From f723ccaff2fb72b71ae8a9fd283f0dee4d9ae7a3 Mon Sep 17 00:00:00 2001 From: Jiayuan Chen Date: Fri, 29 May 2026 23:22:18 +0800 Subject: [PATCH] ipv6: anycast: insert aca into global hash under idev->lock syzbot reported a splat [1]: a slab-use-after-free in ipv6_chk_acast_addr(), which walks the global inet6_acaddr_lst[] hash under RCU and dereferences a struct ifacaddr6 that has already been freed while still linked in the hash, so a later reader walks into a dangling node. In __ipv6_dev_ac_inc() the aca is allocated with refcount 1, then aca_get() bumps it to 2 to keep it alive across the unlocked region. It is published to idev->ac_list under idev->lock, but ipv6_add_acaddr_hash() runs after write_unlock_bh(). A concurrent teardown (ipv6_ac_destroy_dev() from addrconf_ifdown(), under RTNL) can slip into that window: CPU0 __ipv6_dev_ac_inc CPU1 ipv6_ac_destroy_dev (RTNL) ------------------------------ ------------------------------------ aca_alloc() refcnt 1 aca_get() refcnt 2 write_lock_bh(idev->lock) add aca to ac_list write_unlock_bh(idev->lock) write_lock_bh(idev->lock) pull aca off ac_list write_unlock_bh(idev->lock) ipv6_del_acaddr_hash(aca) hlist_del_init_rcu() is a no-op, aca is not in the hash yet aca_put() refcnt 2->1 ipv6_add_acaddr_hash(aca) aca now inserted into the hash aca_put() refcnt 1->0 call_rcu(aca_free_rcu) -> kfree(aca) The hash removal becomes a no-op because the insertion has not happened yet, so once CPU0 inserts and drops the last reference, the aca is freed while still linked in inet6_acaddr_lst[], and readers dereference freed memory after the slab slot is reused. This window opened once RTNL stopped serializing the join path against device teardown. Move ipv6_add_acaddr_hash() inside the idev->lock section so the ac_list and hash insertions are atomic with respect to teardown: a racing remover now either misses the aca entirely or finds it in both lists. acaddr_hash_lock is now nested under idev->lock, which is acquired in softirq context, so switch all acaddr_hash_lock sites to spin_lock_bh() to avoid the irq lock inversion reported in [2]. [1] https://syzkaller.appspot.com/bug?extid=a01df04303c131efbf3a [2] https://lore.kernel.org/netdev/6a194ef7.ba3b1513.1890b4.0000.GAE@google.com/ Reported-by: syzbot+819eb928d120d2bdad0e@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6a191f87.ce022c6e.138e56.0003.GAE@google.com/T/ Reviewed-by: Kuniyuki Iwashima Fixes: eb1ac9ff6c4a ("ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.") Signed-off-by: Jiayuan Chen Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/20260529152219.235475-1-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski --- net/ipv6/anycast.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/net/ipv6/anycast.c b/net/ipv6/anycast.c index 67a42e01dfc3..be6dac8a8566 100644 --- a/net/ipv6/anycast.c +++ b/net/ipv6/anycast.c @@ -243,16 +243,16 @@ static void ipv6_add_acaddr_hash(struct net *net, struct ifacaddr6 *aca) { unsigned int hash = inet6_acaddr_hash(net, &aca->aca_addr); - spin_lock(&acaddr_hash_lock); + spin_lock_bh(&acaddr_hash_lock); hlist_add_head_rcu(&aca->aca_addr_lst, &inet6_acaddr_lst[hash]); - spin_unlock(&acaddr_hash_lock); + spin_unlock_bh(&acaddr_hash_lock); } static void ipv6_del_acaddr_hash(struct ifacaddr6 *aca) { - spin_lock(&acaddr_hash_lock); + spin_lock_bh(&acaddr_hash_lock); hlist_del_init_rcu(&aca->aca_addr_lst); - spin_unlock(&acaddr_hash_lock); + spin_unlock_bh(&acaddr_hash_lock); } static void aca_get(struct ifacaddr6 *aca) @@ -371,10 +371,10 @@ int __ipv6_dev_ac_inc(struct inet6_dev *idev, const struct in6_addr *addr) aca->aca_next = idev->ac_list; rcu_assign_pointer(idev->ac_list, aca); - write_unlock_bh(&idev->lock); - ipv6_add_acaddr_hash(net, aca); + write_unlock_bh(&idev->lock); + ip6_ins_rt(net, f6i); addrconf_join_solict(idev->dev, &aca->aca_addr); @@ -649,8 +649,8 @@ void ipv6_anycast_cleanup(void) { int i; - spin_lock(&acaddr_hash_lock); + spin_lock_bh(&acaddr_hash_lock); for (i = 0; i < IN6_ADDR_HSIZE; i++) WARN_ON(!hlist_empty(&inet6_acaddr_lst[i])); - spin_unlock(&acaddr_hash_lock); + spin_unlock_bh(&acaddr_hash_lock); } -- 2.47.3