Re: [PATCH net v5] ax25: Fix ax25 session cleanup problems

[Paolo Abeni]

On Mon, 2022-05-30 at 23:21 +0800, Duoming Zhou wrote:
> There are session cleanup problems in ax25_release() and
> ax25_disconnect(). If we setup a session and then disconnect,
> the disconnected session is still in "LISTENING" state that
> is shown below.
> 
> Active AX.25 sockets
> Dest       Source     Device  State        Vr/Vs    Send-Q  Recv-Q
> DL9SAU-4   DL9SAU-3   ???     LISTENING    000/000  0       0
> DL9SAU-3   DL9SAU-4   ???     LISTENING    000/000  0       0
> 
> The first reason is caused by del_timer_sync() in ax25_release().
> The timers of ax25 are used for correct session cleanup. If we use
> ax25_release() to close ax25 sessions and ax25_dev is not null,
> the del_timer_sync() functions in ax25_release() will execute.
> As a result, the sessions could not be cleaned up correctly,
> because the timers have stopped.
> 
> In order to solve this problem, this patch adds a device_up flag
> in ax25_dev in order to judge whether the device is up. If there
> are sessions to be cleaned up, the del_timer_sync() in
> ax25_release() will not execute. What's more, we add ax25_cb_del()
> in ax25_kill_by_device(), because the timers have been stopped
> and there are no functions that could delete ax25_cb if we do not
> call ax25_release(). Finally, we reorder the position of
> ax25_list_lock in ax25_cb_del() in order to synchronize among
> different functions that call ax25_cb_del().
> 
> The second reason is caused by improper check in ax25_disconnect().
> The incoming ax25 sessions which ax25->sk is null will close
> heartbeat timer, because the check "if(!ax25->sk || ..)" is
> satisfied. As a result, the session could not be cleaned up properly.
> 
> In order to solve this problem, this patch changes the improper
> check to "if(ax25->sk && ..)" in ax25_disconnect().
> 
> What`s more, the ax25_disconnect() may be called twice, which is
> not necessary. For example, ax25_kill_by_device() calls
> ax25_disconnect() and sets ax25->state to AX25_STATE_0, but
> ax25_release() calls ax25_disconnect() again.
> 
> In order to solve this problem, this patch add a check in
> ax25_release(). If the flag of ax25->sk equals to SOCK_DEAD,
> the ax25_disconnect() in ax25_release() should not be executed.
> 
> Fixes: 82e31755e55f ("ax25: Fix UAF bugs in ax25 timers")
> Fixes: 8a367e74c012 ("ax25: Fix segfault after sock connection timeout")
> Reported-and-tested-by: Thomas Osterried <thomas@xxxxxxxxxxxx>
> Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
> ---
> Changes since v1:
>   - Add ax25_cb_del() in ax25_kill_by_device().
>   - Mitigate race conditions through lock.
>   - Fix session cleanup problem in ax25_disconnect().
>   - Fix ax25_disconnect() may be called twice problem.
>   - Change check in ax25_disconnect() to "if(ax25->sk && ..)".
> 
>  include/net/ax25.h   |  1 +
>  net/ax25/af_ax25.c   | 27 +++++++++++++++++----------
>  net/ax25/ax25_dev.c  |  1 +
>  net/ax25/ax25_subr.c |  2 +-
>  4 files changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/include/net/ax25.h b/include/net/ax25.h
> index 0f9790c455b..a427a05672e 100644
> --- a/include/net/ax25.h
> +++ b/include/net/ax25.h
> @@ -228,6 +228,7 @@ typedef struct ax25_dev {
>  	ax25_dama_info		dama;
>  #endif
>  	refcount_t		refcount;
> +	bool device_up;
>  } ax25_dev;
>  
>  typedef struct ax25_cb {
> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> index 363d47f9453..289f355e185 100644
> --- a/net/ax25/af_ax25.c
> +++ b/net/ax25/af_ax25.c
> @@ -62,12 +62,12 @@ static void ax25_free_sock(struct sock *sk)
