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

[Eric Dumazet]

On Tue, Feb 20, 2024 at 6:38 PM Valentin Schneider <vschneid@redhatcom> wrote:
>
> On 19/02/24 15:42, Eric Dumazet wrote:
> > On Mon, Feb 19, 2024 at 10:57 AM Valentin Schneider <vschneid@xxxxxxxxxx> 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()
>
> Hm so that would indeed prevent a concurrent inet_twsk_schedule() from
> re-arming the timer, but in case the calls are interleaved like so:
>
>                              tcp_time_wait()
>                                inet_twsk_hashdance()
>   inet_twsk_deschedule_put()
>     timer_shutdown_sync()
>                                inet_twsk_schedule()
>
> inet_twsk_hashdance() will have left the refcounts including a count for
> the timer, and we won't arm the timer to clear it via the timer callback
> (via inet_twsk_kill()) - the patch in its current form relies on the timer
> being re-armed for that.
>
> I don't know if there's a cleaner way to do this, but we could catch that
> in inet_twsk_schedule() and issue the inet_twsk_kill() directly if we can
> tell the timer has been shutdown:
> ---
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index 61a053fbd329c..c272da5046bb4 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -227,7 +227,7 @@ void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
>          * have already gone through {tcp,dcpp}_time_wait(), and we can safely
>          * call inet_twsk_kill().
>          */
> -       if (del_timer_sync(&tw->tw_timer))
> +       if (timer_shutdown_sync(&tw->tw_timer))
>                 inet_twsk_kill(tw);
>         inet_twsk_put(tw);
>  }
> @@ -267,6 +267,10 @@ void __inet_twsk_schedule(struct inet_timewait_sock *tw, int timeo, bool rearm)
>                                                      LINUX_MIB_TIMEWAITED);
>                 BUG_ON(mod_timer(&tw->tw_timer, jiffies + timeo));

Would not a shutdown timer return a wrong mod_timer() value here ?

Instead of BUG_ON(), simply release the refcount ?


>                 refcount_inc(&tw->tw_dr->tw_refcount);
> +
> +               /* XXX timer got shutdown */
> +               if (!tw->tw_timer.function)
> +                       inet_twsk_kill(tw);
>         } else {
>                 mod_timer_pending(&tw->tw_timer, jiffies + timeo);
>         }
>