linux-kernel - Re: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to CAP_FS

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Date:	Mon, 16 Mar 2009 18:54:24 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Igor Zhbanov <izh1979@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	neilb@...e.de, Trond.Myklebust@...app.com,
	David Howells <dhowells@...hat.com>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	Miklos Szeredi <miklos@...redi.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: VFS, NFS security bug? Should CAP_MKNOD and
	CAP_LINUX_IMMUTABLE be added to CAP_FS_MASK?

On Mon, Mar 16, 2009 at 12:04:33PM -0500, Serge E. Hallyn wrote:
> Quoting J. Bruce Fields (bfields@...ldses.org):
> > If filesystem permissions similarly never affected the ability to create
> > device nodes, that might also be an argument against including
> > CAP_MKNOD, but it would be interesting to know the pre-capabilities
> > behavior of a uid 0 process with fsuid non-0.
> 
> The sentiment rings true, but again since before capabilities, privilege
> was fully tied to the userid, the question doesn't make sense.  Either
> you had uid 0 and could mknod, or you didn't and couldn't.  And that is
> the behavior which we unfortunately have to emulate when
> !issecure(SECURE_NOROOT|SECURE_NOSUIDFIXUP).

The historical behavior of setfsuid() is still interesting, though.
>From a quick glance at Debian's code for the (long-neglected) userspace
nfsd server, it looks like it depends on setfsuid() and the kernel to
enforce permissions for operations (including mknod).  Might be
interesting to confirm whether it has the same problem, and if so,
whether that was a problem introduced with some capability changes or
whether it always existed.

--b.
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