linux-kernel - Re: [PATCH][SMACK] convert smack rule list to linux list

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Date:	Sun, 22 Feb 2009 09:54:00 -0800
From:	Casey Schaufler <casey@...aufler-ca.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	etienne <etienne.basset@...ericable.fr>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	paul.moore@...com, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH][SMACK] convert smack rule list to linux list

Paul E. McKenney wrote:
> On Sun, Feb 22, 2009 at 02:14:38PM +0100, etienne wrote:
>   
>> Tetsuo Handa wrote:
>>     
>>> etienne wrote:
>>>       
>>>> diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
>>>> index 2e0b83e..3dc312d 100644
>>>> --- a/security/smack/smack_access.c
>>>> +++ b/security/smack/smack_access.c
>>>> @@ -87,7 +87,6 @@ static u32 smack_next_secid = 10;
>>>>  int smk_access(char *subject_label, char *object_label, int request)
>>>>  {
>>>>  	u32 may = MAY_NOT;
>>>> -	struct smk_list_entry *sp;
>>>>  	struct smack_rule *srp;
>>>>  
>>>>  	/*
>>>> @@ -139,8 +138,8 @@ int smk_access(char *subject_label, char *object_label, int request)
>>>>  	 * access (e.g. read is included in readwrite) it's
>>>>  	 * good.
>>>>  	 */
>>>> -	for (sp = smack_list; sp != NULL; sp = sp->smk_next) {
>>>> -		srp = &sp->smk_rule;
>>>> +
>>>> +	list_for_each_entry(srp, &smack_rule_list, list) {
>>>>  
>>>>  		if (srp->smk_subject == subject_label ||
>>>>  		    strcmp(srp->smk_subject, subject_label) == 0) {
>>>>         
>>> Use of standard doubly linked list requires a lock, doesn't it?
>>> What lock protects smack_rule_list?
>>>
>>>       
>> you're right; 
>>
>> what's the best way, using a rcu variant for "list_for_each, container_of ...etc" ?
>> (concurrent list insertion are already protected with a mutex, so rcu must the good idea for the read side) 
>>     
>
> You want list_for_each_entry_rcu() above.  You will need list_add_rcu()
> when adding elements to the list.
>
> Again, if these elements are ever removed, you will need rcu_read_lock()
> and rcu_read_unlock() surrounding their use.  Otherwise, an element can
> be freed out from under a reader who is still referencing it.
>
> 							Thanx, Paul
>   

You'll also need to be very careful that the locking is safe to use
in the networking hooks, in particular smack_socket_sock_rcv_skb. The
amount of care required to get the locking correct is a major factor
in the current list implementation.

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