Re: [syzbot] [net?] WARNING: refcount bug in inet_twsk_kill

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Florian Westphal <fw@strlen.de>
Date: Mon, 12 Aug 2024 16:01:04 +0200
> Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > From: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Date: Sun, 11 Aug 2024 16:00:29 -0700
> > > From: Florian Westphal <fw@strlen.de>
> > > Date: Sun, 11 Aug 2024 18:28:50 +0200
> > > > Florian Westphal <fw@strlen.de> wrote:
> > > > > https://syzkaller.appspot.com/x/log.txt?x=117f3182980000
> > > > > 
> > > > > ... shows at two cores racing:
> > > > > 
> > > > > [ 3127.234402][ T1396] CPU: 3 PID: 1396 Comm: syz-executor.3 Not
> > > > > and
> > > > > [ 3127.257864][   T13] CPU: 1 PID: 13 Comm: kworker/u32:1 Not tainted 6.9.0-syzkalle (netns cleanup net).
> > > > > 
> > > > > 
> > > > > first splat backtrace shows invocation of tcp_sk_exit_batch() from
> > > > > netns error unwinding code.
> > > > > 
> > > > > Second one lacks backtrace, but its also in tcp_sk_exit_batch(),
> > > > 
> > > > ... which doesn't work.  Does this look like a plausible
> > > > theory/exlanation?
> > > 
> > > Yes!  The problem here is that inet_twsk_purge() operates on twsk
> > > not in net_exit_list, but I think such a check is overkill and we
> > > can work around it in another way.
> 
> I'm not so sure.  Once 'other' inet_twsk_purge() found the twsk and
> called inet_twsk_kill(), 'our' task has to wait for that to complete.
> 
> We need to force proper ordering so that all twsk found
> 
> static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
> {
>         struct net *net;
> 
> /*HERE*/tcp_twsk_purge(net_exit_list);
> 
>         list_for_each_entry(net, net_exit_list, exit_list) {
>                 inet_pernet_hashinfo_free(net->ipv4.tcp_death_row.hashinfo);
> 
> .... have gone through inet_twsk_kill() so tw_refcount managed to
> drop back to 1 before doing
>                 WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
> .
> 
> > > We need to sync two inet_twsk_kill(), so maybe give up one
> > > if twsk is not hashed ?
> 
> Not sure, afaiu only one thread enters inet_twsk_kill()
> (the one that manages to deactivate the timer).

Ah, you're right.

> 
> > > ---8<---
> > > diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> > > index 337390ba85b4..51889567274b 100644
> > > --- a/net/ipv4/inet_timewait_sock.c
> > > +++ b/net/ipv4/inet_timewait_sock.c
> > > @@ -52,7 +52,10 @@ static void inet_twsk_kill(struct inet_timewait_sock *tw)
> > >  	struct inet_bind_hashbucket *bhead, *bhead2;
> > >  
> > >  	spin_lock(lock);
> > > -	sk_nulls_del_node_init_rcu((struct sock *)tw);
> > > +	if (!sk_nulls_del_node_init_rcu((struct sock *)tw)) {
> > > +		spin_unlock(lock);
> > > +		return false;
> > 
> > forgot to remove false, just return :)
> 
> I don't see how this helps, we need to wait until 'stolen' twsk
> has gone through inet_twsk_kill() and decremented tw_refcount.
> Obviously It would be a bit simpler if we had a reliable reproducer :-)
> 
> Possible solutions I came up with so far:
> 
> 1) revert b099ce2602d8 ("net: Batch inet_twsk_purge").
> 
> This commit replaced a net_eq(twsk_net(tw) ... with a check for
> dead netns (ns.count == 0),
> 
> Downside: We need to remove the purged_once trick that calls
> inet_twsk_purge(&tcp_hashinfo) only once per exiting batch in
> tcp_twsk_purge() as well.
> 
> As per b099ce2602d8 changelog, likely increases netns dismantle times.

Yes, so I think we should avoid this way although user could mitigate
it with per-net ehash.


