netdev - Re: Crash with SO_REUSEPORT and ef456144da8ef507c8cf504284b6042e9201a05c

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Date:	Tue, 19 Jan 2016 14:51:53 -0400
From:	Marc Dionne <marc.c.dionne@...il.com>
To:	Craig Gallek <kraig@...gle.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: Crash with SO_REUSEPORT and ef456144da8ef507c8cf504284b6042e9201a05c

On Tue, Jan 19, 2016 at 2:11 PM, Craig Gallek <kraig@...gle.com> wrote:
> On Tue, Jan 19, 2016 at 12:08 PM, Marc Dionne <marc.c.dionne@...il.com> wrote:
>> On Tue, Jan 19, 2016 at 12:31 PM, Craig Gallek <kraig@...gle.com> wrote:
>>>
>>> I need to think about how to handle setsockopt-after-bind condition a
>>> bit more, but the NULL pointer dereference is obviously wrong.  Do you
>>> have a way to easily reproduce this?  I've only managed to get it to
>>> happen once so far...
>>
>> The attached code reliably triggers the crash for me.
>
> I think the patch below will address this issue (sorry in advance if
> gmail screws up the whitespace...).  I'll send it for formal review
> once I finish testing it.
>
> Craig
>
> diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> index 1df98c557440..004cb2c974ac 100644
> --- a/net/core/sock_reuseport.c
> +++ b/net/core/sock_reuseport.c
> @@ -97,6 +97,11 @@ int reuseport_add_sock(struct sock *sk, const
> struct sock *sk2)
>  {
>   struct sock_reuseport *reuse;
>
> +  if (!rcu_access_pointer(sk2->sk_reuseport_cb)) {
> +   int err = reuseport_alloc(sk2);
> +   if (err) return err;
> +  }
> +
>   spin_lock_bh(&reuseport_lock);
>   reuse = rcu_dereference_protected(sk2->sk_reuseport_cb,
>    lockdep_is_held(&reuseport_lock)),

That works fine, thanks..

Just wondering though, is there a bit of a race there?  Seems like it
might be safer to have a version of reuseport_alloc that doesn't take
the lock and use it here, moving the block after the lock is taken.

Marc