netdev - Re: [syzbot] [netfilter?] possible deadlock in nf_tables_dumpreset

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Message-ID: <42671512-7b14-57ac-7722-a5739bb59976@blackhole.kfki.hu>
Date: Tue, 23 Dec 2025 13:32:10 +0100 (CET)
From: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To: Florian Westphal <fw@...len.de>
cc: syzbot <syzbot+ff16b505ec9152e5f448@...kaller.appspotmail.com>, 
    coreteam@...filter.org, davem@...emloft.net, edumazet@...gle.com, 
    horms@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org, 
    netdev@...r.kernel.org, netfilter-devel@...r.kernel.org, pabeni@...hat.com, 
    pablo@...filter.org, phil@....cc, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [netfilter?] possible deadlock in
 nf_tables_dumpreset_obj

Hi,

On Mon, 22 Dec 2025, Florian Westphal wrote:

> syzbot <syzbot+ff16b505ec9152e5f448@...kaller.appspotmail.com> wrote:
> > syz.3.970/9330 is trying to acquire lock:
> > ffff888012d4ccd8 (&nft_net->commit_mutex){+.+.}-{4:4}, at: nf_tables_dumpreset_obj+0x6f/0xa0 net/netfilter/nf_tables_api.c:8491
> > 
> > but task is already holding lock:
> > ffff88802bce36f0 (nlk_cb_mutex-NETFILTER){+.+.}-{4:4}, at: __netlink_dump_start+0x150/0x990 net/netlink/af_netlink.c:2404
> > 
> > which lock already depends on the new lock.
> 
> I think this is a real bug:
> 
> CPU0: 'nft reset'.
> CPU1: 'ipset list' (anything in ipset doing a netlink dump op)
> CPU2: 'iptables-nft -A ... -m set ...'
> 
> ... can result in:
> 
> CPU0                    CPU1                            CPU2
> ----                    ----                            ----
> lock(nlk_cb_mutex-NETFILTER);
>                         lock(nfnl_subsys_ipset);
>                                                        lock(&nft_net->commit_mutex);
>                         lock(nlk_cb_mutex-NETFILTER);
>                                                        lock(nfnl_subsys_ipset);
> lock(&nft_net->commit_mutex);
> 
> CPU0 is waiting for CPU2 to release transaction mutex.
> CPU1 is waiting for CPU0 to release the netlink dump mutex
> CPU2 is waiting for CPU1 to release the ipset subsys mutex
> 
> This bug was added when 'nft reset' started to grab the transaction
> mutex from the dump callback path in nf_tables.
> 
> Not yet sure how to avoid it.
> Maybe we could get rid of 'lock(nfnl_subsys_ipset);'
> from the xt_set module call paths.

I don't know how calling it could be avoided: userspace commands (ipset + 
iptables checkentry using ipset match/target) are serialized by 
nfnl_subsys_ipset.

Is there a way to force acquiring nlk_cb_mutex-NETFILTER first and then 
nfnl_subsys_ipset when doing a netlink dump?

> Or add a new lock (spinlock?) to protect the 'reset' object info
> instead of using the transaction mutex.
> 
> I haven't given it much thought yet and will likely not
> investigate further for the next two weeks.

Best regards,
Jozsef
-- 
E-mail : kadlec@...filter.org, kadlec@...ckhole.kfki.hu, kadlecsik.jozsef@...ner.hu
Address: Wigner Research Centre for Physics
          H-1525 Budapest 114, POB. 49, Hungary