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Date: Mon, 16 Mar 2026 11:53:56 +0100
From: Florian Westphal <fw@strlen.de>
To: Hyunwoo Kim <imv4bel@gmail.com>
Cc: pablo@netfilter.org, phil@nwl.cc, davem@davemloft.net,
	edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
	horms@kernel.org, netfilter-devel@vger.kernel.org,
	coreteam@netfilter.org, netdev@vger.kernel.org
Subject: Re: [PATCH net] netfilter: nf_flow_table_offload: fix heap overflow
 in flow_action_entry_next()
Message-ID: <abfhRFfZ1LOgWEsf@strlen.de>
References: <aaxe-uH2Qr6qM4E9@v4bel>
 <aax2yZtJce0d19gd@strlen.de>
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In-Reply-To: <aax2yZtJce0d19gd@strlen.de>

Florian Westphal <fw@strlen.de> wrote:
> Hyunwoo Kim <imv4bel@gmail.com> wrote:
> > flow_action_entry_next() increments num_entries and returns a pointer
> > into the flow_action_entry array without any bounds checking.  The array
> > is allocated with a fixed size of NF_FLOW_RULE_ACTION_MAX (16) entries,
> > but certain combinations of IPv6 + SNAT + DNAT + double VLAN (QinQ)
> > require 17 or more entries, causing a slab-out-of-bounds write in the
> > kmalloc-4k slab.
> > 
> > The maximum possible entry count is:
> >   tunnel(2) + eth(4) + VLAN(4) + IPv6_NAT(10) + redirect(1) = 21
> > 
> > Increase NF_FLOW_RULE_ACTION_MAX to 24 (with headroom) to cover the
> >  
> > -#define NF_FLOW_RULE_ACTION_MAX	16
> > +#define NF_FLOW_RULE_ACTION_MAX	24
> 
> This fix looks rather fragile.
> 
> What guarantees that this stays right-sized?
> 
> Can you add a BUILD_BUG_ON or if needed, run-time check?

Ping.  I'm not even sure if there is a bug to begin with, see Pablos
response.  How did you conclude there is a missing bounds check and that
this increase is the best fix?

Normally there should be a check that prevents such a configuration.
If thats missing, please add one instead of increasing this define.