linux-kernel - [PATCH net] ipv4: route: Prevent rt_bind_exception() from rebinding stale fnhe

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Message-ID: <CACueBy7yNo4jq4HbiLXn0ez14w8CUTtTpPHmpSB-Ou6jhhNypA@mail.gmail.com>
Date: Tue, 4 Nov 2025 11:09:08 +0800
From: chuang <nashuiliang@...il.com>
Cc: Chuang Wang <nashuiliang@...il.com>, stable@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Networking <netdev@...r.kernel.org>, 
	open list <linux-kernel@...r.kernel.org>
Subject: [PATCH net] ipv4: route: Prevent rt_bind_exception() from rebinding
 stale fnhe

>From 35dbc9abd8da820007391b707bd2c1a9c99ee67d Mon Sep 17 00:00:00 2001
From: Chuang Wang <nashuiliang@...il.com>
Date: Tue, 4 Nov 2025 02:52:11 +0000
Subject: [PATCH net] ipv4: route: Prevent rt_bind_exception() from rebinding
 stale fnhe

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

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.

Cc: stable@...r.kernel.org
Fixes: 67d6d681e15b ("ipv4: make exception cache less predictible")
Signed-off-by: Chuang Wang <nashuiliang@...il.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);
--