linux-kernel - Re: [PATCH] ima: fix infinite loop within "ima_match

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Message-ID: <c02ff60205fdb343cb5a2ff0e4384fc7b47635a3.camel@linux.ibm.com>
Date:   Mon, 23 Aug 2021 07:57:58 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     THOBY Simon <Simon.THOBY@...eris.fr>,
        liqiong <liqiong@...china.com>
Cc:     "dmitry.kasatkin@...il.com" <dmitry.kasatkin@...il.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ima: fix infinite loop within "ima_match_policy"
 function.

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