Re: Re: NFSv4 ACL and POSIX interaction / mask, draft-ietf-nfsv4-acls-00 not ready

[Andreas Gruenbacher <agruen@suse.de>]

On Tuesday, 25. July 2006 06:26, Sam Falkner wrote:
> On Jul 24, 2006, at 6:32 PM, a.gruenbacher@computer.org wrote:
> > Looking at draft-ietf-nfsv4-minorversion1-04.txt, I disagree with
> > secion
> > 5.4.1. "Discussion of EVERYONE@".
>
> The fact that you disagree with it is not a flaw.  That section is
> very simple -- it says that EVERYONE@ means "everyone".
>
> If you are arguing that EVERYONE@ ACEs should be affected by the
> POSIX "other" class upon chmod(), then that's not in disagreement
> with 5.4.1.

Alright, fine then.

> > Section 5.5. "Mode Attribute" claims that
> > MODE4_RGRP, MODE4_WGRP, MODE4_XGRP apply to the principals
> > identified in the
> > owner_group attribute, while it's really the File Group Class
> > according to POSIX.
>
> Since when does POSIX define NFSv4?  RFC 3530 is the current
> authority on NFSv4, and it says that the mode attribute is based upon
> the UNIX mode -- seems sensible to me.

POSIX does not define NFSv4. The mode attribute directly maps to the POSIX 
file mode though, and I assume we have agreement on that.

My argument is that there is a conceptual difference between the MODE4_RGRP, 
MODE4_WGRP, MODE4_XGRP permission bits (which map to the POSIX File Group 
Class) and the owner_group permissions: this has to do with the 
classification of ACEs, and with the file access control mechanisms that 
POSIX allows.

RFC3530 does not go very deeply into the interactions between the mode 
attribute and POSIX. In Section 5.11.5. "Mode Attribute" RFC2530 defines that 
the MODE4_RGRP, MODE4_WGRP, MODE4_XGRP bits apply to the principals 
identified in the owner_group attribute. This is a less general definition 
than how POSIX defines File Permission Bits, which are part of the file mode:

[] File Permission Bits
[]
[] Information about a file that is used, along with other information, to
[] determine whether a process has read, write, or execute/search permission
[] to a file. The bits are divided into three parts: owner, group, and other.
[] Each part is used with the corresponding file class of processes. These
[] bits are contained in the file mode.

On POSIX systems that only support the POSIX File Permission Bits and no other 
file access control mechanisms, both definitions amount to the same. On 
systems that implement additional file access control mechanisms, the File 
Group Class permission bits are no longer necessarily identical with the 
permissions granted to the owning group: the definition of Additional File 
Access Control Mechanisms allows further restrictions to be imposed. In other 
words, the owning group may only be granted a subset of the File Group Class 
permissions.

You are arguing that users have been confused by this definition, and that 
changing the File Group Class permissions (=changing the MODE4_RGRP, 
MODE4_WGRP, MODE4_XGRP bits in the mode attribute) should automatically also 
change the owner_group permissions, and that this is how chmod behaves with 
POSIX ACLs on Solaris (and which differs from how implementations of POSIX 
1003.1e draft 17 (withdrawn) are supposed to behave). In Section 4.6. "Mode 
Changes and the OWNER@, GROUP@, and EVERYONE@ ACEs" of 
http://www.suse.de/~agruen/nfs4acl/mapping-nfsv4-acls-to-posix-00.html I am 
pointing out two reasons why the Solaris behavior is a really bad idea. (Note 
that these problems may well be the reason why Solaris appears to behave 
differently at the system call level.)

I would be sorry if you persisted in implementing the current Solaris chmod 
behavior in NFSv4, but in case you really do, this hack still does not belong 
into the NFSv4.1 specification, and I would in that case much prefer if you 
could just break Solaris, and not everybody else as well.

> > From that pretty much follows that the algorithm defined in Section
> > 5.6.1. is not POSIX compliant: the File Group Class permissions
> > must be a superset of the permissions of the permissions granted to all
> > ACEs for a who other than OWNER@ and EVERYONE@, and the 5.6.1. algorithm
> > does not guarantee this.
>
> You are arguing that NFSv4 should define MODE4_RGRP, MODE4_WGRP, and
> MODE4_XGRP to be something other than the UNIX mode bits.

No, this is a misunderstanding. I am arguing that the NFSv4 mode attribute map 
to the POSIX mode bits, in the precise semantics which POSIX assigns to these 
bits.

> The fact that you argue this does not make it a requirement.  As stated
> above, RFC 3530 defines these bits as reflecting the mode.

I have above argued within which limits the definition in RFC3530 is correct 
with respect to POSIX, and that this definition is not general enough as soon 
as ACLs, in the form of additional file access control mechanisms, are used 
on POSIX compliant systems.

