From 0346ec2f080b40d95ed05b853bb9226289e75212 Mon Sep 17 00:00:00 2001 From: Kuniyuki Iwashima Date: Fri, 18 Sep 2026 08:22:05 +0000 Subject: [PATCH] ipv6: Prevent rt6_insert_exception() for dying fib6_info. Before the cited commit, fib6_nh_flush_exceptions() always set from->exception_bucket_flushed = 1 under rt6_exception_lock to prevent rt6_insert_exception() from inserting a new exception for a dying fib6_info. The flag was replaced with the FIB6_EXCEPTION_BUCKET_FLUSHED bit stored in nh->rt6i_exception_bucket. The problem is that now the bit is only set when the bucket is not NULL and fib6_nh_flush_exceptions() is called from fib6_nh_release() after fib6_ref has already reached zero. If rt6_insert_exception() is called while the target fib6_info is being removed via fib6_purge_rt(), a new exception could be created successfully because rt6_flush_exceptions() no longer sets the bit. This creates a reference cycle between the fib6_info and the exception route, leaking the fib6_info, its nexthop device, and all per-CPU routes in fib6_nh->rt6i_pcpu, which stalls netdev unregistration. [ 34.680602] unregister_netdevice: waiting for gre6 to become free. Usage count = 68 [ 44.920675] unregister_netdevice: waiting for gre6 to become free. Usage count = 68 [ 55.176582] unregister_netdevice: waiting for gre6 to become free. Usage count = 68 Let's call fib6_drop_pcpu_from() before rt6_flush_exceptions(), to set fib6_destroying before rt6_exception_lock, and check f6i->fib6_destroying in rt6_insert_exception(). Note that FIB6_EXCEPTION_BUCKET_FLUSHED logic is dead and we can clean it up in net-next. Fixes: cc5c073a693f ("ipv6: Move exception bucket to fib6_nh") Signed-off-by: Kuniyuki Iwashima Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/20260918082209.2853582-1-kuniyu@google.com Signed-off-by: Jakub Kicinski --- net/ipv6/ip6_fib.c | 2 +- net/ipv6/route.c | 5 +++++ 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c index 9ea75703b38d..9ff761962b45 100644 --- a/net/ipv6/ip6_fib.c +++ b/net/ipv6/ip6_fib.c @@ -1043,8 +1043,8 @@ static void fib6_purge_rt(struct fib6_info *rt, struct fib6_node *fn, struct fib6_table *table = rt->fib6_table; /* Flush all cached dst in exception table */ - rt6_flush_exceptions(rt); fib6_drop_pcpu_from(rt); + rt6_flush_exceptions(rt); if (rt->nh) { spin_lock(&rt->nh->lock); diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 08bd68f1b5bb..884d9ab0d50d 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -1729,6 +1729,11 @@ static int rt6_insert_exception(struct rt6_info *nrt, spin_lock_bh(&rt6_exception_lock); + if (f6i->fib6_destroying) { + err = -ENOENT; + goto out; + } + bucket = rcu_dereference_protected(nh->rt6i_exception_bucket, lockdep_is_held(&rt6_exception_lock)); if (!bucket) {