From: "Serge E. Hallyn" <serue@us.ibm.com>
Subject: Re: [PATCH] add some long-missing capabilities to fs_mask
Date: Mon, 13 Apr 2009 16:50:03 -0500
Message-ID: <20090413215003.GB25085@us.ibm.com>
References: <20090413145614.GA15342@us.ibm.com> <13502.1239656608@turing-police.cc.vt.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	mtk.manpages@gmail.com, Stephen Smalley <sds@epoch.ncsc.mil>,
	Andrew Morgan <morgan@kernel.org>,
	linux-security-module@vger.kernel.org,
	lkml <linux-kernel@vger.kernel.org>, linux-nfs@vger.kernel.org,
	Igor Zhbanov <izh1979@gmail.com>,
	"J. Bruce Fields" <bfields@citi.umich.edu>, stable@kernel.org,
	linux-api@vger.kernel.org, Chris Wright <chrisw@sous-sol.org>
To: Valdis.Kletnieks@vt.edu
Return-path: <linux-security-module-owner@vger.kernel.org>
In-Reply-To: <13502.1239656608@turing-police.cc.vt.edu>
Sender: linux-security-module-owner@vger.kernel.org
List-ID: <linux-nfs.vger.kernel.org>

Quoting Valdis.Kletnieks@vt.edu (Valdis.Kletnieks@vt.edu):
> On Mon, 13 Apr 2009 09:56:14 CDT, "Serge E. Hallyn" said:
> > When POSIX capabilities were introduced during the 2.1 Linux
> > cycle, the fs mask, which represents the capabilities which having
> > fsuid==0 is supposed to grant, did not include CAP_MKNOD and
> > CAP_LINUX_IMMUTABLE.  However, before capabilities the privilege
> > to call these did in fact depend upon fsuid==0.
> 
> Wow. How did this manage to stay un-noticed for this long?

I guess setfsuid() is mainly used by NFS, and not a lot of people
do mknod over NFS?

To run into this, you'd have to do something like

	1. run as root
	2. setresuid(500,500,0);
	3. (...)
	4. setfsuid(0);
	5. mknod(path, mode, dev);

so I suspect the simpler (cross-platform) thing to do was
seteuid(0) for the mknod anyway...  Plus there is nowhere I've
found where the precise capabilities afforded to fsuid=0 are
documented, so noone would complain, they'd just accept it and
do seteuid()?

I'm guessing of course.

-serge