[RFC/PATCH] Bluetooth: prevent double l2cap_chan_destroy

[RFC/PATCH] Bluetooth: prevent double l2cap_chan_destroy

[Minho Ban]

l2cap_sock_kill can be called in l2cap_sock_release and l2cap_sock_close_cb
either. This lead l2cap_chan_destroy to be called twice for same channel.
To prevent double list_del and double chan_put, chan_destroy should be protected
with chan->refcnt and chan_list_lock so that reentrance could be forbidden.

Signed-off-by: Minho Ban <mhban@samsung.com>
---
 net/bluetooth/l2cap_core.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 24f144b..156ca14 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -400,10 +400,14 @@ struct l2cap_chan *l2cap_chan_create(void)
 void l2cap_chan_destroy(struct l2cap_chan *chan)
 {
 	write_lock(&chan_list_lock);
+	/* Check if channel is valid */
+	if (!atomic_read(&chan->refcnt)) {
+		write_unlock(&chan_list_lock);
+		return;
+	}
 	list_del(&chan->global_l);
-	write_unlock(&chan_list_lock);
-
 	l2cap_chan_put(chan);
+	write_unlock(&chan_list_lock);
 }
 
 void l2cap_chan_set_defaults(struct l2cap_chan *chan)
-- 
1.7.5.4

Re: [RFC/PATCH] Bluetooth: prevent double l2cap_chan_destroy

[Gustavo Padovan]

Hi Minho,

* Minho Ban <mhban@samsung.com> [2012-05-21 09:56:40 +0900]:

> l2cap_sock_kill can be called in l2cap_sock_release and l2cap_sock_close_cb
> either. This lead l2cap_chan_destroy to be called twice for same channel.
> To prevent double list_del and double chan_put, chan_destroy should be protected
> with chan->refcnt and chan_list_lock so that reentrance could be forbidden.

Even if l2cap_sock_kill() is called twice it will call l2cap_chan_destroy()
only once. If this is not happening we just have a broken piece of code
somewhere else and not here.

	Gustavo

Re: [RFC/PATCH] Bluetooth: prevent double l2cap_chan_destroy

[Minho Ban]

On 05/22/2012 01:21 AM, Gustavo Padovan wrote:
> Hi Minho,
> 
> * Minho Ban <mhban@samsung.com> [2012-05-21 09:56:40 +0900]:
> 
>> l2cap_sock_kill can be called in l2cap_sock_release and l2cap_sock_close_cb
>> either. This lead l2cap_chan_destroy to be called twice for same channel.
>> To prevent double list_del and double chan_put, chan_destroy should be protected
>> with chan->refcnt and chan_list_lock so that reentrance could be forbidden.
> 
> Even if l2cap_sock_kill() is called twice it will call l2cap_chan_destroy()
> only once. If this is not happening we just have a broken piece of code
> somewhere else and not here.
> 
> 	Gustavo
> 

Thanks for comment but I could not found any suitable code in l2cap_sock_kill that 
can make l2cap_chan_destroy to be called just once. sock flag test is not enough to 
do it.

I agree this path should not be the fix. Testing chan->refcnt is nonsense because 
chan might have been freed already. So I looked for another point,

@@ -1343,10 +1343,10 @@ static void l2cap_conn_del(struct hci_conn *hcon, int err)
                l2cap_chan_lock(chan);
 
                l2cap_chan_del(chan, err);
+               chan->ops->close(chan->data);
 
                l2cap_chan_unlock(chan);
 
-               chan->ops->close(chan->data);
                l2cap_chan_put(chan);
        }

close callback must locate within chan_lock unless it can be scheduled to other thread 
which may wait for chan_lock in l2cap_sock_shutdown and this lead to duplicate sock_kill.

 static void l2cap_sock_kill(struct sock *sk)
 {
-       if (!sock_flag(sk, SOCK_ZAPPED) || sk->sk_socket)
+       if (!sock_flag(sk, SOCK_ZAPPED) || sock_flag(sk, SOCK_DEAD) ||
+                       sk->sk_socket)
                return;
 
        BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));

Duplicate sock_kill may happen anyway, need test SOCK_DEAD if chan_destroy is already called.

Regards,
Minho Ban

Re: [RFC/PATCH] Bluetooth: prevent double l2cap_chan_destroy

[Chanyeol Park]

Hi

On 2012년 05월 22일 11:50, Minho Ban wrote:
> @@ -1343,10 +1343,10 @@ static void l2cap_conn_del(struct hci_conn *hcon, int err)
>                  l2cap_chan_lock(chan);
>
>                  l2cap_chan_del(chan, err);
> +               chan->ops->close(chan->data);
>
>                  l2cap_chan_unlock(chan);
>
> -               chan->ops->close(chan->data);
>                  l2cap_chan_put(chan);
>          }
I think this patch does not make sense Because inside chan->ops->close() 
"chan" could be freed in the l2cap_chan_destroy().

> close callback must locate within chan_lock unless it can be scheduled to other thread
> which may wait for chan_lock in l2cap_sock_shutdown and this lead to duplicate sock_kill.
>
>   static void l2cap_sock_kill(struct sock *sk)
>   {
> -       if (!sock_flag(sk, SOCK_ZAPPED) || sk->sk_socket)
> +       if (!sock_flag(sk, SOCK_ZAPPED) || sock_flag(sk, SOCK_DEAD) ||
> +                       sk->sk_socket)
>                  return;
>
>          BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));
>
> Duplicate sock_kill may happen anyway, need test SOCK_DEAD if chan_destroy is already called.
Even l2cap_sock_kill() is called twice, " if (!sock_flag(sk, 
SOCK_ZAPPED) || sk->sk_socket)" can't filter it.
I tested Mr.ban case. it works fine with me.

BR
Chanyeol Park.

Re: [RFC/PATCH] Bluetooth: prevent double l2cap_chan_destroy

[Minho Ban]

On 05/22/2012 09:23 PM, Chanyeol Park wrote:
> Hi
> 
> On 2012년 05월 22일 11:50, Minho Ban wrote:
>> @@ -1343,10 +1343,10 @@ static void l2cap_conn_del(struct hci_conn *hcon, int err)
>>                  l2cap_chan_lock(chan);
>>
>>                  l2cap_chan_del(chan, err);
>> +               chan->ops->close(chan->data);
>>
>>                  l2cap_chan_unlock(chan);
>>
>> -               chan->ops->close(chan->data);
>>                  l2cap_chan_put(chan);
>>          }
> I think this patch does not make sense Because inside chan->ops->close() "chan" could be freed in the l2cap_chan_destroy().
> 

I agree, thanks for pointing out.