netdev - Re: [PATCH] mpls: Add missing RCU-bh read side critical section locking in output path

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Message-ID: <5247037b-65c6-f50f-415a-e3dd7a53cdc8@cumulusnetworks.com>
Date:	Mon, 20 Jun 2016 12:14:52 -0600
From:	David Ahern <dsa@...ulusnetworks.com>
To:	Lennert Buytenhek <buytenh@...tstofly.org>, netdev@...r.kernel.org,
	Roopa Prabhu <roopa@...ulusnetworks.com>,
	Robert Shearman <rshearma@...cade.com>
Subject: Re: [PATCH] mpls: Add missing RCU-bh read side critical section
 locking in output path

On 6/20/16 12:05 PM, Lennert Buytenhek wrote:
> From: David Barroso <dbarroso@...tly.com>
>
> When locally originated IP traffic hits a route that says to push
> MPLS labels, we'll get a call chain dst_output() -> lwtunnel_output()
> -> mpls_output() -> neigh_xmit() -> ___neigh_lookup_noref() where the
> last function in this chain accesses a RCU-bh protected struct
> neigh_table pointer without us ever having declared an RCU-bh read
> side critical section.
>
> As in case of locally originated IP traffic we'll be running in process
> context, with softirqs enabled, we can be preempted by a softirq at any
> time, and RCU-bh considers the completion of a softirq as signaling
> the end of any pending read-side critical sections, so if we do get a
> softirq here, we can end up with an unexpected RCU grace period and
> all the nastiness that that comes with.
>
> This patch makes neigh_xmit() take rcu_read_{,un}lock_bh() around the
> code that expects to be treated as an RCU-bh read side critical section.
>

Fixes: 4fd3d7d9e868f ("neigh: Add helper function neigh_xmit")

> Signed-off-by: David Barroso <dbarroso@...tly.com>
> Signed-off-by: Lennert Buytenhek <lbuytenhek@...tly.com>
>
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index f18ae91..769cece 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -2467,13 +2467,17 @@ int neigh_xmit(int index, struct net_device *dev,
>  		tbl = neigh_tables[index];
>  		if (!tbl)
>  			goto out;
> +		rcu_read_lock_bh();
>  		neigh = __neigh_lookup_noref(tbl, addr, dev);
>  		if (!neigh)
>  			neigh = __neigh_create(tbl, addr, dev, false);
>  		err = PTR_ERR(neigh);
> -		if (IS_ERR(neigh))
> +		if (IS_ERR(neigh)) {
> +			rcu_read_unlock_bh();
>  			goto out_kfree_skb;
> +		}
>  		err = neigh->output(neigh, skb);
> +		rcu_read_unlock_bh();
>  	}
>  	else if (index == NEIGH_LINK_TABLE) {
>  		err = dev_hard_header(skb, dev, ntohs(skb->protocol),
>

LGTM.

Acked-by: David Ahern <dsa@...ulusnetworks.com>