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Date: Tue, 26 Aug 2025 16:18:43 +0200
From: Florian Westphal <fw@strlen.de>
To: Zhang Tengfei <zhtfdev@gmail.com>
Cc: Simon Horman <horms@verge.net.au>, Julian Anastasov <ja@ssi.bg>,
	lvs-devel@vger.kernel.org, netfilter-devel@vger.kernel.org,
	Pablo Neira Ayuso <pablo@netfilter.org>,
	Jozsef Kadlecsik <kadlec@netfilter.org>,
	"David S . Miller" <davem@davemloft.net>,
	David Ahern <dsahern@kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	coreteam@netfilter.org,
	syzbot+1651b5234028c294c339@syzkaller.appspotmail.com
Subject: Re: [PATCH] net/netfilter/ipvs: Fix data-race in ip_vs_add_service /
 ip_vs_out_hook
Message-ID: <aK3CQ1yNTtP4NgP4@strlen.de>
References: <20250826133104.212975-1-zhtfdev@gmail.com>
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In-Reply-To: <20250826133104.212975-1-zhtfdev@gmail.com>

Zhang Tengfei <zhtfdev@gmail.com> wrote:
> A data-race was detected by KCSAN between ip_vs_add_service() which
> acts as a writer, and ip_vs_out_hook() which acts as a reader. This
> can lead to unpredictable behavior and crashes.

Really?  How can this cause a crash?

> The race occurs on the `enable` flag within the `netns_ipvs`
> struct. This flag was being written in the configuration path without
> any protection, while concurrently being read in the packet processing
> path. This lack of synchronization means a reader on one CPU could see a
> partially initialized service, leading to incorrect behavior.
> 
> To fix this, convert the `enable` flag from a plain integer to an
> atomic_t. This ensures that all reads and writes to the flag are atomic.
> More importantly, using atomic_set() and atomic_read() provides the
> necessary memory barriers to guarantee that changes to other fields of
> the service are visible to the reader CPU before the service is marked
> as enabled.

> -	int			enable;		/* enable like nf_hooks do */
> +	atomic_t	enable;		/* enable like nf_hooks do */

Julian, Simon, I will defer to your judgment but I dislike this,
because I see no reason for atomic_t.  To me is seems better to use
READ/WRITE_ONCE as ->enable is only ever set but not modified
(increment for instance).