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

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Message-ID: <87prb7v0dr.fsf@devron.myhome.or.jp>
Date:	Sat, 08 Aug 2009 05:23:44 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Eric Paris <eparis@...hat.com>
Cc:	Amerigo Wang <amwang@...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
Subject: Re: [Patch v3] vfs: allow file truncations when both suid and write permissions set

Eric Paris <eparis@...hat.com> writes:

> On Fri, 2009-08-07 at 06:05 -0400, Amerigo Wang wrote:
>> V2 -> V3:
>> Call notify_change() before clearing suid/sgid.
>> Thanks to OGAWA Hirofumi.
>> 
>> V1 -> V2:
>> Introduce dentry_remove_suid(), and use it in do_truncate().
>> Thanks to Eric Paris.
>> 
>> 
>> When suid is set and the non-owner user has write permission,
>> any writing into this file should be allowed and suid should be
>> removed after that.
>> 
>> However, current kernel only allows writing without truncations,
>> when we do truncations on that file, we get EPERM. This is a bug.
>> 
>> Steps to reproduce this bug:
>> 
>> % ls -l rootdir/file1
>> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
>> % echo h > rootdir/file1
>> zsh: operation not permitted: rootdir/file1
>> % ls -l rootdir/file1
>> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
>> % echo h >> rootdir/file1
>> % ls -l rootdir/file1
>> -rwxrwxrwx 1 root root 5 Jun 25 16:34 rootdir/file1
>> 
>> This patch fixes it.
>> 
>> Signed-off-by: WANG Cong <amwang@...hat.com>
>> Cc: Eric Sandeen <esandeen@...hat.com>
>> Cc: Eric Paris <eparis@...hat.com>
>> Cc: Eugene Teo <eteo@...hat.com>
>> Cc: Al Viro <viro@...iv.linux.org.uk>
>> Cc: hirofumi@...l.parknet.co.jp
>
> 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?

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