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 12:05:20 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
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>
Subject: Re: VFS, NFS security bug? Should CAP_MKNOD and
	CAP_LINUX_IMMUTABLE be added to CAP_FS_MASK?

Quoting J. Bruce Fields (bfields@...ldses.org):
> On Mon, Mar 16, 2009 at 12:36:11PM -0400, bfields wrote:
> > That may be reasonable, but I'd like to see clearer criteria for
> > choosing those.  Some considerations:
> > 
> > 	1. As capabilities(7) says, we must "preserve the traditional
> > 	   semantics for transitions between 0 and non-zero user IDs".
> > 	   The setfsuid() interface predates capabilities, so the
> > 	   introduction of capabilities shouldn't have changed the
> > 	   behavior of a program written in ignorance of capabilities.
> > 	2. Users of the interface (like nfsd!) would be less likely to
> > 	   make mistakes if we had a simpler, more conceptual
> > 	   description of CAP_FS_MASK than just "the following list of
> > 	   capabilities".
> > 	3. If there's a possibility new capabilities will be added again
> > 	   in the future, then we should document CAP_FS_MASK in a way
> > 	   that makes it clear how those new bits will be treated.
> > 	4. We must fix nfsd in any case, either by changing the nfsd
> > 	   code or CAP_FS_MASK, but we should err on the side of not
> > 	   changing CAP_FS_MASK, for obvious backwards-compatibility
> > 	   reasons.
> 
> Also, thinking of the nfsd case: it violates the principal of least
> surprise if dropping CAP_FS_MASK still leaves it possible to make a
> change to the filesystem that would normally require special
> privileges....

Agreed, and so between that and the labeled nfs work, I think we
should add all 4 capabilities to both the CAP_FS_MASK and CAP_NFSD_MASK.

-serge
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