linux-kernel - Re: [BUG][PATCH -mm] avoid BUG() in __stop_machine

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Date:	Thu, 19 Jun 2008 08:51:06 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	linux-mm@...ck.org, sugita <yumiko.sugita.yf@...achi.com>,
	Satoshi OSHIMA <satoshi.oshima.fk@...achi.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [BUG][PATCH -mm] avoid BUG() in __stop_machine_run()

Rusty Russell wrote:
> On Thursday 19 June 2008 16:59:50 Hidehiro Kawai wrote:
>   
>> When a process loads a kernel module, __stop_machine_run() is called, and
>> it calls sched_setscheduler() to give newly created kernel threads highest
>> priority.  However, the process can have no CAP_SYS_NICE which required
>> for sched_setscheduler() to increase the priority.  For example, SystemTap
>> loads its module with only CAP_SYS_MODULE.  In this case,
>> sched_setscheduler() returns -EPERM, then BUG() is called.
>>     
>
> Hi Hidehiro,
>
> 	Nice catch.  This can happen in the current code, it just doesn't
> BUG().
>
>   
>> Failure of sched_setscheduler() wouldn't be a real problem, so this
>> patch just ignores it.
>>     
>
> 	Well, it can mean that the stop_machine blocks indefinitely.  Better
> than a BUG(), but we should aim higher.
>
>   
>> Or, should we give the CAP_SYS_NICE capability temporarily?
>>     
>
>         I don't think so.  It can be seen from another thread, and in theory
> that should not see something random.  Worse, they can change it from
> another thread.
>
> How's this?
>
> sched_setscheduler: add a flag to control access checks
>
> Hidehiro Kawai noticed that sched_setscheduler() can fail in
> stop_machine: it calls sched_setscheduler() from insmod, which can
> have CAP_SYS_MODULE without CAP_SYS_NICE.
>
> This simply introduces a flag to allow us to disable the capability
> checks for internal callers (this is simpler than splitting the
> sched_setscheduler() function, since it loops checking permissions).
>   
What about?

int sched_setscheduler(struct task_struct *p, int policy,
		       struct sched_param *param)
{
	return __sched_setscheduler(p, policy, param, true);
}


int sched_setscheduler_nocheck(struct task_struct *p, int policy,
		               struct sched_param *param)
{
	return __sched_setscheduler(p, policy, param, false);
}


(With the appropriate transformation of sched_setscheduler -> __)

Better than scattering stray true/falses around the code.

    J
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