Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del()
From: 周多明 <hidden>
Date: 2022-03-15 14:11:54
Also in:
linux-hams, lkml
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.
quoted hunk ↗ jump to hunk
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