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Message-Id: <201011101455.47455.sgrubb@redhat.com>
Date: Wed, 10 Nov 2010 14:55:47 -0500
From: Steve Grubb <sgrubb@...hat.com>
To: Kees Cook <kees@...ntu.com>
Cc: oss-security@...ts.openwall.com
Subject: Re: filesystem capabilities
On Wednesday, November 10, 2010 02:40:14 pm Kees Cook wrote:
> On Wed, Nov 10, 2010 at 02:18:33PM -0500, Steve Grubb wrote:
> > Not that I know of. The library cannot know what the application's threat
> > model is. The library can make it simple to access the correct things.
> > The following should be the model that I think solves the problem for
> > either setuid or fs based capabilities.
> >
> > Assuming you needed CAP_CHOWN:
> > capng_get_caps_process();
> >
> > switch (capng_have_capabilities(CAPNG_SELECT_CAPS)) {
> >
> > case CAPNG_FULL:
> > capng_clear(CAPNG_SELECT_BOTH);
> > capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED,
> >
> > CAP_CHOWN);
> >
> > if (capng_apply(CAPNG_SELECT_BOTH))
> >
> > exit(0);
> >
> > break;
> >
> > case CAPNG_PARTIAL:
> > // Paranoid double check that we have what we expect
> > if (capng_have_capability(CAPNG_EFFECTIVE, CAP_CHOWN)==0)
> >
> > exit(0);
> >
> > // Now to make sure that is ALL that we have...let's drop it and
> > // see if we are empty
> > capng_update(CAPNG_DROP, CAPNG_EFFECTIVE|CAPNG_PERMITTED,
> >
> > CAP_CHOWN);
> >
> > if (capng_have_capabilities(CAPNG_SELECT_CAPS) != CAPNG_NONE)
> >
> > exit(0);
> >
> > break;
> >
> > case CAPNG_FAIL:
> >
> > case CAPNG_NONE:
> > exit(0);
> >
> > }
> > // At this point both setuid and fs based caps should have the same
> > thing
>
> What about dropping setuid once it has the needed caps, etc? It seems like
> it'd be nice to have something like:
>
> validate_I_have_only_these_caps(CAP_WHATEVER);
Validating the caps is what I did in CAPNG_PARTIAL. But for setuid root apps, you
would have all capabilites. And that is why I have the switch statement above. In that
case, you would simply drop to the capabilities you needed.
> ...do stuff...
> drop_all_my_privs();
drop all privs is a 2 liner:
capng_clear(CAPNG_SELECT_CAPS);
if (capng_apply(CAPNG_SELECT_CAPS))
exit(0);
Not sure anything that small needs a library function.
> and those two routines could be in the cap library, and it would handle
> both fscaps and setuid style of priv escalation.
>
> > You can lead a horse to water, but you cannot make them drink.
> > http://lists.gnu.org/archive/html/bug-tar/2006-08/msg00004.html
> >
> > We did our part. Its up to them to accept the patch or keep talking about
> > it. Reading the thread, it sounded like they were going to take it. No
> > idea why they decided against it unless it was seen as a Linux only
> > patch. You might poke them and ask why in the last 4 years they never
> > took the patch. Of course since then we've maintained the patch against
> > current tar releases, so they would want a newer patch.
>
> Well, I have to disagree here: the job isn't done until it's upstream
> or it has been categorically rejected. At the time they had identified
> real crashes in the code, and were working on it. All this said, anyone can
> drive it, which is why I added the TODO item to Ubuntu's list of what was
> needed for fscaps, and it would be great to try to get it in again. Since
> the fscap push is higher on Fedora's list than Ubuntu's, would it be
> possible to try to push that patch upstream again?
I asked the maintainer if he's had any discussion lately.
> Has there been any discussion of making rsync, cp, and cpio default to
> copying xattrs and acls too? I know at least with rsync they are explicitly
> not included in the "-a" option. :(
My rsync man page shows a -X option and cp has a --preserve=xattr. cpio doesn't but no
one seems to have been missing that.
-Steve
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