netdev - Re: [PATCH v3 1/1] tcp/dcpp: Un-pin tw

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Message-ID: <CANn89i+3-zgAkWukFavu1wgf1XG+K9U4BhJWw7H+QKwsfYL4WA@mail.gmail.com>
Date: Mon, 19 Feb 2024 15:42:37 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: dccp@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-rt-users@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, mleitner@...hat.com, 
	David Ahern <dsahern@...nel.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Tomas Glozar <tglozar@...hat.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/1] tcp/dcpp: Un-pin tw_timer

On Mon, Feb 19, 2024 at 10:57 AM Valentin Schneider <vschneid@...hat.com> wrote:
>
> The TCP timewait timer is proving to be problematic for setups where scheduler
> CPU isolation is achieved at runtime via cpusets (as opposed to statically via
> isolcpus=domains).
>

...

>  void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
>  {
> +       /* This can race with tcp_time_wait() and dccp_time_wait(), as the timer
> +        * is armed /after/ adding it to the hashtables.
> +        *
> +        * If this is interleaved between inet_twsk_hashdance() and inet_twsk_put(),
> +        * then this is a no-op: the timer will still end up armed.
> +        *
> +        * Conversely, if this successfully deletes the timer, then we know we
> +        * have already gone through {tcp,dcpp}_time_wait(), and we can safely
> +        * call inet_twsk_kill().
> +        */
>         if (del_timer_sync(&tw->tw_timer))
>                 inet_twsk_kill(tw);

I really do not think adding a comment will prevent races at netns dismantle.

We need to make sure the timer is not rearmed, we want to be absolutely
sure that after inet_twsk_purge() we have no pending timewait sockets,
otherwise UAF will happen on the netns structures.

I _think_ that you need timer_shutdown_sync() here, instead of del_timer_sync()