Re: [PATCH v2] cgroup: fix panic in netprio_cgroup

[Neil Horman]

On Sun, Jul 08, 2012 at 09:50:43PM +0200, Eric Dumazet wrote:
> On Thu, 2012-07-05 at 17:28 +0800, Gao feng wrote:
> > we set max_prioidx to the first zero bit index of prioidx_map in
> > function get_prioidx.
> > 
> > So when we delete the low index netprio cgroup and adding a new
> > netprio cgroup again,the max_prioidx will be set to the low index.
> > 
> > when we set the high index cgroup's net_prio.ifpriomap,the function
> > write_priomap will call update_netdev_tables to alloc memory which
> > size is sizeof(struct netprio_map) + sizeof(u32) * (max_prioidx + 1),
> > so the size of array that map->priomap point to is max_prioidx +1,
> > which is low than what we actually need.
> > 
> > fix this by adding check in get_prioidx,only set max_prioidx when
> > max_prioidx low than the new prioidx.
> > 
> > Signed-off-by: Gao feng <gaofeng@xxxxxxxxxxxxxx>
> > ---
> >  net/core/netprio_cgroup.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> > 
> > diff --git a/net/core/netprio_cgroup.c b/net/core/netprio_cgroup.c
> > index 5b8aa2f..aa907ed 100644
> > --- a/net/core/netprio_cgroup.c
> > +++ b/net/core/netprio_cgroup.c
> > @@ -49,8 +49,9 @@ static int get_prioidx(u32 *prio)
> >  		return -ENOSPC;
> >  	}
> >  	set_bit(prioidx, prioidx_map);
> > +	if (atomic_read(&max_prioidx) < prioidx)
> > +		atomic_set(&max_prioidx, prioidx);
> >  	spin_unlock_irqrestore(&prioidx_map_lock, flags);
> > -	atomic_set(&max_prioidx, prioidx);
> >  	*prio = prioidx;
> >  	return 0;
> >  }
> 
> This patch seems fine to me.
> 
> Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> Neil, looking at this file, I believe something is wrong.
> 
> dev->priomap is allocated by extend_netdev_table() called from
> update_netdev_tables(). And this is only called if write_priomap() is
> called.
> 
> But if write_priomap() is not called, it seems we can have out of bounds
> accesses in cgrp_destroy() and read_priomap()
> 
> What do you think of following patch ?
> 
> diff --git a/net/core/netprio_cgroup.c b/net/core/netprio_cgroup.c
> index 5b8aa2f..80150d2 100644
> --- a/net/core/netprio_cgroup.c
> +++ b/net/core/netprio_cgroup.c
> @@ -141,7 +141,7 @@ static void cgrp_destroy(struct cgroup *cgrp)
>  	rtnl_lock();
>  	for_each_netdev(&init_net, dev) {
>  		map = rtnl_dereference(dev->priomap);
> -		if (map)
> +		if (map && cs->prioidx < map->priomap_len)
>  			map->priomap[cs->prioidx] = 0;
>  	}
>  	rtnl_unlock();
> @@ -165,7 +165,7 @@ static int read_priomap(struct cgroup *cont, struct cftype *cft,
>  	rcu_read_lock();
>  	for_each_netdev_rcu(&init_net, dev) {
>  		map = rcu_dereference(dev->priomap);
> -		priority = map ? map->priomap[prioidx] : 0;
> +		priority = (map && prioidx < map->priomap_len) ? map->priomap[prioidx] : 0;
>  		cb->fill(cb, dev->name, priority);
>  	}
>  	rcu_read_unlock();
> 
> 
> 
You're right,  If we create a cgroup after a net device is registered the group
priority index will likely be out of bounds for those devices.  We can fix it
like you propose above (including the additional prioidx < map->priomap_len
check in skb_update_prio as Gao notes), or we can call update_netdev_tables
iteratively for every net device in cgrp_create, and on device_registration in
netprio_device_event.

I'm not sure how adventageous one is over the other, but it does seem that,
given that skb_update_prio is in the transmit path, it might be nice to avoid
the additional length check there if possible.

Thanks!
Neil

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