Re: [PATCH] ima: fix infinite loop within "ima_match_policy" function.

[liqiong <liqiong@nfschina.com>]

Hi Simon :

在 2021年08月23日 16:14, THOBY Simon 写道:
> Hi Liqiong,
>
> On 8/23/21 10:06 AM, liqiong wrote:
>> Hi Simon :
>>
>> Using a temporary ima_rules variable is not working for "ima_policy_next". 
>>
>>  void *ima_policy_next(struct seq_file *m, void *v, loff_t *pos)
>>  {
>>  	struct ima_rule_entry *entry = v;
>> -
>> +	struct list_head *ima_rules_tmp = rcu_dereference(ima_rules);
>>  	rcu_read_lock();
>>  	entry = list_entry_rcu(entry->list.next, struct ima_rule_entry, list);
>>  	rcu_read_unlock();
>>  	(*pos)++;
>>  
>> -	return (&entry->list == ima_rules) ? NULL : entry;
>> +	return (&entry->list == ima_rules_tmp) ? NULL : entry;
>>  }
>>
>> It seems no way to fix "ima_rules" change within this function, it will alway
>> return a entry if "ima_rules" being changed.
> - I think rcu_dereference() should be called inside the RCU read lock
> - Maybe we could cheat with:
> 	return (&entry->list == &ima_policy_rules || &entry->list == &ima_default_rules) ? NULL : entry;
>   as that's the only two rulesets IMA ever use?
>   Admittedly, this is not as clean as previously, but it should work too.
>
> The way I see it, the semaphore solution would not work here either,
> as ima_policy_next() is called repeatedly as a seq_file
> (it is set up in ima_fs.c) and we can't control the locking there:
> we cannot lock across the seq_read() call (that cure could end up be
> worse than the disease, deadlock-wise), so I fear we cannot protect
> against a list update while a user is iterating with a lock.
>
> So in both cases a cheat like "&entry->list == &ima_policy_rules || &entry->list == &ima_default_rules"
> maybe need to be considered.
>
> What do you think?

Yes,  semaphore solution not work here,  splicing two list is a little complex.
This solution is  simple and clear,  should  work.  I will work on that, test and 
confirm  the patch. 

"rcu_dereference() should be called inside the RCU read lock", I will correct this.

Thanks for your help.


Regrads,

liqiong

>
>
>> Regrads,
>>
>> liqiong
> Thanks,
> Simon