Message-ID: <CANn89i+Zp=_F0tHTgQKuk1+5MV8MU+N=JV35vSTwKLFmi_5dNg@mail.gmail.com>
Date: Mon, 3 Jun 2024 12:30:35 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, mleitner@...hat.com, 
	juri.lelli@...hat.com, vschneid@...hat.com, tglozar@...hat.com, 
	dsahern@...nel.org, bigeasy@...utronix.de, tglx@...utronix.de
Subject: Re: [PATCH net-next v6 1/3] net: tcp/dcpp: prepare for tw_timer un-pinning

On Mon, Jun 3, 2024 at 11:37 AM Florian Westphal <fw@...len.de> wrote:
>
> From: Valentin Schneider <vschneid@...hat.com>
>
> 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).
>
> What happens there is a CPU goes through tcp_time_wait(), arming the
> time_wait timer, then gets isolated. TCP_TIMEWAIT_LEN later, the timer
> fires, causing interference for the now-isolated CPU. This is conceptually
> similar to the issue described in commit e02b93124855 ("workqueue: Unbind
> kworkers before sending them to exit()")
>
> Move inet_twsk_schedule() to within inet_twsk_hashdance(), with the ehash
> lock held. Expand the lock's critical section from inet_twsk_kill() to
> inet_twsk_deschedule_put(), serializing the scheduling vs descheduling of
> the timer. IOW, this prevents the following race:
>
>                              tcp_time_wait()
>                                inet_twsk_hashdance()
>   inet_twsk_deschedule_put()
>     del_timer_sync()
>                                inet_twsk_schedule()
>
> Thanks to Paolo Abeni for suggesting to leverage the ehash lock.
>
> This also restores a comment from commit ec94c2696f0b ("tcp/dccp: avoid
> one atomic operation for timewait hashdance") as inet_twsk_hashdance() had
> a "Step 1" and "Step 3" comment, but the "Step 2" had gone missing.
>
> inet_twsk_deschedule_put() now acquires the ehash spinlock to synchronize
> vs. reschedule and timer firing: timer_del_sync() is replaced with
> timer_shutdown().
>
> This means that tw_timer may still be running on another CPU.  However, as
> the timer owns a reference on tw sk that is only put at the end this
> should be fine.
>
> To ease possible regression search, actual un-pin is done in next patch.
>
> Link: https://lore.kernel.org/all/ZPhpfMjSiHVjQkTk@localhost.localdomain/
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>  include/net/inet_timewait_sock.h |  6 ++-
>  net/dccp/minisocks.c             |  3 +-
>  net/ipv4/inet_timewait_sock.c    | 74 ++++++++++++++++++++++++++------
>  net/ipv4/tcp_minisocks.c         |  3 +-
>  4 files changed, 68 insertions(+), 18 deletions(-)
>
> diff --git a/include/net/inet_timewait_sock.h b/include/net/inet_timewait_sock.h
> index 2a536eea9424..5b43d220243d 100644
> --- a/include/net/inet_timewait_sock.h
> +++ b/include/net/inet_timewait_sock.h
> @@ -93,8 +93,10 @@ struct inet_timewait_sock *inet_twsk_alloc(const struct sock *sk,
>                                            struct inet_timewait_death_row *dr,
>                                            const int state);
>
> -void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
> -                        struct inet_hashinfo *hashinfo);
> +void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
> +                                 struct sock *sk,
> +                                 struct inet_hashinfo *hashinfo,
> +                                 int timeo);
>
>  void __inet_twsk_schedule(struct inet_timewait_sock *tw, int timeo,
>                           bool rearm);
> diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
> index 251a57cf5822..deb52d7d31b4 100644
> --- a/net/dccp/minisocks.c
> +++ b/net/dccp/minisocks.c
> @@ -59,11 +59,10 @@ void dccp_time_wait(struct sock *sk, int state, int timeo)
>                  * we complete the initialization.
>                  */
>                 local_bh_disable();
> -               inet_twsk_schedule(tw, timeo);
>                 /* Linkage updates.
>                  * Note that access to tw after this point is illegal.
>                  */
> -               inet_twsk_hashdance(tw, sk, &dccp_hashinfo);
> +               inet_twsk_hashdance_schedule(tw, sk, &dccp_hashinfo, timeo);
>                 local_bh_enable();
>         } else {
>                 /* Sorry, if we're out of memory, just CLOSE this
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index e28075f0006e..2b1d836df64e 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -44,14 +44,14 @@ void inet_twsk_bind_unhash(struct inet_timewait_sock *tw,
>         __sock_put((struct sock *)tw);
>  }
>
> -/* Must be called with locally disabled BHs. */
> -static void inet_twsk_kill(struct inet_timewait_sock *tw)
> +static void __inet_twsk_kill(struct inet_timewait_sock *tw, spinlock_t *lock)
> +__releases(lock)
>  {
>         struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
> -       spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
>         struct inet_bind_hashbucket *bhead, *bhead2;
>
> -       spin_lock(lock);
> +       lockdep_assert_held(lock);
> +
>         sk_nulls_del_node_init_rcu((struct sock *)tw);
>         spin_unlock(lock);
>
> @@ -71,6 +71,16 @@ static void inet_twsk_kill(struct inet_timewait_sock *tw)
>         inet_twsk_put(tw);
>  }
>
> +/* Must be called with locally disabled BHs. */
> +static void inet_twsk_kill(struct inet_timewait_sock *tw)
> +{
> +       struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
> +       spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
