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

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Date:   Mon, 23 Aug 2021 08:14:44 +0000
From:   THOBY Simon <Simon.THOBY@...eris.fr>
To:     liqiong <liqiong@...china.com>, Mimi Zohar <zohar@...ux.ibm.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.

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?


> 
> Regrads,
> 
> liqiong

Thanks,
Simon