Subject: Re: [PATCH v3 06/11] rlimits: do security check under task_lock
From: Stephen Smalley <sds@tycho.nsa.gov>
To: Jiri Slaby <jirislaby@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>, akpm@linux-foundation.org,
       adobriyan@gmail.com, nhorman@tuxdriver.com,
       linux-kernel@vger.kernel.org, James Morris <jmorris@namei.org>,
       Eric Paris <eparis@parisplace.org>
In-Reply-To: <4C224804.7030809@gmail.com>
References: <20100513155621.51ca77a4.akpm@linux-foundation.org>
	 <1275855783-27316-1-git-send-email-jslaby@suse.cz>
	 <20100607180855.GA6689@redhat.com> <4C222644.4040601@gmail.com>
	 <20100623161254.GA10098@redhat.com>  <4C224804.7030809@gmail.com>
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Organization: National Security Agency
Date: Wed, 23 Jun 2010 14:37:43 -0400
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On Wed, 2010-06-23 at 19:44 +0200, Jiri Slaby wrote:
> 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 :).

IMO, just do them both under task_lock.  All of the core SELinux
permission checking code should be safe under task lock and the audit
path is the exceptional case (only upon denial).

> >>> 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,

-- 
Stephen Smalley
National Security Agency

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