From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta0.migadu.com (out-183.mta0.migadu.com [91.218.175.183]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C95952E739C for ; Fri, 29 May 2026 03:20:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780024845; cv=none; b=JEVCfeVo74pdK0ky8kppfMPmJAIyeUc8mTero5HTmqqjkXIQPiPY/JIWEFGrkDoWQ5pBHDuPE2Kty/dY/JBuKIWKn+Whfe8nYywGCbd1F2+GVmVp9QMUXBV/zf9rFY12W3abqTm7ifb2m33l72s3juVNmXo9tLNuzVNvyb3c114= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780024845; c=relaxed/simple; bh=RB3qOs2Ohx6Lg4He1Ag2kH5Y/QXyI/UlS5MiaZD55JI=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=OYFcMfnTm6z/rmMG+KboztTJ6bHvw4bACtXdKsogEnVDBP7QyaGxPLhaRAyhRc8rg6L6dDKSXIrH2wXks4lBvY5/gkMBRQi27glaaTkgXI8X6GGhVYBto/M/Gr5QfDkHjZTS6Vmq4JMMaBM1J4HX2VPRUjjqtcTmIKbOI1BeKjU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ZmWvFfw1; arc=none smtp.client-ip=91.218.175.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ZmWvFfw1" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780024840; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=Jx7dJ3LrSYHkDmTM3C5e8iRNjYq89+NASBYIOXgzJoU=; b=ZmWvFfw1S5HaZrtgg4Q6iNLHX/ASXIzovM5ogMfXVEwEFIzkvg2vd30VMFbeGb/HWn9wb3 foJFayJAzLRjC+TX0EhDaAE1dVjdStrcLhzH8heSNrZzdGLnK1gnEwBnhIqARTBBa03+Kz ZSwHj/KilAjlDhrzgRH3d4xid4C1XgY= From: Jiayuan Chen To: netdev@vger.kernel.org Cc: Jiayuan Chen , David Ahern , Ido Schimmel , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Kuniyuki Iwashima , linux-kernel@vger.kernel.org Subject: [PATCH net] ipv6: anycast: insert aca into global hash under idev->lock Date: Fri, 29 May 2026 11:20:25 +0800 Message-ID: <20260529032026.363856-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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. [1] https://syzkaller.appspot.com/bug?extid=a01df04303c131efbf3a Fixes: eb1ac9ff6c4a ("ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.") Signed-off-by: Jiayuan Chen --- net/ipv6/anycast.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/net/ipv6/anycast.c b/net/ipv6/anycast.c index 67a42e01dfc3..cd8c02a1ad4c 100644 --- a/net/ipv6/anycast.c +++ b/net/ipv6/anycast.c @@ -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); -- 2.43.0