> ACL is not a required attribute in NFSv4.  If we adopted your
> proposal, a client that supported mode but not ACL would be unable to
> set the mode of a file.

Not at all. Maybe you can explain what makes you think so. An application can 
set the mode with a mode SETATTR at any time, etc.

> But let's pretend that all NFSv4 clients do support the ACL
> attribute.  Having the chmod command unable to set the mode was a
> source of many customer complaints when that was the behavior in
> Solaris (http://xrl.us/pd7c and http://xrl.us/pd7b are just two
> examples of bugs).  The Solaris chmod command was fixed to alter the
> POSIX-draft ACL (in file systems that support POSIX-draft ACLs), so
> that chmod was actually able to change the mode.  Since making this
> change to chmod, complaints have stopped.

Maybe nobody explained to users how to properly use ACLs to prevent this from 
happening? The behavior of Solaris chmod(1) is a potential security hole, 
although a small one only.

> So, if we were to do as your proposed design says, and have
> MODE4_RGRP, etc., not reflect the mode, then once again, we would
> have to modify the chmod command to do more than the chmod() system
> call.  This is madness.

This is not what I am proposing at all.

> Or, we could simply have MODE4_RGRP, etc., reflect the mode of the file.

This *is* what I am proposing. A mode SETATTR also affects which permissions 
are effective in the ACL and which must be disabled, though. Both proposals 
disable permissions, but they do so using different mechanisms: your proposal 
introduces DENY ACEs spread throughout the ACL. My proposal sets masks, which 
are only applied in the access check algorithm. I argue that simply updating 
the masks is a much nicer way of achieving the same effect, and that it 
avoids the problems inherent to those DENY ACEs.

> > Algorithm 5.6.3. is at least problematic because it enforces implementing
> > masking of permissions using DENY ACEs, while an alternative design exists
> > that achieves the same effects without the disadvantages of those
> > DENY ACEs.
>
> That alternative introduces a new file attribute, which causes
> problems for NFSv4.0, and for Windows servers.

That's not true. There are several ways how we can deal with clients that do 
not understand masks:

 (1) we can apply the masks before exposing the ACL to such clients, or

 (2) we can introduce DENY ACEs where necessary, as per your proposal.

With (2), we are back to the problems with DENY ACEs: clients which reorder 
ACEs as (at least some versions of) Windows do will cause havoc. *But* we 
will never run into any such effects with clients that *do* understand masks, 
we don't have to bloat ACLs all the time, 

On the other hand, with (1), an ACL read-modify-write cycle will potentially 
lose some information, but the huge benefit is that the ACLs that clients 
will see will actually make some sense to them, instead of being littered 
with DENY ACEs that nobody will know how to deal with. Of course clients that 
understand masks will be fine as well.

With both mechanisms, knocking the right mask bits in place in the ACL will 
cause these bits to become effective (provided that other ACEs don't deny 
them earlier in the ACL).

> > The paragraphs below "3. To conform with POSIX" is lossy, even with the
> > six-entry ACL that "2. If there are at least six ACEs" would
> > create, which is a very unfavorable property.
>
> No, it is not lossy with the six-entry ACE in "2. ...".  That's
> because the six-entry ACE in "2." will be adjusted in the "3."
> immediately following "2.".
>
> In general, the algorithm can be lossy, when modes such as 077 are
> assigned to files having ACLs that grant read/write/execute to
> supplemental groups.

Exactly, the algorithm is lossy with supplemental groups. I misread that "3. 
To conform with POSIX" only applies to explicit groups. My point still stands 
though.

Where does the algorithm deal with explicit users?

Masks are not lossy per se, they will only become lossy when talking to 
clients that don't understand masks (and in those cases, the effects will 
never be worse than with DENY ACEs).

> > It also does not cover the case where the owner
> > is granted permissions from EVERYONE@ ACEs, or where users or
> > groups matching
> > user@domain or group@domain are granted permissions from EVERYONE@
> > ACEs.
>
> EVERYONE@ ACEs are covered.  Re-read the algorithm.

Ah, I see now, "3. To conform with POSIX" only refers to explicit groups.

> > I can't see a compelling reason for requiring the exact six-entry
> > ACL as specified in "2. If there are at least six ACEs".
>
> The goal of that step is to re-use the final six ACEs if possible, so
> that successive changes to the mode don't cause a growing ACL.

There is no reason for requiring this exact six-entry ACL other that that the 
algorithm you have chosen relies on this exact form. Other algorithms may be 
just fine with something completely different. I don't think that NFSv4.1 
should limit implementations in ways like this.

> > Finally, I have argued why it is wrong to have a mode SETATTR write
> > through to USER@, GROUP@, and EVERYONE@ ACEs.
>
> To not do so leaves you in the situation where a client that supports
> the mode attribute and not the ACL attribute cannot set the mode.

Not true. What makes you think so?

Andreas

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