linux-kernel - Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

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Date:	Wed, 18 Feb 2015 20:50:21 +0100
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Guenter Roeck <linux@...ck-us.net>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

On 18.02.2015 20:38, Guenter Roeck wrote:
> On Tue, Feb 17, 2015 at 03:15:11PM -0800, Andrew Morton wrote:
>> On Tue, 17 Feb 2015 20:01:38 +0100 Heinrich Schuchardt <xypron.glpk@....de> wrote:
>>
>>> PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
>>> THREAD_SIZE.
>>>
>>> E.g. architecture hexagon may have page size 1M and thread size 4096.
>>>
>>> This would lead to a division by zero.
>>>
>>> The futex implementation assumes that tids fit into the FUTEX_TID_MASK.
>>> This limits the number of allowable threads.
>>>
>>> ...
>>>
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -74,6 +74,7 @@
>>>  #include <linux/uprobes.h>
>>>  #include <linux/aio.h>
>>>  #include <linux/compiler.h>
>>> +#include <linux/math64.h>
>>>  
>>>  #include <asm/pgtable.h>
>>>  #include <asm/pgalloc.h>
>>> @@ -255,6 +256,8 @@ void __init __weak arch_task_cache_init(void) { }
>>>  
>>>  void __init fork_init(unsigned long mempages)
>>>  {
>>> +	u64 temp;
>>
>> That's a really poor name.  We should always try to make names
>> meaningful.  Here, something like "threads" would be better.
>>
>>>  #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
>>>  #ifndef ARCH_MIN_TASKALIGN
>>>  #define ARCH_MIN_TASKALIGN	L1_CACHE_BYTES
>>> @@ -273,7 +276,16 @@ void __init fork_init(unsigned long mempages)
>>>  	 * value: the thread structures can take up at most half
>>>  	 * of memory.
>>>  	 */
>>> -	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
>>> +	temp = div64_u64((u64) mempages * (u64) PAGE_SIZE,
>>> +			 (u64) THREAD_SIZE * 8UL);
>>> +
>>> +	/*
>>> +	 * The futex code assumes that tids fit into the FUTEX_TID_MASK.
>>> +	 */
>>> +	if (temp < FUTEX_TID_MASK)
>>> +		max_threads = temp;
>>> +	else
>>> +		max_threads = FUTEX_TID_MASK;
>>
>> Seems rather complicated.  How about
>>
>> 	max_threads = mempages / (8 * THREAD_SIZE);
> 
> Apparently it is possible that
> 	mempages < 8 * THREAD_SIZE
> which would result in max_threads == 0.
> 
> How about the following ?
> 
> 	max_threads = mempages / DIV_ROUND_UP(8 * THREAD_SIZE, PAGE_SIZE);

For PAGE_SIZE 2MB and THREAD_SIZE 4kB this is wrong by factor 64.

Best regards

Heinrich

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