netdev - Re: [PATCH] netfilter: nf_tables: always synchronize with readers before releasing tables

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Message-ID: <20230228105929.GB6107@breakpoint.cc>
Date:   Tue, 28 Feb 2023 11:59:29 +0100
From:   Florian Westphal <fw@...len.de>
To:     Alexander Atanasov <alexander.atanasov@...tuozzo.com>
Cc:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH] netfilter: nf_tables: always synchronize with readers
 before releasing tables

Alexander Atanasov <alexander.atanasov@...tuozzo.com> wrote:
> On 28.02.23 1:31, Florian Westphal wrote:
> > Alexander Atanasov <alexander.atanasov@...tuozzo.com> wrote:
> > > As i said i am still trying to figure out the basechain place,
> > > where is that synchronize_rcu() call done?
> > 
> > cleanup_net() in net/core/net_namespace.c.
> > 
> > pre_exit handlers run, then synchronize_rcu, then the
> > normal exit handlers, then exit_batch.
> 
> It prevents anyone new to find the namespace but it does not guard against
> the ones that have already found it.

The netns is being dismantled, how can there be any process left?

> What stops them to enter a rcu_read_lock() section after the synchronize
> call in cleanup_net() is done and race with the exit handler?

There should be no task in the first place.

> synchronize_rcu() must be called with the commit_mutex held to be safe
> against lock less readers using data protected with commit_mutext.

Sorry, I do not understand this bug nor the fix.