Re: SELinux and access(2), we want to know.

[Stephen Smalley <sds@tycho.nsa.gov>]

On Fri, 2009-05-08 at 08:27 -0400, Stephen Smalley wrote:
> On Thu, 2009-05-07 at 20:51 -0700, Casey Schaufler wrote:
> > Serge E. Hallyn wrote:
> > > Quoting Eric Paris (eparis@redhat.com):
> > >   
> > >> 3) I've also heard it hinted that we could do this with audit by just
> > >> having audit drop the denials that include the access(2) syscall and the
> > >> scontext and tcontext for the slew of things the SELinux policy writers
> > >> know we are not interested in.  And while it seems good, now we have
> > >>     
> > >
> > > What is the difference whether an attacker does access(2) to check for
> > > /etc/shadow rights, or does a failed open()?
> > >   
> > 
> > I have been studiously ignoring the discussions on the SELinux list because
> > in the end it really doesn't matter, as Serge (appears to) point out here.
> > The access() system call was a major thorn in the side of the POSIX security
> > working group because its behavior is not really very rational. By design
> > it does not take into account read-only file systems, ACLs, MAC labels,
> > TOMOYO policy, or anything other than the mode bits. A successful return
> > from access() gives you no assurance whatever that if you actually try the
> > operation it will succeed. My recollection is that every version of
> > "trusted unix" written treats the system call the same way it would a
> > call to lstat(), because that's really all it is doing.
> 
> Casey, please go read the access(2) / faccessat(2) code in Linux and
> then come back to the discussion.  It does in fact take into account all
> of those things presently (and notes in a comment that SuS v2 requires
> that it report a read-only fs).

...and it is precisely for this reason that programs like nautilus use
access(2) rather than lstat(2) in order to determine accessibility of
the files.  As do other things like the kerberos libraries.  So we can't
just change the behavior to fit your model of access(2) only checking
mode bits.

-- 
Stephen Smalley
National Security Agency