linux-kernel - Re: [Patch v3] vfs: allow file truncations when both suid and write permissions set

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Message-ID: <877hxc6z8n.fsf@devron.myhome.or.jp>
Date:	Mon, 10 Aug 2009 13:59:04 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Amerigo Wang <amwang@...hat.com>
Cc:	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org,
	esandeen@...hat.com, eteo@...hat.com,
	linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, sds@...ho.nsa.gov,
	linux-security-module@...r.kernel.org
Subject: Re: [Patch v3] vfs: allow file truncations when both suid and write permissions set

Amerigo Wang <amwang@...hat.com> writes:

> OGAWA Hirofumi wrote:
>> Eric Paris <eparis@...hat.com> writes:
>>
>>   
>>>>> I was thinking about this and kept telling myself I was going to test v2
>>>>> before I ack/nak.  Clearly we shouldn't for the dropping of SUID if the
>>>>> process didn't have permission to change the ATTR_SIZE.
>>>>>
>>>>> Acked-by: Eric Paris <eparis@...hat.com>
>>>>>         
>>>> BTW, Do you know why doesn't security modules fix the handling of
>>>> do_truncate() (i.e. ATTR_MODE | ATTR_SIZE). And why doesn't it allow to
>>>> pass ATTR_FORCE for it?
>>>>       
>>> I'm not sure what you mean.  I understood ATTR_FORCE to mean 'I am magic
>>> and get to override all security checks."  Which is why nothing should
>>> ever be using ATTR_FORCE with things other than SUID.
>>>
>>> I guess we could somehow force logic into the LSM to make it only apply
>>> to SUID and friends but I'm not sure it buys us anything.
>>>     
>>
>> Yes, I think it's good way. Don't we want to do the following?
>>
>> 	if (permission check of job)
>> 		return error;
>> 	if (do job at once)
>>         	return error;
>>
>> But currently way is,
>>
>> 	if (permission check of first part)
>>         	return error
>> 	if (do first part of job)
>>         	return error
>> 	if (permission check of second part)
>>         	return error
>> 	if (do second part of job)
>>         	return error
>>
>> So, if second part was error, we may want to undo the job of first part
>> in theory. But, to undo is just hard and strange.
>>   
>
> Yeah, the problem is currently we don't have such wrappers, only 
> notify_change(). :-/

I'm not sure you are meaning what wrappers though, I'm still thinking
changing LSM (or something) like Eric said is the way to do it easily
(and define ATTR_FORCE is not for ATTR_SIZE at least).

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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