Subject: Re: [PATCH net 1/5] net/smc: fix dangling sock under state SMC_APPFINCLOSEWAIT

From: Dust Li <[email-masked]>
Date: Fri, 13 Oct 2023 13:32:14 +0800

On Thu, Oct 12, 2023 at 01:51:54PM +0200, Wenjia Zhang wrote:
>
>
>On 12.10.23 04:37, D. Wythe wrote:
>> 
>> 
>> On 10/12/23 4:31 AM, Wenjia Zhang wrote:
>> > 
>> > 
>> > On 11.10.23 09:33, D. Wythe wrote:
>> > > From: "D. Wythe" <[email-masked]>
>> > > 
>> > > Considering scenario:
>> > > 
>> > >                 smc_cdc_rx_handler_rwwi
>> > > __smc_release
>> > >                 sock_set_flag
>> > > smc_close_active()
>> > > sock_set_flag
>> > > 
>> > > __set_bit(DEAD)            __set_bit(DONE)
>> > > 
>> > > Dues to __set_bit is not atomic, the DEAD or DONE might be lost.
>> > > if the DEAD flag lost, the state SMC_CLOSED  will be never be reached
>> > > in smc_close_passive_work:
>> > > 
>> > > if (sock_flag(sk, SOCK_DEAD) &&
>> > >     smc_close_sent_any_close(conn)) {
>> > >     sk->sk_state = SMC_CLOSED;
>> > > } else {
>> > >     /* just shutdown, but not yet closed locally */
>> > >     sk->sk_state = SMC_APPFINCLOSEWAIT;
>> > > }
>> > > 
>> > > Replace sock_set_flags or __set_bit to set_bit will fix this problem.
>> > > Since set_bit is atomic.
>> > > 
>> > I didn't really understand the scenario. What is
>> > smc_cdc_rx_handler_rwwi()? What does it do? Don't it get the lock
>> > during the runtime?
>> > 
>> 
>> Hi Wenjia,
>> 
>> Sorry for that, It is not smc_cdc_rx_handler_rwwi() but
>> smc_cdc_rx_handler();
>> 
>> Following is a more specific description of the issues
>> 
>> 
>> lock_sock()
>> __smc_release
>> 
>> smc_cdc_rx_handler()
>> smc_cdc_msg_recv()
>> bh_lock_sock()
>> smc_cdc_msg_recv_action()
>> sock_set_flag(DONE) sock_set_flag(DEAD)
>> __set_bit __set_bit
>> bh_unlock_sock()
>> release_sock()
>> 
>> 
>> 
>> Note : |bh_lock_sock|and |lock_sock|are not mutually exclusive. They are
>> actually used for different purposes and contexts.
>> 
>> 
>ok, that's true that |bh_lock_sock|and |lock_sock|are not really mutually
>exclusive. However, since bh_lock_sock() is used, this scenario you described
>above should not happen, because that gets the sk_lock.slock. Following this
>scenarios, IMO, only the following situation can happen.
>
>lock_sock()
>__smc_release
>
>smc_cdc_rx_handler()
>smc_cdc_msg_recv()
>bh_lock_sock()
>smc_cdc_msg_recv_action()
>sock_set_flag(DONE)
>bh_unlock_sock()
>sock_set_flag(DEAD)
>release_sock()

Hi wenjia,

I think I know what D. Wythe means now, and I think he is right on this.

IIUC, in process context, lock_sock() won't respect bh_lock_sock() if it
acquires the lock before bh_lock_sock(). This is how the sock lock works.

    PROCESS CONTEXT                                 INTERRUPT CONTEXT
------------------------------------------------------------------------
lock_sock()
    spin_lock_bh(&sk->sk_lock.slock);
    ...
    sk->sk_lock.owned = 1;
    // here the spinlock is released
    spin_unlock_bh(&sk->sk_lock.slock);
__smc_release()
                                                   bh_lock_sock(&smc->sk);
                                                   smc_cdc_msg_recv_action(smc, cdc);
                                                       sock_set_flag(&smc->sk, SOCK_DONE);
                                                   bh_unlock_sock(&smc->sk);

    sock_set_flag(DEAD)  <-- Can be before or after sock_set_flag(DONE)
release_sock()

The bh_lock_sock() only spins on sk->sk_lock.slock, which is already released
after lock_sock() return. Therefor, there is actually no lock between
the code after lock_sock() and before release_sock() with bh_lock_sock()...bh_unlock_sock().
Thus, sock_set_flag(DEAD) won't respect bh_lock_sock() at all, and might be
before or after sock_set_flag(DONE).

