From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta0.migadu.com (out-180.mta0.migadu.com [91.218.175.180]) (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 4A54039E6D4 for ; Fri, 29 May 2026 05:12:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780031558; cv=none; b=LBxko5fm4vcyHgqxljPpt0lRBwKXm0BIHfxC+sx7WY2V58bOFXC4JFJI4Vteny8ImsJrJSnaKvv5Dxv2Svu148y0Vubl6S+pEqsxdYpgo4Cu3BuAAId3pddk20Uu6Pvy6CE4RSgpGo2Shpp5xahVnxBjvBRRWiWaVxHyIkuI+ho= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780031558; c=relaxed/simple; bh=oU2AMy4t4gZMI5sHNKxb46+ca5KXphKznVnyxwi9PUg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HiIzjyoPI8Ke2Vj1ChzOKlX84snMzo1LkMha0IeNmx/O1vrq3a+wq/xy94cqVK9DPUwPBAH3AywgC52faJA5+rKUO4sbEdNLQqFkq8wPbUDfXMWHSKSqshG1cvS18yOk0uDHNKMpZ/o7DywRx1PY/wupnyBZd01c3gsbqUiUNTA= 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=uqfYwV3/; arc=none smtp.client-ip=91.218.175.180 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="uqfYwV3/" Message-ID: <2a49dc12-da42-4113-b191-41ebd00c8d90@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780031554; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=7vkXWXXjuY1xM+N6I834Ud1A/LAg/+P/Lc0BN6iguHg=; b=uqfYwV3/vJyMF20u9Jy3Er6sCdA9/xaFHnGdJ8wOjbehNHsAdvPeL5g+SC48kYTVo2F31k d4VYcHPUlm2xX8Sfbr5gFh7UJQjGSxdwS6XAqr//lmJ0As4u561lTZmyYzOdbLC4rrLbht e+uT+k9WiyAjiHZzWhQmxsL2ljPldN0= Date: Fri, 29 May 2026 13:12:17 +0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH net] ipv6: anycast: insert aca into global hash under idev->lock To: Kuniyuki Iwashima Cc: netdev@vger.kernel.org, David Ahern , Ido Schimmel , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , linux-kernel@vger.kernel.org References: <20260529032026.363856-1-jiayuan.chen@linux.dev> <2a64dc4a-ad65-4fbf-a505-f4813886629b@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Jiayuan Chen In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 5/29/26 1:10 PM, Kuniyuki Iwashima wrote: > On Thu, May 28, 2026 at 10:03 PM Jiayuan Chen wrote: >> On 5/29/26 11:41 AM, Kuniyuki Iwashima wrote: >>> On Thu, May 28, 2026 at 8:20 PM Jiayuan Chen wrote: >>>> 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 >>> Closes: and Reported-by ? >> >> >> It has already been closed automatically, so I didn't add a Closes tag. >> As for the Reported-by tag, it should be carried along with the patch. > Closes: is not related to the status in the syzbot dashboard. > > Also, the moderation is the stage before being sent upstream, > not that the report is closed, I just published it. > > So please add both tags using this link and Reported-by > tag there. > > https://lore.kernel.org/netdev/6a191f87.ce022c6e.138e56.0003.GAE@google.com/T/ Thanks, I find this report now. I will carry both tags. > >> >> I'll pay attention next time. :) >> >>>> Fixes: eb1ac9ff6c4a ("ipv6: anycast: Don't hold RTNL for IPV6_JOIN_ANYCAST.") >>>> Signed-off-by: Jiayuan Chen >>> Change itself looks good, thanks >>> >>> Reviewed-by: Kuniyuki Iwashima >>> >>> >>>> --- >>>> 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 >>>>