netdev - Re: neigh use-after-free

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Date:	Fri, 03 Apr 2015 16:34:13 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Potashnik <alexei@...estorage.com>
Cc:	Joern Engel <joern@...estorage.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: neigh use-after-free

On Fri, 2015-04-03 at 13:52 -0700, Alexei Potashnik wrote:
> Would this be an appropriate solution:
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index b49e8ba..38265f2 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -168,7 +168,8 @@ static int neigh_forced_gc(struct neigh_table *tbl)
> 
>  static void neigh_add_timer(struct neighbour *n, unsigned long when)
>  {
> -       neigh_hold(n);
> +       if (!atomic_inc_not_zero(&n->refcnt))
> +               return;
>         if (unlikely(mod_timer(&n->timer, when))) {
>                 printk("NEIGH: BUG, double timer add, state is %x\n",
>                        n->nud_state);

This is a very ugly hack. Please find root cause.

The correct way to implement refcount on a timer is :

if (!mod_timer(&n->timer, when))
	neigh_hold(&n->refcnt);

And current code seems to do that (It dumps a printk() and stack trace
otherwise)


Look at sk_reset_timer() for a good example.

Caller of this function must own a reference.
(Or the socket lock in case of sk_reset_timer())

If refcnt is 0 at this point, it means there is another bug, and we
should fix it instead of trying to work around it.



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