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Message-ID: <2533f9c8.5741.17f8e6a4593.Coremail.duoming@zju.edu.cn>
Date: Wed, 16 Mar 2022 00:30:03 +0800 (GMT+08:00)
From: 周多明 <duoming@....edu.cn>
To: "Eric Dumazet" <eric.dumazet@...il.com>
Cc: linux-hams@...r.kernel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, kuba@...nel.org, davem@...emloft.net,
ralf@...ux-mips.org, jreuter@...na.de
Subject: Re: Re: [PATCH net V4 2/2] ax25: Fix NULL pointer dereferences in
ax25 timers
Hello,
On Mon, 14 Mar 2022 20:03:00 -0700, Eric Dumazet wrote:
> > There are race conditions that may lead to null pointer dereferences in
> > ax25_heartbeat_expiry(), ax25_t1timer_expiry(), ax25_t2timer_expiry(),
> > ax25_t3timer_expiry() and ax25_idletimer_expiry(), when we use
> > ax25_kill_by_device() to detach the ax25 device.
> >
> > One of the race conditions that cause null pointer dereferences can be
> > shown as below:
> >
> > (Thread 1) | (Thread 2)
> > ax25_connect() |
> > ax25_std_establish_data_link() |
> > ax25_start_t1timer() |
> > mod_timer(&ax25->t1timer,..) |
> > | ax25_kill_by_device()
> > (wait a time) | ...
> > | s->ax25_dev = NULL; //(1)
> > ax25_t1timer_expiry() |
> > ax25->ax25_dev->values[..] //(2)| ...
> > ... |
> >
> > We set null to ax25_cb->ax25_dev in position (1) and dereference
> > the null pointer in position (2).
> >
> > The corresponding fail log is shown below:
> > ===============================================================
> > BUG: kernel NULL pointer dereference, address: 0000000000000050
> > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.17.0-rc6-00794-g45690b7d0
> > RIP: 0010:ax25_t1timer_expiry+0x12/0x40
> > ...
> > Call Trace:
> > call_timer_fn+0x21/0x120
> > __run_timers.part.0+0x1ca/0x250
> > run_timer_softirq+0x2c/0x60
> > __do_softirq+0xef/0x2f3
> > irq_exit_rcu+0xb6/0x100
> > sysvec_apic_timer_interrupt+0xa2/0xd0
> > ...
> >
> > This patch uses ax25_disconnect() to delete timers before we set null to
> > ax25_cb->ax25_dev in ax25_kill_by_device().The function ax25_disconnect()
> > will not return until all timers are stopped, because we have changed
> > del_timer() to del_timer_sync(). What`s more, we add condition check in
> > ax25_destroy_socket(), because ax25_stop_heartbeat() will not return,
> > if there is still heartbeat.
> >
> > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
>
> Missing FIxes: tag ?
>
>
> > ---
> > Changes in V4:
> > - Based on [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del().
> >
> > net/ax25/af_ax25.c | 7 ++++---
> > net/ax25/ax25_timer.c | 10 +++++-----
> > 2 files changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> > index 0886109421a..dc6161a75a1 100644
> > --- a/net/ax25/af_ax25.c
> > +++ b/net/ax25/af_ax25.c
> > @@ -89,20 +89,20 @@ static void ax25_kill_by_device(struct net_device *dev)
> > sk = s->sk;
> > if (!sk) {
> > spin_unlock_bh(&ax25_list_lock);
> > - s->ax25_dev = NULL;
> > ax25_disconnect(s, ENETUNREACH);
> > + s->ax25_dev = NULL;
> > spin_lock_bh(&ax25_list_lock);
> > goto again;
> > }
> > sock_hold(sk);
> > spin_unlock_bh(&ax25_list_lock);
> > lock_sock(sk);
> > + ax25_disconnect(s, ENETUNREACH);
> > s->ax25_dev = NULL;
> > if (sk->sk_wq) {
> > dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
> > ax25_dev_put(ax25_dev);
> > }
> > - ax25_disconnect(s, ENETUNREACH);
> > release_sock(sk);
> > spin_lock_bh(&ax25_list_lock);
> > sock_put(sk);
> > @@ -307,7 +307,8 @@ void ax25_destroy_socket(ax25_cb *ax25)
> >
> > ax25_cb_del(ax25);
> >
> > - ax25_stop_heartbeat(ax25);
> > + if (!ax25->sk || !sock_flag(ax25->sk, SOCK_DESTROY))
> > + ax25_stop_heartbeat(ax25);
> > ax25_stop_t1timer(ax25);
> > ax25_stop_t2timer(ax25);
> > ax25_stop_t3timer(ax25);
> > diff --git a/net/ax25/ax25_timer.c b/net/ax25/ax25_timer.c
> > index 85865ebfdfa..99af3d1aeec 100644
> > --- a/net/ax25/ax25_timer.c
> > +++ b/net/ax25/ax25_timer.c
> > @@ -78,27 +78,27 @@ void ax25_start_idletimer(ax25_cb *ax25)
> >
> > void ax25_stop_heartbeat(ax25_cb *ax25)
> > {
> > - del_timer(&ax25->timer);
> > + del_timer_sync(&ax25->timer);
> > }
> >
> > void ax25_stop_t1timer(ax25_cb *ax25)
> > {
> > - del_timer(&ax25->t1timer);
> > + del_timer_sync(&ax25->t1timer);
> > }
> >
> > void ax25_stop_t2timer(ax25_cb *ax25)
> > {
> > - del_timer(&ax25->t2timer);
> > + del_timer_sync(&ax25->t2timer);
> > }
> >
> > void ax25_stop_t3timer(ax25_cb *ax25)
> > {
> > - del_timer(&ax25->t3timer);
> > + del_timer_sync(&ax25->t3timer);
> > }
> >
> > void ax25_stop_idletimer(ax25_cb *ax25)
> > {
> > - del_timer(&ax25->idletimer);
> > + del_timer_sync(&ax25->idletimer);
> > }
> >
> > int ax25_t1timer_running(ax25_cb *ax25)
>
>
>
> Are you sure calling del_time_sync() wont deadlock ?
>
>
> If the timer handlers need a lock owned by the thread calling
> del_timer_sync(),
>
> then this will block forever.
I think there is no deadlock.
Function ax25_kill_by_device() will only hold lock_sock(sk) before using
ax25_disconnect() to call del_timer_sync().
In timers, we hold bh_lock_sock(sk) or spin_lock_bh(&ax25_list_lock) which
is different from lock_sock(sk).
Best wishes,
Duoming Zhou
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