linux-kernel - Re: [PATCH 1/2] kernel/sys.c: return the current gid when error occurs

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Date:	Thu, 08 Aug 2013 09:48:06 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Al Viro <viro@...iv.linux.org.uk>, holt@....com,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] kernel/sys.c: return the current gid when error occurs

On 08/08/2013 09:35 AM, Andy Lutomirski wrote:
> On Wed, Aug 7, 2013 at 6:30 PM, Chen Gang <gang.chen@...anux.com> wrote:
>> On 08/08/2013 12:58 AM, Andy Lutomirski wrote:
>>> On Wed, Aug 7, 2013 at 9:21 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>>>> On 08/06, Andy Lutomirski wrote:
>>>>>
>>>>> I assume that what the man page means is that the return value is
>>>>> whatever fsgid was prior to the call.  On error, fsgid isn't changed, so
>>>>> the return value is still "current".
>>>>
>>>> Probably... Still
>>>>
>>>>         On success, the previous value of fsuid is returned.
>>>>         On error, the current value of fsuid is returned.
>>>>
>>>> looks confusing. sys_setfsuid() always returns the old value.
>>>>
>>>>> (FWIW, this behavior is awful and is probably the cause of a security
>>>>> bug or three, since success and failure are indistinguishable.
>>>>
>>>> At least this all looks strange.
>>>>
>>>> I dunno if we can change this old behaviour. I won't be surprized
>>>> if someone already uses setfsuid(-1) as getfsuid().
>>>
>>
>> Oh, really it is.
>>
>> Hmm... as a pair function, we need add getfsuid() too, if we do not add
>> it, it will make negative effect with setfsuid().
>>
>> Since it is a system call, we have to keep compitable.
>>
>> So in my opinion, better add getfsuid2()/setfsuid2() instead of current
>> setfsuid()
> 
> How about getfsuid() and setfsuid2()?
> 

Hmm... I have 2 reasons, please check.

1st reason: I checked history (just like Kees Cook suggested),
getfsuid() is mentioned before (you can google to find it), so need use
getfsuid2() to bypass the history complex.

And 2nd reason: getfsuid() seems more like the pair of setfsuid(), not
for setfsuid2().



> --Andy
> 
> 

Thanks.
-- 
Chen Gang
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