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

[Valentin Schneider]

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));
 		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);
 	}