linux-kernel - Re: [ 42/79] vlan: fix a race in egress prio management

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Message-ID: <1374944157.13555.43.camel@deadeye.wl.decadent.org.uk>
Date:	Sat, 27 Jul 2013 17:55:57 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [ 42/79] vlan: fix a race in egress prio management

On Fri, 2013-07-26 at 13:47 -0700, Greg Kroah-Hartman wrote:
> 3.10-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Eric Dumazet <edumazet@...gle.com>
> 
> [ Upstream commit 3e3aac497513c669e1c62c71e1d552ea85c1d974 ]
> 
> egress_priority_map[] hash table updates are protected by rtnl,
> and we never remove elements until device is dismantled.
> 
> We have to make sure that before inserting an new element in hash table,
> all its fields are committed to memory or else another cpu could
> find corrupt values and crash.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Patrick McHardy <kaber@...sh.net>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  net/8021q/vlan_dev.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> --- a/net/8021q/vlan_dev.c
> +++ b/net/8021q/vlan_dev.c
> @@ -73,6 +73,8 @@ vlan_dev_get_egress_qos_mask(struct net_
>  {
>  	struct vlan_priority_tci_mapping *mp;
>  
> +	smp_rmb(); /* coupled with smp_wmb() in vlan_dev_set_egress_priority() */
> +
>  	mp = vlan_dev_priv(dev)->egress_priority_map[(skb->priority & 0xF)];

This barrier is the wrong type in the wrong place; there should be a
smp_read_barrier_depends() *after* reading the pointer from
egress_priority_map, and after reading each next pointer.

(In practice, I think this error is harmless except on Alpha.  So the
patch is worth applying anyway.)

Ben.

>  	while (mp) {
>  		if (mp->priority == skb->priority) {
> @@ -249,6 +251,11 @@ int vlan_dev_set_egress_priority(const s
>  	np->next = mp;
>  	np->priority = skb_prio;
>  	np->vlan_qos = vlan_qos;
> +	/* Before inserting this element in hash table, make sure all its fields
> +	 * are committed to memory.
> +	 * coupled with smp_rmb() in vlan_dev_get_egress_qos_mask()
> +	 */
> +	smp_wmb();
>  	vlan->egress_priority_map[skb_prio & 0xF] = np;
>  	if (vlan_qos)
>  		vlan->nr_egress_mappings++;

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.

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