linux-kernel - Re: [PATCH v3 06/11] rlimits: do security check under task

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Date:	Wed, 23 Jun 2010 17:20:36 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	akpm@...ux-foundation.org, adobriyan@...il.com,
	nhorman@...driver.com, linux-kernel@...r.kernel.org,
	Stephen Smalley <sds@...ho.nsa.gov>,
	James Morris <jmorris@...ei.org>,
	Eric Paris <eparis@...isplace.org>
Subject: Re: [PATCH v3 06/11] rlimits: do security check under task_lock

On 06/07/2010 08:08 PM, Oleg Nesterov wrote:
> First of all, my apologies for the huge delay. And I still didn't
> read the whole series, sorry.

Hi, never mind, my RTT of 2 weeks doesn't look like very short too :).

> On 06/06, Jiri Slaby wrote:
>> @@ -1339,13 +1364,19 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
>>  
>>  	rlim = tsk->signal->rlim + resource;
>>  	task_lock(tsk->group_leader);
>> +again:
>> +	retval = 0;
>>  	if (new_rlim) {
>>  		if ((new_rlim->rlim_max > rlim->rlim_max) &&
>>  					!capable(CAP_SYS_RESOURCE))

BTW this capable() has the exactly same problem with being called with
task_lock held. Is it OK to move it completely out of critical section?
I'm asking because it sets a current->flags SU bit used for accounting.
If I move it out of the section, it will set the bit always.

>>  			retval = -EPERM;
>> -		if (!retval)
>> -			retval = security_task_setrlimit(tsk, resource,
>> -					new_rlim);
>> +		if (!retval) {
>> +			retval = check_security_task_setrlimit_unlocked(tsk,
>> +					resource, new_rlim, rlim);
>> +			if (retval == -EAGAIN) {
>> +				goto again;
>> +			}
>> +		}
> 
> Oh. Can't we just ignore this (imho minor) race ? Or just verify/document that
> current_has_perm() can be called under task_lock. Actually, I do not think
> we have a race, selinux_task_setrlimit() only checks that the caller has
> rights to change the rlimits.

But does so only if current limits are different to the new ones. My
opinion is that we can ignore it anyway.

> And. Given that avc_has_perm() can be called from irq context (say,
> selinux_file_send_sigiotask or selinux_task_kill), we can assume it is safe
> to call it under task_lock() which is not irq-safe.
> 
> But. OTOH, if we are really worried about security_ ops, then we have another
> reason to call this hook under task_lock(), and we probably want to modify
> selinux_bprm_committing_creds() to take this lock too:
> 
> 	--- security/selinux/hooks.c
> 	+++ security/selinux/hooks.c
> 	@@ -2333,11 +2333,14 @@ static void selinux_bprm_committing_cred
> 		rc = avc_has_perm(new_tsec->osid, new_tsec->sid, SECCLASS_PROCESS,
> 				  PROCESS__RLIMITINH, NULL);
> 		if (rc) {
> 	+		/* protects against do_prlimit() */
> 	+		task_lock(current);
> 			for (i = 0; i < RLIM_NLIMITS; i++) {
> 				rlim = current->signal->rlim + i;
> 				initrlim = init_task.signal->rlim + i;
> 				rlim->rlim_cur = min(rlim->rlim_max, initrlim->rlim_cur);
> 			}
> 	+		task_unlock(current);
> 			update_rlimit_cpu(current->signal->rlim[RLIMIT_CPU].rlim_cur);
> 		}
> 	 }

Makes sense to me.

> Finally. selinux_task_setrlimit(p) uses __task_cred(p) for the check.
> This looks a bit strange, different threads can have different creds
> but obviously rlimits are per-process.

Sorry I can't see it. Could you point out in which function this is done?

thanks,
-- 
js
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