Re: [PATCH][RFC] 2.5.44 (1/2): Filesystem capabilities kernel patch

[Andreas Gruenbacher <agruen@suse.de>]

On Tuesday 29 October 2002 00:36, chris@scary.beasts.org wrote:
> On Mon, 28 Oct 2002, Olaf Dietsche wrote:
> > Solving the last issue (checking access to the capabilities database)
> > involves filesystem support, I guess. So, this will be the next step
> > to address.
> >
> > If you're careful with giving away capabilities however, this patch
> > can make your system more secure as it is. But this isn't fully
> > explored, so you might achieve the opposite and open new security
> > holes.
>
> Have you checked how glibc handles an executable with filesystem
> capabilities? e.g. can an LD_PRELOAD hack subvert the privileged
> executable?
> I'm not sure what the current glibc security check is, but it used to be
> simple *uid() vs. *euid() checks. This would not catch an executable with
> filesystem capabilities.
> Have a look at
> http://security-archive.merton.ox.ac.uk/security-audit-199907/0192.html

It seems an additional mechanism is needed to prevent LD_PRELOAD from loading 
non-standard libraries for executables that are not suid/sgid, if those 
executables have any effective or permitted capabilities that the calling 
process doesn't have already.

This shouldn't be too hard; perhaps Ulrich has an opinion on that.

> I think the eventual plan was that we pass the kernel's current->dumpable
> as an ELF note. Not sure if it got done. Alternatively glibc could use
> prctl(PR_GET_DUMPABLE).

Sorry, I don't know exactly what was your plan here. Could you please explain?

A perhaps unrelated note: We once had Pavel Machek's elfcap implementation, in 
which capabilities were stored in ELF. This was a bad idea because being able 
to create executables does not imply the user is capable of CAP_SETFCAP, and 
users shouldn't be able to freely choose their capabilities :-] We still want 
to be able to grant additional capabilities to a binary that are not owned by 
root though. Extended attributes to overcome this limitation.

There also has to be a mechanism to drop capabilities off binaries if they are 
written to (on write or perhaps on open).

The final goal would be the `incapable root user', i.e., we would not give 
suid root binaries any capabilities except those that are explicitly defined.

--Andreas.