Subject: Re: [PATCH v4] net: cgroup: fix access the unallocated memory in netprio cgroup

From: John Fastabend <[email-masked]>
Date: Wed, 18 Jul 2012 07:10:18 -0700

On 7/18/2012 5:21 AM, Neil Horman wrote:
> On Tue, Jul 17, 2012 at 01:47:25PM -0700, John Fastabend wrote:
>> On 7/12/2012 12:50 AM, Gao feng wrote:
>>> there are some out of bound accesses in netprio cgroup.
>>>
>>> now before accessing the dev->priomap.priomap array,we only check
>>> if the dev->priomap exist.and because we don't want to see
>>> additional bound checkings in fast path, so we should make sure
>>> that dev->priomap is null or array size of dev->priomap.priomap
>>> is equal to max_prioidx + 1;
>>>
>>> so in write_priomap logic,we should call extend_netdev_table when
>>> dev->priomap is null and dev->priomap.priomap_len < max_len.
>>> and in cgrp_create->update_netdev_tables logic,we should call
>>> extend_netdev_table only when dev->priomap exist and
>>> dev->priomap.priomap_len < max_len.
>>>
>>> and it's not needed to call update_netdev_tables in write_priomap,
>>> we can only allocate the net device's priomap which we change through
>>> net_prio.ifpriomap.
>>>
>>> this patch also add a return value for update_netdev_tables &
>>> extend_netdev_table, so when new_priomap is allocated failed,
>>> write_priomap will stop to access the priomap,and return -ENOMEM
>>> back to the userspace to tell the user what happend.
>>>
>>> Change From v3:
>>> 1. add rtnl protect when reading max_prioidx in write_priomap.
>>>
>>> 2. only call extend_netdev_table when map->priomap_len < max_len,
>>>     this will make sure array size of dev->map->priomap always
>>>     bigger than any prioidx.
>>>
>>> 3. add a function write_update_netdev_table to make codes clear.
>>>
>>> Change From v2:
>>> 1. protect extend_netdev_table by RTNL.
>>> 2. when extend_netdev_table failed,call dev_put to reduce device's refcount.
>>>
>>> Signed-off-by: Gao feng <[email-masked]>
>>> Cc: Neil Horman <[email-masked]>
>>> Cc: Eric Dumazet <[email-masked]>
>>> ---
>>>   net/core/netprio_cgroup.c |   71 ++++++++++++++++++++++++++++++++++-----------
>>>   1 files changed, 54 insertions(+), 17 deletions(-)
>>>
>>
>> [...]
>>
>>> +
>>> +static int update_netdev_tables(void)
>>> +{
>>> +	int ret = 0;
>>>   	struct net_device *dev;
>>> -	u32 max_len = atomic_read(&max_prioidx) + 1;
>>> +	u32 max_len;
>>>   	struct netprio_map *map;
>>
>>
>> need to check if net subsystem is initialized before we try
>> to use it here...
>>
>> 	if (some_check)     -> need to lookup what this check is
>> 		return ret;
>>
>>>
>>>   	rtnl_lock();
>>> +	max_len = atomic_read(&max_prioidx) + 1;
>>>   	for_each_netdev(&init_net, dev) {
>>>   		map = rtnl_dereference(dev->priomap);
>>> -		if ((!map) ||
>>> -		    (map->priomap_len < max_len))
>>> -			extend_netdev_table(dev, max_len);
>>> +		/*
>>> +		 * don't allocate priomap if we didn't
>>> +		 * change net_prio.ifpriomap (map == NULL),
>>> +		 * this will speed up skb_update_prio.
>>> +		 */
>>> +		if (map && map->priomap_len < max_len) {
>>> +			ret = extend_netdev_table(dev, max_len);
>>> +			if (ret < 0)
>>> +				break;
>>> +		}
>>>   	}
>>>   	rtnl_unlock();
>>> +	return ret;
>>>   }
>>>
>>>   static struct cgroup_subsys_state *cgrp_create(struct cgroup *cgrp)
>>>   {
>>>   	struct cgroup_netprio_state *cs;
>>> -	int ret;
>>> +	int ret = -EINVAL;
>>>
>>>   	cs = kzalloc(sizeof(*cs), GFP_KERNEL);
>>>   	if (!cs)
>>>   		return ERR_PTR(-ENOMEM);
>>>
>>> -	if (cgrp->parent && cgrp_netprio_state(cgrp->parent)->prioidx) {
>>> -		kfree(cs);
>>> -		return ERR_PTR(-EINVAL);
>>> -	}
>>> +	if (cgrp->parent && cgrp_netprio_state(cgrp->parent)->prioidx)
>>> +		goto out;
>>>
>>>   	ret = get_prioidx(&cs->prioidx);
>>> -	if (ret != 0) {
>>> +	if (ret < 0) {
>>>   		pr_warn("No space in priority index array\n");
>>> -		kfree(cs);
>>> -		return ERR_PTR(ret);
>>> +		goto out;
>>> +	}
>>> +
>>> +	ret = update_netdev_tables();
>>> +	if (ret < 0) {
>>> +		put_prioidx(cs->prioidx);
>>> +		goto out;
>>>   	}
>>
>> Gao,
>>
>> This introduces a null ptr dereference when netprio_cgroup is built
>> into the kernel because update_netdev_tables() depends on init_net.
>> However cgrp_create is being called by cgroup_init before
>> do_initcalls() is called and before net_dev_init().
>>
>> .John
>>
> Not sure I follow here John.  Shouldn't init_net be initialized prior to any
> network devices getting registered?  In other words, shouldn't for_each_netdev
> just result in zero iterations through the loop?
> Neil
>

init_net _is_ initialized prior to any network devices getting
registered but not before cgrp_create called via cgroup_init.

#define for_each_netdev(net, d)         \
                 list_for_each_entry(d, &(net)->dev_base_head, dev_list)

but dev_base_head is zeroed at this time. In netdev_init we have,

         INIT_LIST_HEAD(&net->dev_base_head);

but we haven't got that far yet because cgroup_init is called
before do_initcalls().