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

Lisa,

On Thursday, 27. July 2006 02:59, Andreas Gruenbacher wrote:
> 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!
>
> Therefore, there cannot be any file access control mechanisms which are
> neither defined by POSIX, nor additional or alternate.

I forgot to mention that an analogous case can be made for additional file 
access control mechanisms as well: POSIX applications cannot know which 
additional file access control mechanisms a particular implementation may 
support; they are free to rely on invariants under POSIX, such as a chmod 
from one mode to another, which should be a no-op. Additional file access 
control mechanisms must make sure that POSIX invariants will not change the 
permissions granted.

This contradicts with having a chmod write through to OWNER@, GROUP@, and 
EVERYONE@ ACEs: consider the same ACE we had before with a file mode of 0740 
(rwxr-----):

  OWNER@:READ_DATA/APPEND_DATA/EXECUTE::ALLOW

If a chmod to 0640 and back to 0740 writes through to the OWNER@ ACE, the 
entry will change to:

  OWNER@:READ_DATA/WRITE_DATA/APPEND_DATA/EXECUTE::ALLOW

So a POSIX application has added mask flags in a supposed invariant. That's 
also a security hole, but not one as severe as the first because the only 
mask flag affected is APPEND_DATA (I believe).

I have pointed this out before in Section 4.7. "Mode Changes and the OWNER@, 
GROUP@, and EVERYONE@ ACEs".

Andreas

-- 
Andreas Gruenbacher <agruen@suse.de>
Novell / SUSE Labs

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