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

[Mimi Zohar]

Hi Simon,

On Mon, 2021-08-23 at 12:02 +0000, THOBY Simon wrote:
> Hi Mimi,
> 
> On 8/23/21 1:57 PM, Mimi Zohar wrote:
> > 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()?
> 
> I was thinking only of ima_policy_next(), I don't think (from what I could see in a short glance)
> that other functions need such a treatment. The ima_rules_tmp dance is probably safe for the
> other uses of ima_rules.

Thanks, just making sure it is limited to here.

Mimi