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

liqiong
2021-08-23T12:57:13Z

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