Re: [PATCH] net/packet: fix race condition between fanout_add and __unregister_prot_hook

[Cong Wang]

On Thu, Sep 14, 2017 at 7:35 AM, Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
> On Thu, Sep 14, 2017 at 10:07 AM, nixiaoming <nixiaoming@xxxxxxxxxx> wrote:
>> From: l00219569 <lisimin@xxxxxxxxxx>
>>
>> If fanout_add is preempted after running po-> fanout = match
>> and before running __fanout_link,
>> it will cause BUG_ON when __unregister_prot_hook call __fanout_unlink
>>
>> so, we need add mutex_lock(&fanout_mutex) to __unregister_prot_hook
>
> The packet socket code has no shortage of locks, so there are many
> ways to avoid the race condition between fanout_add and packet_set_ring.
>
> Another option would be to lock the socket when calling fanout_add:
>
>     -               return fanout_add(sk, val & 0xffff, val >> 16);
>     +               lock_sock(sk);
>     +               ret = fanout_add(sk, val & 0xffff, val >> 16);
>     +               release_sock(sk);
>     +               return ret;
>

I don't think this is an option, because __unregister_prot_hook()
can be called without lock_sock(), for example in packet_notifier().


> But, for consistency, and to be able to continue to make sense of the
> locking policy, we should use the most appropriate lock. This
> is po->bind_lock, as it ensures atomicity between testing whether
> a protocol hook is active through po->running and the actual existence
> of that hook on the protocol hook list.

Yeah, register_prot_hook() and unregister_prot_hook() already assume
bind_lock.

[...]

>>  out:
>>         mutex_unlock(&fanout_mutex);
>> +       spin_unlock(&po->bind_lock);
>
> This function can call kzalloc with GFP_KERNEL, which may sleep. It is
> not correct to sleep while holding a spinlock. Which is why I take the lock
> later and test po->running again.


Right, no need to mention the mutex_unlock() before the spin_unlock()
is clearly wrong.


>
> I will clean up that patch and send it for review.

How about the following patch?


diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index c26172995511..f5c696a548ed 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1754,10 +1754,14 @@ static int fanout_add(struct sock *sk, u16 id,
u16 type_flags)
            match->prot_hook.dev == po->prot_hook.dev) {
                err = -ENOSPC;
                if (refcount_read(&match->sk_ref) < PACKET_FANOUT_MAX) {
+                       spin_lock(&po->bind_lock);
                        __dev_remove_pack(&po->prot_hook);
-                       po->fanout = match;
-                       refcount_set(&match->sk_ref,
refcount_read(&match->sk_ref) + 1);
-                       __fanout_link(sk, po);
+                       if (po->running) {
+                               refcount_set(&match->sk_ref,
refcount_read(&match->sk_ref) + 1);
+                               po->fanout = match;
+                               __fanout_link(sk, po);
+                       }
+                       spin_unlock(&po->bind_lock);
                        err = 0;
                }
        }