>   */
>  static void ax25_cb_del(ax25_cb *ax25)
>  {
> +	spin_lock_bh(&ax25_list_lock);
>  	if (!hlist_unhashed(&ax25->ax25_node)) {
> -		spin_lock_bh(&ax25_list_lock);
>  		hlist_del_init(&ax25->ax25_node);
> -		spin_unlock_bh(&ax25_list_lock);
>  		ax25_cb_put(ax25);
>  	}
> +	spin_unlock_bh(&ax25_list_lock);
>  }
>  
>  /*
> @@ -81,6 +81,7 @@ static void ax25_kill_by_device(struct net_device *dev)
>  
>  	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
>  		return;
> +	ax25_dev->device_up = false;
>  
>  	spin_lock_bh(&ax25_list_lock);
>  again:
> @@ -91,6 +92,7 @@ static void ax25_kill_by_device(struct net_device *dev)
>  				spin_unlock_bh(&ax25_list_lock);
>  				ax25_disconnect(s, ENETUNREACH);
>  				s->ax25_dev = NULL;
> +				ax25_cb_del(s);
>  				spin_lock_bh(&ax25_list_lock);
>  				goto again;
>  			}
> @@ -103,6 +105,7 @@ static void ax25_kill_by_device(struct net_device *dev)
>  				dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
>  				ax25_dev_put(ax25_dev);
>  			}
> +			ax25_cb_del(s);
>  			release_sock(sk);
>  			spin_lock_bh(&ax25_list_lock);
>  			sock_put(sk);
> @@ -995,9 +998,11 @@ static int ax25_release(struct socket *sock)
>  	if (sk->sk_type == SOCK_SEQPACKET) {
>  		switch (ax25->state) {
>  		case AX25_STATE_0:
> -			release_sock(sk);
> -			ax25_disconnect(ax25, 0);
> -			lock_sock(sk);
> +			if (!sock_flag(ax25->sk, SOCK_DEAD)) {
> +				release_sock(sk);
> +				ax25_disconnect(ax25, 0);
> +				lock_sock(sk);
> +			}
>  			ax25_destroy_socket(ax25);
>  			break;
>  
> @@ -1053,11 +1058,13 @@ static int ax25_release(struct socket *sock)
>  		ax25_destroy_socket(ax25);
>  	}
>  	if (ax25_dev) {
> -		del_timer_sync(&ax25->timer);
> -		del_timer_sync(&ax25->t1timer);
> -		del_timer_sync(&ax25->t2timer);
> -		del_timer_sync(&ax25->t3timer);
> -		del_timer_sync(&ax25->idletimer);
> +		if (!ax25_dev->device_up) {
> +			del_timer_sync(&ax25->timer);
> +			del_timer_sync(&ax25->t1timer);
> +			del_timer_sync(&ax25->t2timer);
> +			del_timer_sync(&ax25->t3timer);
> +			del_timer_sync(&ax25->idletimer);
> +		}
>  		dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
>  		ax25_dev_put(ax25_dev);
>  	}
> diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
> index d2a244e1c26..5451be15e07 100644
> --- a/net/ax25/ax25_dev.c
> +++ b/net/ax25/ax25_dev.c
> @@ -62,6 +62,7 @@ void ax25_dev_device_up(struct net_device *dev)
>  	ax25_dev->dev     = dev;
>  	dev_hold_track(dev, &ax25_dev->dev_tracker, GFP_ATOMIC);
>  	ax25_dev->forward = NULL;
> +	ax25_dev->device_up = true;
>  
>  	ax25_dev->values[AX25_VALUES_IPDEFMODE] = AX25_DEF_IPDEFMODE;
>  	ax25_dev->values[AX25_VALUES_AXDEFMODE] = AX25_DEF_AXDEFMODE;
> diff --git a/net/ax25/ax25_subr.c b/net/ax25/ax25_subr.c
> index 3a476e4f6cd..9ff98f46dc6 100644
> --- a/net/ax25/ax25_subr.c
> +++ b/net/ax25/ax25_subr.c
> @@ -268,7 +268,7 @@ void ax25_disconnect(ax25_cb *ax25, int reason)
>  		del_timer_sync(&ax25->t3timer);
>  		del_timer_sync(&ax25->idletimer);
>  	} else {
> -		if (!ax25->sk || !sock_flag(ax25->sk, SOCK_DESTROY))
> +		if (ax25->sk && !sock_flag(ax25->sk, SOCK_DESTROY))
>  			ax25_stop_heartbeat(ax25);
>  		ax25_stop_t1timer(ax25);
>  		ax25_stop_t2timer(ax25);

Side note outside the scope of this patch: I think the ax25
implementation is prone to other races, as it looks like a bit of code
lives under the assumption that once acquired bh_lock_sock() it can
modify the socket status arbitrary. It can't: it will still race with a
different process context acquiring the plain socket lock.

As said, outside the scope of this patch.

Paolo