Messages in this thread Patch in this message | | | Date | Tue, 15 Mar 2022 22:11:10 +0800 (GMT+08:00) | From | 周多明 <> | Subject | Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del() |
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Hello,
On Tue, 15 Mar 2022 13:26:57 +0300, Dan Carpenter wrote: > I'm happy that this is simpler. I'm not super happy about the > if (sk->sk_wq) check. That seems like a fragile side-effect condition > instead of something deliberate. But I don't know networking so maybe > this is something which we can rely on.
The variable sk->sk_wq is the address of waiting queue of sock, it is initialized to the address of sock->wq through the following path: sock_create->__sock_create->ax25_create()->sock_init_data()->RCU_INIT_POINTER(sk->sk_wq, &sock->wq). Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or the address of sock->wq is not null. What`s more, sk->sk_wq is set to null only in sock_orphan().
Another solution: We could also use sk->sk_socket to check. We set sk->sk_socket to sock in the following path: sock_create()->__sock_create()->ax25_create()->sock_init_data()->sk_set_socket(sk, sock). Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or sk->sk_socket is not null. What`s more, sk->sk_socket is set to null only in sock_orphan().
I will change the if (sk->sk_wq) check to if(sk->sk_socket) check, because I think it is easier to understand.
> When you sent the earlier patch then I asked if the devices in > ax25_kill_by_device() were always bound and if we could just use a local > variable instead of something tied to the ax25_dev struct. I still > wonder about that. In other words, could we just do this? > > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c > index 6bd097180772..4af9d9a939c6 100644 > --- a/net/ax25/af_ax25.c > +++ b/net/ax25/af_ax25.c > @@ -78,6 +78,7 @@ static void ax25_kill_by_device(struct net_device *dev) > ax25_dev *ax25_dev; > ax25_cb *s; > struct sock *sk; > + bool found = false; > > if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL) > return; > @@ -86,6 +87,7 @@ static void ax25_kill_by_device(struct net_device *dev) > again: > ax25_for_each(s, &ax25_list) { > if (s->ax25_dev == ax25_dev) { > + found = true; > sk = s->sk; > if (!sk) { > spin_unlock_bh(&ax25_list_lock); > @@ -115,6 +117,11 @@ static void ax25_kill_by_device(struct net_device *dev) > } > } > spin_unlock_bh(&ax25_list_lock); > + > + if (!found) { > + dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker); > + ax25_dev_put(ax25_dev); > + } > }
If we just use ax25_dev_device_up() to bring device up without using ax25_bind(), the "found" flag could be false when we enter ax25_kill_by_device() and the refcounts underflow will happen. So we should use two additional variables.
If we use additional variables to fix the bug, I think there is a problem. In the real world, the device could be detached only once. If the following race condition happens, we could not deallocate ax25_dev and net_device anymore, because we could not call ax25_kill_by_device() again.
(Thread 1) | (Thread 2) ax25_bind() | | ax25_kill_by_device() //decrease refcounts (Thread 3) | ax25_bind() | ... | ... ax25_dev_hold() //(1) | dev_hold_track() //(2) | | ax25_dev_device_down() In patch "[PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del()", even the device has been detached, we could also decrease the refcouns by using ax25_release(), which could ensure ax25_dev and net_device could be deallocated. So I think "[PATCH net V4 1/2]" is better.
Best wishes, Duoming Zhou | |