linux-kernel - Re: [PATCH -V6 00/26] New ACL format for better NFSv4 acl interoperability

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Date:	Mon, 12 Sep 2011 19:53:39 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	Valdis.Kletnieks@...edu,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	agruen@...nel.org, akpm@...ux-foundation.org, dhowells@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	LSM <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH -V6 00/26] New ACL format for better NFSv4 acl
 interoperability

On Mon, Sep 12, 2011 at 04:23:24PM -0700, Casey Schaufler wrote:
> On 9/12/2011 3:43 PM, J. Bruce Fields wrote:
> > On Mon, Sep 12, 2011 at 03:38:24PM -0700, Casey Schaufler wrote:
> >> On 9/12/2011 3:20 PM, J. Bruce Fields wrote:
> >>> On Mon, Sep 12, 2011 at 02:34:04PM -0700, Casey Schaufler wrote:
> >>>> On 9/7/2011 5:46 PM, Valdis.Kletnieks@...edu wrote:
> >>>>> On Mon, 05 Sep 2011 15:42:17 PDT, Casey Schaufler said:
> >>>>>> On 9/5/2011 10:25 AM, Aneesh Kumar K.V wrote:
> >>>>>>> The following set of patches implements VFS and ext4 changes needed to implement
> >>>>>>> a new acl model for linux. Rich ACLs are an implementation of NFSv4 ACLs,
> >>>>>>> extended by file( masks to fit into the standard POSIX file permission model.
> >>>>>>> They are designed to work seamlessly locally as well as across the NFSv4 and
> >>>>>>> CIFS/SMB2 network file system protocols.
> >>>>>> POSIX ACLs predate the LSM and can't be done as an LSM due to
> >>>>>> the interactions between mode bits and ACLs as defined by the
> >>>>>> POSIX DRAFT specification.
> >>> I don't know LSM so don't understand what you mean when you say that
> >>> interactions between mode bits and ACLs would make an ACL model hard to
> >>> implement as an LSM.
> >> POSIX ACLs require that the file permission bits change when
> >> the ACL changes. This interaction violates the strict "additional
> >> restriction" model of the LSM.
> > Oh, OK.  Yes, rich ACLs are the same as POSIX ACLs in this respect.
> > When you set an ACL the mode bits are reset to represent an "upper
> > bound" on the permissions granted by the ACL.
> 
> One of the areas in which the POSIX group was careful almost beyond
> reason was the program that uses chmod() judiciously in the absence
> of ACLs and how the presence of ACLs might result in a less secure
> situation. Thus, a program that does
>     stat(..., &buf);
>     chmod(..., 0);
>     chmod(..., buf.st_mode)
> 
> should get the exact same access at the end as it had at the beginning
> and the file must be completely inaccessible after the chmod(..., 0)
> regardless of the content of the ACL. Without this requirement the ACL
> scheme would have worked fine as an LSM. If rich ACLs can't make these
> claims, they aren't safe.

Yes, see the patches--rich ACLs have the same property, using a similar
mechanism.  (They're essentially windows/NFSv4 ACLs + mask bits.)

--b.
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