netdev - Re: [PATCH net] Fix race condition between vxlan_sock_add and vxlan_sock

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Message-Id: <20141210.131133.729833991671277249.davem@davemloft.net>
Date:	Wed, 10 Dec 2014 13:11:33 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	mleitner@...hat.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net] Fix race condition between vxlan_sock_add and
 vxlan_sock_release

From: Marcelo Ricardo Leitner <mleitner@...hat.com>
Date: Tue,  9 Dec 2014 12:28:28 -0200

> Currently, when trying to reuse a socket, vxlan_sock_add will grab
> vn->sock_lock, locate a reusable socket, inc refcount and release
> vn->sock_lock.
> 
> But vxlan_sock_release() will first decrement refcount, and then grab
> that lock. refcnt operations are atomic but as currently we have
> deferred works which hold vs->refcnt each, this might happen, leading to
> a use after free (specially after vxlan_igmp_leave):
> 
>   CPU 1                            CPU 2
> 
> deferred work                    vxlan_sock_add

Just make vxlan_sock_add() do atomic_add_unless(x, 1, 0), that way
if vxlan_sock_add() sees the count at zero it can just act as if
no such reusable socket exists.
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