> +
> +       spin_lock(lock);
> +       __inet_twsk_kill(tw, lock);
> +}
> +
>  void inet_twsk_free(struct inet_timewait_sock *tw)
>  {
>         struct module *owner = tw->tw_prot->owner;
> @@ -96,9 +106,13 @@ static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
>   * Enter the time wait state. This is called with locally disabled BH.
>   * Essentially we whip up a timewait bucket, copy the relevant info into it
>   * from the SK, and mess with hash chains and list linkage.
> + *
> + * The caller must not access @tw anymore after this function returns.
>   */
> -void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
> -                          struct inet_hashinfo *hashinfo)
> +void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
> +                                 struct sock *sk,
> +                                 struct inet_hashinfo *hashinfo,
> +                                 int timeo)
>  {
>         const struct inet_sock *inet = inet_sk(sk);
>         const struct inet_connection_sock *icsk = inet_csk(sk);
> @@ -129,26 +143,33 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
>
>         spin_lock(lock);
>
> +       /* Step 2: Hash TW into tcp ehash chain */
>         inet_twsk_add_node_rcu(tw, &ehead->chain);
>
>         /* Step 3: Remove SK from hash chain */
>         if (__sk_nulls_del_node_init_rcu(sk))
>                 sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
>
> -       spin_unlock(lock);
>
> +       /* Ensure above writes are committed into memory before updating the
> +        * refcount.
> +        * Provides ordering vs later refcount_inc().
> +        */
> +       smp_wmb();
>         /* tw_refcnt is set to 3 because we have :
>          * - one reference for bhash chain.
>          * - one reference for ehash chain.
>          * - one reference for timer.
> -        * We can use atomic_set() because prior spin_lock()/spin_unlock()
> -        * committed into memory all tw fields.
>          * Also note that after this point, we lost our implicit reference
>          * so we are not allowed to use tw anymore.
>          */
>         refcount_set(&tw->tw_refcnt, 3);
> +
> +       inet_twsk_schedule(tw, timeo);
> +
> +       spin_unlock(lock);
>  }
> -EXPORT_SYMBOL_GPL(inet_twsk_hashdance);
> +EXPORT_SYMBOL_GPL(inet_twsk_hashdance_schedule);
>
>  static void tw_timer_handler(struct timer_list *t)
>  {
> @@ -217,8 +238,37 @@ EXPORT_SYMBOL_GPL(inet_twsk_alloc);
>   */
>  void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
>  {
> -       if (del_timer_sync(&tw->tw_timer))
> -               inet_twsk_kill(tw);
> +       struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
> +       spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
> +
> +       /* inet_twsk_purge() walks over all sockets, including tw ones,
> +        * and removes them via inet_twsk_deschedule_put() after a
> +        * refcount_inc_not_zero().
> +        *
> +        * inet_twsk_hashdance_schedule() must (re)init the refcount before
> +        * arming the timer, i.e. inet_twsk_purge can obtain a reference to
> +        * a twsk that did not yet schedule the timer.
> +        *
> +        * The ehash lock synchronizes these two:
> +        * After acquiring the lock, the timer is always scheduled (else
> +        * timer_shutdown returns false).
> +        *
> +        * With plain timer_shutdown_sync() and without grabbing the ehash
> +        * lock, we can get:
> +        * 1) cpu x sets twsk refcount to 3
> +        * 2) cpu y bumps refcount to 4
> +        * 3) cpu y calls inet_twsk_deschedule_put() and shuts timer down
> +        * 4) cpu x tries to start timer, but mod_timer is a noop post-shutdown
> +        * -> timer refcount is never decremented.
> +        */
> +       spin_lock(lock);
> +       if (timer_shutdown(&tw->tw_timer)) {
> +               /* releases @lock */
> +               __inet_twsk_kill(tw, lock);
> +       } else {

If we do not have a sync variant here, I think that inet_twsk_purge()
could return while ongoing timers are alive.

tcp_sk_exit_batch() would then possibly hit :

WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));

The alive timer are releasing tw->tw_dr->tw_refcount at the end of
inet_twsk_kill()

Presumably, adding some udelay(200) in inet_twsk_kill() could help a
syzkaller instance to trigger a bug.

> +               spin_unlock(lock);
> +       }
> +
>         inet_twsk_put(tw);
>  }
>  EXPORT_SYMBOL(inet_twsk_deschedule_put);
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index 538c06f95918..47de6f3efc85 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -344,11 +344,10 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
>                  * we complete the initialization.
>                  */
>                 local_bh_disable();
> -               inet_twsk_schedule(tw, timeo);
>                 /* Linkage updates.
>                  * Note that access to tw after this point is illegal.
>                  */
> -               inet_twsk_hashdance(tw, sk, net->ipv4.tcp_death_row.hashinfo);
> +               inet_twsk_hashdance_schedule(tw, sk, net->ipv4.tcp_death_row.hashinfo, timeo);
>                 local_bh_enable();
>         } else {
>                 /* Sorry, if we're out of memory, just CLOSE this
> --
> 2.44.2
>

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