linux-kernel - Re: [NFS] [PATCH 2/7] NFS: if ATTR_KILL_S*ID bits are set, then skip mode change

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Date:	Fri, 14 Sep 2007 09:38:46 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Greg Banks <gnb@....com>
Cc:	reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	ecryptfs-devel@...ts.sourceforge.net, nfs@...ts.sourceforge.net,
	linux-fsdevel@...r.kernel.org, unionfs@...esystems.org,
	linux-cifs-client@...ts.samba.org
Subject: Re: [NFS] [PATCH 2/7] NFS: if ATTR_KILL_S*ID bits are set, then
 skip mode change

On Fri, 14 Sep 2007 23:09:24 +1000
Greg Banks <gnb@....com> wrote:

> On Fri, Sep 14, 2007 at 07:02:58AM -0400, Jeff Layton wrote:
> > On Fri, 14 Sep 2007 20:25:45 +1000
> > Greg Banks <gnb@....com> wrote:
> > 
> > > I'm curious about the reasons behind this change.  You mention
> > > credential issues; how exactly is it that you have the correct creds
> > > to perform a WRITE rpc but not a SETATTR rpc?
> > > 
> > 
> > Consider this case. user1 and user2 are both members of group
> > "allusers":
> > 
> > user1$ echo foo > foo
> > user1$ chgrp allusers foo
> > user1$ chmod 04770 foo
> > user2$ echo bar >> foo
> > 
> > On most local filesystems, this would work correctly. The end result
> > would be a file with mode 0770 and the expected contents. On NFS
> > though, the write by user2 fails. When the write is attempted, the
> > kernel tries to squash the setuid bit using the credentials of user2,
> > who's not allowed to change the mode. The write then fails because the
> > setattr fails.
> 
> Ok, I ran an experiment and I see this failure mode.
> 
> So the SETATTR rpc is really a side effect of the client kernel's
> behaviour and not an operation directly requested by the user process
> on the client.  Is there any reason why that rpc needs to have user2's
> creds?  Why not do the rpc with a fake set of creds with uid and gid
> set to the uid and gid of the file, in this case user1/allusers ?
> That way the rpc will most likely pass the server's permission check.
> 

That might work in some cases, but there are many where it wouldn't...

Suppose user1 here is root and all of the user1 operations are being
done on the server. If the server has root squashing enabled, then
user2's operation would still fail.

Another problem:

Suppose we're using gssapi. There's no guarantee that the client will
have the proper credentials to fake up a call as user1 (you might need
user1 krb5 tickets, etc).

-- 
Jeff Layton <jlayton@...hat.com>
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