[Ocfs2-devel] [RFC] The reflink(2) system call v4.

[Stephen Smalley]

On Wed, 2009-05-13 at 23:57 -0400, Andy Lutomirski wrote:
> Joel Becker wrote:
> > +
> > +Preserving the security context of the source file obviously requires
> > +the privilege to do so.  Callers that do not own the source file and do
> > +not have CAP_CHOWN will get a new reflink with all non-security
> > +attributes preserved; the security context of the new reflink will be
> > +as a newly created file by that user.
> > +
> 
> There are plenty of syscalls that require some privilege and fail if the 
> caller doesn't have it.  But I can think of only one syscall that does 
> *something different* depending on who called it: setuid.
> 
> Please search the web and marvel at the disasters caused by setuid's 
> magical caller-dependent behavior (the sendmail bug is probably the most 
> famous [1]).  This proposal for reflink is just asking for bugs where an 
> attacker gets some otherwise privileged program to call reflink but to 
> somehow lack the privileges (CAP_CHOWN, selinux rights, or whatever) to 
> copy security attributes, thus exposing a link with the wrong permissions.
> 
> Would it really be that hard to have two syscalls, or a flag, or 
> whatever, where one of them preserves all security attributes and 
> *fails* if the caller isn't allowed to do that and the other one makes 
> the caller own the new link?
> 
> 
> [1] http://www.cs.berkeley.edu/~daw/papers/setuid-usenix02.pdf

Yes, I agree - the selection of whether or not to preserve the security
attributes should be an explicit part of the kernel interface.  Then the
application still has the freedom to fall back on the non-preserving
form of the call if that is truly what it wants.

-- 
Stephen Smalley
National Security Agency