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

[Mimi Zohar]

On Mon, 2021-08-23 at 08:14 +0000, THOBY Simon wrote:
> 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?

Is this an overall suggestion or limited to just ima_policy_next()?

thanks,

Mimi