linux-kernel - Re: [PATCH] add some long-missing capabilities to fs

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Message-ID: <20090413215003.GB25085@us.ibm.com>
Date:	Mon, 13 Apr 2009 16:50:03 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Valdis.Kletnieks@...edu
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	mtk.manpages@...il.com, Stephen Smalley <sds@...ch.ncsc.mil>,
	Andrew Morgan <morgan@...nel.org>,
	linux-security-module@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>, linux-nfs@...r.kernel.org,
	Igor Zhbanov <izh1979@...il.com>,
	"J. Bruce Fields" <bfields@...i.umich.edu>, stable@...nel.org,
	linux-api@...r.kernel.org, Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH] add some long-missing capabilities to fs_mask

Quoting Valdis.Kletnieks@...edu (Valdis.Kletnieks@...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
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