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

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Date:	Wed, 7 Aug 2013 09:58:09 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	Chen Gang <gang.chen@...anux.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 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().

I suspect that changing this without introducing security or other
bugs is impossible.  If someone wanted to add new_setfsuid that
returned an error when it failed, that would be a different story.
(I'm not going to do that myself.)

>
> And perhaps the man page should be changed. Add Michael.

Agreed.  The text is a bit odd.

>
> Oleg.
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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