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

[Lisa Week <Lisa.Week at Sun.COM>]

Hi Andreas,

Some initial comments inline...

Thanks,
Lisa

Andreas Gruenbacher wrote:

> Lisa,
>
> On Tuesday, 11. July 2006 08:50, Lisa Week wrote:
>  
>
> > POSIX defines Additional Access Control Mechanism as follows:
> > "An implementation-defined mechanism that is layered upon the access
> > control mechanisms defined here, but which do not grant permissions
> > beyond those defined herein, although they may further restrict them."
> >
> > POSIX defines Alternate Access Control Mechanisms are as follows:
> > "An implementation-defined mechanism that is independent of the
> > access control mechanisms defined herein, and which if enabled on a
> > file may either restrict or extend the permissions of a given user.
> > IEEE Std 1003.1-2001 defines when such mechanisms can be enabled and
> > when they are disabled."
> >
> > So, the question that comes to my mind when reading that text is:
> > What does it mean when they say "access mechanisms defined herein" in
> > both of those definitions?  Well, the access mechanisms defined in
> > the POSIX spec are the "File Access Permissions", right?  So, can we
> > agree that "access mechanisms defined herein" would be the "File
> > Access Permissions"?
> >    
>
> the "access mechanisms defined herein" refer to all file access control 
> mechanisms that POSIX defines. This includes permissions always granted to 
> the owner such as chmod, but also mechanisms such as the restricted deletion 
> flag bit (S_ISVTX) for directories, etc.
>
> Trust me, POSIX is very careful about wording, and if "access mechanisms 
> defined herein" did indeed refer to the file access permissions, then POSIX 
> would say exactly that.
>
>  
>
> > As documented in the algorithm definition in section 3.16.6.3 -
> > "Applying a Mode to an Existing ACL" of the minor version doc, the
> > following is a clarification of the classification:
> >
> > ALLOW ACEs with ACE4_READ_DATA/ACE4_LIST_DIRECTORY, ACE4_WRITE_DATA/
> > ACE4_ADD_FILE, ACE4_APPEND_DATA/ACE4_ADD_SUBDIRECTORY, and
> > ACE4_EXECUTE access mask bits are Alternate security mechanisms.
> > These access mask bits will be disabled upon chmod.
> >
> > DENY ACEs with those access mask bits are Additional access control
> > mechanisms.  These access mask bits will be left unchanged  after a
> > chmod, unless, as described in 3.16.6.3 it is one of the special
> > identifiers, OWNER@, GROUP@, EVERYONE at .
> >    
>
> No, that's still messed up. We don't care about permissions that are not 
> granted, so we *could* only look at ALLOW ACEs, but this distinction is not 
> at all relevant.
>  
What?  I don't understand how you can say that "we don't care about 
permissions that are not granted".  We definitely care about permissions 
that are explicitly not granted. 

Also, I believe it is relevant to make a distinction between ALLOW and 
DENY ACEs.  Here's why:

The reasoning behind classifying those DENY ACEs as additional access 
control mechanisms are 1.) because they further restrict the permissions 
of users and groups (which is what additional does by definition) and 
2.) we don't want them to have to be disabled upon chmod.  Try to think 
about it in a way that you have just set an explicit entry to deny 
certain permissions to a supplemental user or group.  It is important 
that those users or groups don't have their permissions elevated after 
chmod.

For example, if you have an ACL such as this:

fred:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::DENY
OWNER@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW
GROUP@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW
EVERYONE@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW


The mode would be 777.  User fred is denied the ability to read, write 
and execute because of the explicit deny ACE.  After chmod, fred should 
still not be able to read, write and execute.  That is the goal that we 
were trying to meet.

I realize that the mapping that you describe in section 4.4 of 
http://www.suse.de/~agruen/nfs4acl/mapping-nfsv4-acls-to-posix-00.html 
would also give us the capabilities that  I have described above.

With that said, I will go on to describe the capabilities that I feel 
the mapping in section 4.4 lacks.  The capabilities that it lacks is 
where the relevance of having those ALLOW ACEs that I described before 
be considered Alternate access control mechanisms comes in.

The mapping in section 4.4 makes it impossible to do the following by 
*only* modifying the ACL.

The mode of the file is 644 and you want to give fred the ability to 
read, write and execute.

fred:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW
OWNER@:READ_DATA/WRITE_DATA/APPEND_DATA::ALLOW
OWNER@:EXECUTE::DENY
GROUP@:READ_DATA::ALLOW
GROUP@:WRITE_DATA/APPEND_DATA/EXECUTE::DENY
EVERYONE@:READ_DATA::ALLOW
EVERYONE@:WRITE_DATA/APPEND_DATA/EXECUTE::DENY

With the classification in 4.4, you can set that ACL, but it won't take 
effect until you also explicitly modify the file group class permissions 
(via a mask or whatever).

The capability to set the type of ACL that is above was the goal when 
classifying ALLOW ACEs as Alternate security access control mechanisms.  
We can do this because as Alternate is defined, it can extend the 
permissions of a user.  Therefore, setting the ACL attribute is all that 
is needed for this to take effect.

---

I am trying to figure out why there has been such a disagreement here 
and perhaps this is another thing that needs to be cleared up.  What 
does the disabling of an alternate access control mechanism mean?  
Certainly it is not that we have to get rid of everything that is in an 
ALLOW ACE every time a chmod is done.

If you had the following ACL:

fred:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW
OWNER@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW
GROUP@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW
EVERYONE@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW

The mode would be 777.  Disabling can be meant that after the chmod, the 
user "fred" should not have more permissions that his respective class.  
So, after a chmod to 770, fred should still be allowed the ability to 
read, write and execute.  This is in how the permissions for fred are 
disabled.  They are being masked down to what the permissions of the 
file group class are.

