> fnhe_hash_bucket *hash)
>                         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);
> --
>