Actually, in TCP, the interrupt context will check sock_owned_by_user().
If it returns true, the softirq just defer the process to backlog, and process
that in release_sock(). Which avoid the race between softirq and process
when visiting the 'struct sock'.

tcp_v4_rcv()
         bh_lock_sock_nested(sk);
         tcp_segs_in(tcp_sk(sk), skb);
         ret = 0;
         if (!sock_owned_by_user(sk)) {
                 ret = tcp_v4_do_rcv(sk, skb);
         } else {
                 if (tcp_add_backlog(sk, skb, &drop_reason))
                         goto discard_and_relse;
         }
         bh_unlock_sock(sk);

But in SMC we don't have a backlog, that means fields in 'struct sock'
might all have race, and this sock_set_flag() is just one of the cases.

Best regards,
Dust

>
>> 
>> > > Signed-off-by: D. Wythe <[email-masked]>
>> > > ---
>> > >   net/smc/af_smc.c    | 4 ++--
>> > >   net/smc/smc.h       | 5 +++++
>> > >   net/smc/smc_cdc.c   | 2 +-
>> > >   net/smc/smc_close.c | 2 +-
>> > >   4 files changed, 9 insertions(+), 4 deletions(-)
>> > > 
>> > > diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
>> > > index bacdd97..5ad2a9f 100644
>> > > --- a/net/smc/af_smc.c
>> > > +++ b/net/smc/af_smc.c
>> > > @@ -275,7 +275,7 @@ static int __smc_release(struct smc_sock *smc)
>> > >         if (!smc->use_fallback) {
>> > >           rc = smc_close_active(smc);
>> > > -        sock_set_flag(sk, SOCK_DEAD);
>> > > +        smc_sock_set_flag(sk, SOCK_DEAD);
>> > >           sk->sk_shutdown |= SHUTDOWN_MASK;
>> > >       } else {
>> > >           if (sk->sk_state != SMC_CLOSED) {
>> > > @@ -1742,7 +1742,7 @@ static int smc_clcsock_accept(struct
>> > > smc_sock *lsmc, struct smc_sock **new_smc)
>> > >           if (new_clcsock)
>> > >               sock_release(new_clcsock);
>> > >           new_sk->sk_state = SMC_CLOSED;
>> > > -        sock_set_flag(new_sk, SOCK_DEAD);
>> > > +        smc_sock_set_flag(new_sk, SOCK_DEAD);
>> > >           sock_put(new_sk); /* final */
>> > >           *new_smc = NULL;
>> > >           goto out;
>> > > diff --git a/net/smc/smc.h b/net/smc/smc.h
>> > > index 24745fd..e377980 100644
>> > > --- a/net/smc/smc.h
>> > > +++ b/net/smc/smc.h
>> > > @@ -377,4 +377,9 @@ void smc_fill_gid_list(struct smc_link_group *lgr,
>> > >   int smc_nl_enable_hs_limitation(struct sk_buff *skb, struct
>> > > genl_info *info);
>> > >   int smc_nl_disable_hs_limitation(struct sk_buff *skb, struct
>> > > genl_info *info);
>> > >   +static inline void smc_sock_set_flag(struct sock *sk, enum
>> > > sock_flags flag)
>> > > +{
>> > > +    set_bit(flag, &sk->sk_flags);
>> > > +}
>> > > +
>> > >   #endif    /* __SMC_H */
>> > > diff --git a/net/smc/smc_cdc.c b/net/smc/smc_cdc.c
>> > > index 89105e9..01bdb79 100644
>> > > --- a/net/smc/smc_cdc.c
>> > > +++ b/net/smc/smc_cdc.c
>> > > @@ -385,7 +385,7 @@ static void smc_cdc_msg_recv_action(struct
>> > > smc_sock *smc,
>> > >           smc->sk.sk_shutdown |= RCV_SHUTDOWN;
>> > >           if (smc->clcsock && smc->clcsock->sk)
>> > >               smc->clcsock->sk->sk_shutdown |= RCV_SHUTDOWN;
>> > > -        sock_set_flag(&smc->sk, SOCK_DONE);
>> > > +        smc_sock_set_flag(&smc->sk, SOCK_DONE);
>> > >           sock_hold(&smc->sk); /* sock_put in close_work */
>> > >           if (!queue_work(smc_close_wq, &conn->close_work))
>> > >               sock_put(&smc->sk);
>> > > diff --git a/net/smc/smc_close.c b/net/smc/smc_close.c
>> > > index dbdf03e..449ef45 100644
>> > > --- a/net/smc/smc_close.c
>> > > +++ b/net/smc/smc_close.c
>> > > @@ -173,7 +173,7 @@ void smc_close_active_abort(struct smc_sock *smc)
>> > >           break;
>> > >       }
>> > >   -    sock_set_flag(sk, SOCK_DEAD);
>> > > +    smc_sock_set_flag(sk, SOCK_DEAD);
>> > >       sk->sk_state_change(sk);
>> > >         if (release_clcsock) {
>>