Re: RFC: disablenetwork facility. (v4)

[Serge E. Hallyn]

Quoting Bryan Donlan (bdonlan@xxxxxxxxx):
> On Mon, Dec 28, 2009 at 3:55 PM, Pavel Machek <pavel@xxxxxx> wrote:
> > On Mon 2009-12-28 09:37:24, Valdis.Kletnieks@xxxxxx wrote:
> >> On Mon, 28 Dec 2009 11:10:06 +0100, Pavel Machek said:
> >>
> >> > a) make disablenetwork reset to "enablenetwork" during setuid exec
> >>
> >> That won't work either.  If you only make it 'setuid==0' binaries, you still
> >> break 'setuid-FOO' binaries that require the net. If you just check the setuid
> >> bit, it allows a trivial escape by creating a setuid-yourself binary and using
> >> that to exec something else (now with network access, because we apparently
> >> don't have a way to remember the previous setting).
> >
> >
> > it is really only required for binaries setuid to someone else, but
> > that would be too ugly. (Plus, as someone said, ping is great for
> > leaking data out.)
> 
> No, this is not sufficient; one needs only to find a setuid process
> that can be convinced to run a program with the original (pre-suid)
> privileges. For example, one could invoke gpg (older versions setuid
> so it can lock memory, executes user code for the passphrase input
> agent) or pulseaudio (in some cases setuid to go realtime, loads user
> plugins) or screen (setuid for sharing sessions, obviously executes
> user programs) or at/cron (did you remember to deny access to these?)
> ...
> 
> Or one can target a non-root setuid program that may have security
> holes - how about nethack?
> 
> While in modern distros these uses of setuid may be rare, they can
> exist, and under the old security model they were safe. Not so
> anymore. As such, re-enabling network access upon executing a setuid
> program is not acceptable.
> 
> That said, I do feel this is a separate issue. The process should
> first drop its ability to suid; then it can freely apply additional
> restrictions without there being any risk of breaking setuid
> applications.
> 
> In short, how does this sound:
> * Add an API to allow processes to permanently revoke their own
> ability to gain privileges from setuid-exec

Well, this is possible now, but requires privilege:  Remove
any bit not in pP from the bounding set.

Removing the requirement for privilege to do so has some conerns.  Do we
force a task to then run with absolutely no capabilities, or can it just
stop itself from gaining new ones?  If the latter, then we are close to
re-raising the sendmail-capabilities bug.  The main difference would be
that you must already have the capbilities you want to keep, but I'm
not convinced that's sufficient.

A function which can be called without privilege, which empties out
all capability sets and the bounding set, that may be safe.  Still might
cause a setuid-root app which is running as root but with no privilege
to be confused and mess up the system...

> * Add this disablenetwork facility, conditional on dropping
> setuid-exec abilities
> 
> This also paves the way for:
> * Allow processes that have dropped said suid ability to freely create
> new namespaces (and chroot)
> 
> Which, combined with doing whatever audits are necessary to allow
> cross-network-namespace uses of unix domain sockets, actually
> eliminates the need for the disablenetwork API. :)
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