> What counts is which access mask flags are equivalent to the POSIX Read, 
> Write, and Execute permissions, and which are not: those are the access mask 
> flags which may (sometimes) be used as additional access control mechanisms.
>  

What do you mean by sometimes.

> The other issue is which POSIX file class each ACE maps to. Let's assume for 
> now that we only know that OWNER@ maps to the File Owner Class to keep this 
> example short.
>
> Together with these two classifications, we can determine whether an ACE is 
> within the bounds of an additional file access control mechanism. Let's look 
> at the following ACE, and a file which has a file mode of 0740 (rwxr-----):
>
>  OWNER@:READ_DATA/APPEND_DATA/EXECUTE::ALLOW
>
>
> The OWNER@ ACE maps to the File Owner Class. This means that as an additional 
> access control mechanism, the ACE may not grant more than Read, Write, and 
> Execute. The READ_DATA access mask flag is equivalent to POSIX Read, the 
> APPEND_DATA access mask flag is a subset of POSIX Write, and the EXECUTE 
> access mask flag is equivalent to POSIX Execute, so we are fine.
>
> Now, let's assume that the user does a chmod to mode 0640 (rw-r-----). The 
> permissions of the File Owner Class now no longer cover the EXECUTE 
> permission. There are two options to fix this problem:
>
> 1. either accept that the ACE cannot be part of an additional file
>    access control mechanism, or
>
> 2. disable the EXECUTE access mask flag, either temporarily or
>    permanently, which gets us back into the bounds of additional file
>    access control mechanisms.
>
> In case 1, the only choice that remains is that the ACE must be part of an 
> alternate file access control mechanism (see more on that below). POSIX 
> states that we must disable alternate file access control mechanisms upon 
> chmod and for new files, and they must only be re-enabled per explicit user 
> action. So the EXECUTE permission would be just gone until the user 
> explicitly re-enables it (with something other than a chmod). This would 
> render ACLs practically useless under POSIX.
>  

Why wouldn't the EXECUTE permission be able to be re-enabled with 
chmod?  Of course, it can.  If you are setting a mode of 740, that says 
that the owner can execute.  That is an explicit action and EXECUTE 
permission should be granted to the owner.

> In case 2, the ACE still remains within the bounds of additional file access 
> control mechanisms. If we disable EXECUTE only temporarily, then a future 
> chmod back to mode 0740 can simply re-enable EXECUTE, and we will not have 
> lost anything. (Remember, chmod does not disable additional file access 
> control mechanisms.)

> There are several possibilities how we can disable access mode flags 
> temporarily:
>
> 1. we could add an explicit DENY ACE before the ALLOW ACE that has
>    the affected access mask flags disabled (this is the
>    draft-ietf-nfsv4-acls-00 approach),
>
> 2. we could declare that all access mask flags which go beyond
>    the corresponding File Class are automatically disabled, or
>
> 3. we could introduce file masks as I am proposing.
>
> The file masks have the advantage that the user can easily manually enable 
> more access mask flags in them. This would construe an explicit user action 
> as per POSIX. So depending on which flags the user enables, that may amount 
> to an additional or alternate file access control mechanism. POSIX is 
> perfectly fine with that, even for enabling flags such as WRITE_OWNER. The 
> only thing we must ensure is that alternate flags are disabled upon chmod as 
> well as for new files.
>
>  
>
> > All other access mask bits listed below are outside of the access
> > control mechanisms specified by POSIX and are therefore unclassified
> > and unrestricted:
> > ACE4_READ_NAMED_ATTRS
> > ACE4_WRITE_NAMED_ATTRS
> > ACE4_DELETE_CHILD
> > ACE4_READ_ATTRIBUTES
> > ACE4_WRITE_ATTRIBUTES
> > ACE4_DELETE
> > ACE4_READ_ACL
> > ACE4_WRITE_ACL
> > ACE4_WRITE_OWNER
> > ACE4_SYNCHRONIZE
> >
> > This means that if an ACE defines one of those access mask bits, the
> > access granted or denied will remain unchanged after a chmod.
> >    
>
> I am afraid this is impossible: there is no such thing as being outside the 
> file access control mechanisms defined by POSIX. The reason is simple: POSIX 
> applications absolutely *must* be able to rely on POSIX semantics in order to 
> be able to keep the system secure.
>
> Let's assume that ACE4_WRITE_OWNER was indeed outside of the access control 
> mechanisms defined by POSIX as a counter example. A POSIX application does a 
> chmod to mode 0600 (rw-------) in order to restrict access to the file owner. 
> After that, the application can perfectly well write information that is 
> confidential to the owner into the file -- after all, only the owner will 
> have access, right?
>  
>
> Well, not quite, because an ACL might still grant ACE4_WRITE_OWNER to some 
> other user on the system. That other user could simply take over ownership of 
> the file, and do whatever he wants with it. Here we have it, a perfect 
> security hole; all POSIX applications potentially broken!
The owner of the file had taken *explicit* action to grant 
ACE4_WRITE_OWNER to another user on the system.  You say that it is a 
security hole if the user that was explicitly granted the ability to 
write owner is now able to do what they were told they could do?

> Therefore, there cannot be any file access control mechanisms which are 
> neither defined by POSIX, nor additional or alternate.
>
> By definition, pure POSIX applications have no idea which additional or 
> alternate file access control mechanisms a particular POSIX implementation 
> may define,


> so they cannot know about such sideband mechanisms, and they also 
> have no way of disabling them.
>
> Andreas
>
>  

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