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

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Date:	Mon, 10 Aug 2009 10:30:06 +0800
From:	Amerigo Wang <amwang@...hat.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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

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(). :-/

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