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

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Date:	Wed, 23 Jun 2010 19:44:36 +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 06/23/2010 06:12 PM, Oleg Nesterov wrote:
> On 06/23, Jiri Slaby wrote:
>>
>> On 06/07/2010 08:08 PM, Oleg Nesterov wrote:
>>> 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.
> 
> Well, with all these delays I do not know what "exactly same problem"
> means ;) Please explain?

As I wrote: that the capable() is called with task_lock held. With
security enabled, capable() goes through all the avc_has_perm_noaudit,
avc_audit and similar (in selinux), the same as security_task_setrlimit
which we were writing about -- Andrew complaining about doing very long
security checks while holding spinlocks.

I mean we should do either none of capable and selinux_task_setrlimit
under task_lock or both :).

>>> 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?
> 
> selinux_task_setrlimit()->current_has_perm()->current_sid()->current_cred()

I still see no way how this is wrong. We want to check whether current
thread has capabilities to change (someone else's) rlimits. Maybe I'm
missing something?

thanks,
-- 
js
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