> 
> Upside: simpler code, so this is my preferred solution.
> 
> No concurrent runoff anymore, by time tcp_twsk_purge() returns it has
> called refcount_dec(->tw_refcount) for every twsk in the exiting netns
> list, without other task stealing twsks owned by exiting netns.
> 
> Solution 2: change tcp_sk_exit_batch like this:
> 
>    tcp_twsk_purge(net_exit_list);
> 
> +  list_for_each_entry(net, net_exit_list, exit_list) {
> +      while (refcount_read(&net->ipv4.tcp_death_row.tw_refcount) > 1)
> +         schedule();
> +
> +  }
> 
>     list_for_each_entry(net, net_exit_list, exit_list) {
>        inet_pernet_hashinfo_free(net->ipv4.tcp_death_row.hashinfo);
>        WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
> 
> This synchronizes two concurrent tcp_sk_exit_batch() calls via
> existing refcount; if netns setup error unwinding ran off with one of
> 'our' twsk, it will wait until other task has completed the refcount decrement.
> 
> I don't expect it to increase netns dismantle times, else we'd have seen
> the WARN_ON_ONCE splat frequently.
> 
> Solution 3:

I prefer this because in all of my local reports I see two splats from
exit_batch calls.

If the same issue still happens with solution 3, we'll see the same
splats with mostly no risk.  But solution 2 will give us hung task,
possibly in cleanup_net, which is worse.

Then, we can consider other possibilities explained in solution 4
or like that kernel twsk is put into ehash of dying netns.

Another option would be add reftracker for tcp_death_row.tw_refcount
to detect the root cause ?


> 
> Similar to 2), but via mutex_lock/unlock pair:
> 
> static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
> {
>         struct net *net;
> 
> 	mutex_lock(&tcp_exit_batch_mutex);
> 
>         tcp_twsk_purge(net_exit_list);
> 
>         list_for_each_entry(net, net_exit_list, exit_list) {
>                 inet_pernet_hashinfo_free(net->ipv4.tcp_death_row.hashinfo);
>                 WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
>                 tcp_fastopen_ctx_destroy(net);
>         }
> 	mutex_unlock(&tcp_exit_batch_mutex);
> }
> 
> Solution 4:
> 
> I have doubts wrt. tcp_twsk_purge() interaction with tw timer firing at
> the 'wrong' time.  This is independent "problem", I might be
> imagining things here.
> 
> Consider:
> 313 void inet_twsk_purge(struct inet_hashinfo *hashinfo)
> 314 {
> [..]
> 321         for (slot = 0; slot <= ehash_mask; slot++, head++) {
> 
> tw sk timer fires on other cpu, inet_twsk_kill() does:
> 
> 56         spin_lock(lock);
> 57         sk_nulls_del_node_init_rcu((struct sock *)tw);
> 58         spin_unlock(lock);
> 
> ... then other cpu gets preempted.
> inet_twsk_purge() resumes and hits empty chain head:
> 
> 322                 if (hlist_nulls_empty(&head->chain))
> 323                         continue;
> 
> so we don't(can't) wait for the timer to run to completion.
> 
> If this sounds plausible to you, this gives us solution 4:
> 
> Restart inet_twsk_purge() loop until tw_dr->tw_refcount) has
> dropped down to 1.
> 
> Alternatively (still assuming the above race is real), sk_nulls_del_node_init_rcu
> needs to be moved down:
> 
>  48 static void inet_twsk_kill(struct inet_timewait_sock *tw)
> ...
>  58     /* Disassociate with bind bucket. */
> ...
>  68     spin_unlock(&bhead->lock);
> 
>  70     refcount_dec(&tw->tw_dr->tw_refcount);
> 
>  +      spin_lock(lock);
>  +      sk_nulls_del_node_init_rcu((struct sock *)tw);
>  +      spin_unlock(lock);
> 71      inet_twsk_put(tw);
> 72 }
> 
> ... so concurrent purge() call will find us
> the node list (and then wait on timer_shutdown_sync())
> until other cpu executing the timer is done.
> 
> If twsk was unlinked from table already before
> inet_twsk_purge() had chance to find it sk, then in worst
> case call to tcp_twsk_destructor() is missing, but I don't
> see any ordering requirements that need us to wait for this.