Re: [PATCH 0/3] Implement LANDLOCK_RESTRICT_SELF_NNP_ON_EXEC

[Justin Suess <utilityemal77@gmail.com>]

On Thu, Jul 09, 2026 at 12:22:40PM +0100, Simon McVittie wrote:
> On Thu, 09 Jul 2026 at 12:09:25 +0200, Mickaël Salaün wrote:
> > On Wed, Jul 08, 2026 at 09:39:24AM -0400, Justin Suess wrote:
> > > Consider a sandbox launcher that depends on a set-user-ID helper, such
> > > as launching applications through bubblewrap on distributions where
> > > unprivileged user namespaces are disabled and bwrap is installed
> > > set-user-ID root.
> 
> It's perhaps worth noting that the current version 0.11.2 of bubblewrap
> deprecates this mode of use (it will refuse to run while setuid unless
> that was explicitly enabled at compile-time), and the next release 0.12.0
> will also remove the ability to enable it at compile-time.
> 
> When bubblewrap was first written, having it be setuid was a necessary
> workaround for kernels/distros not letting it do its sandboxing job any
> other way; but now that unprivileged user namespaces are more widespread,
> its maintainers have come to the conclusion that when it's setuid,
> the risk of vulnerabilities that allow a root privilege escalation
> (CVE-2020-5291, CVE-2026-41163) is unacceptably high, so being able to
> make it setuid is no longer a good trade-off.
> 
> > > This flag also closes a gap for CAP_SYS_ADMIN callers of
> > > landlock_restrict_self(2) itself.  The no_new_privs/CAP_SYS_ADMIN
> > > requirement exists to keep set-user-ID programs from running confused
> > > inside a sandbox they do not expect.  However, a privileged process
> > > that enforces a domain without setting no_new_privs leaves that hole
> > > open for all of its descendants
> > 
> > In a nutshell, not setting NNP might be risky, even when not strictly
> > needed.  We might want to update the Landlock doc with that.
> 
> If the CAP_SYS_ADMIN caller is setuid or setcap, then it has been
> granted special privileges by the sysadmin or distro, and part of
> the "contract" between the sysadmin/distro and the setuid program is
> that setuid/setcap must only be set on executables that have taken
> responsibility for ensuring that they can't create insecure situations
> (for example bubblewrap always sets PR_SET_NO_NEW_PRIVS, unconditionally,
> for this reason).
> 
> A large part of why bubblewrap no longer supports being setuid is that
> its maintainers don't want it to have this heavy responsibility.
>
Understandable. I guess my usecase proposed was narrower than thought.
And we probably want to avoid creating risky confused deputy scenarios.

So now I'm leaning towards the LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS path,
dropping the "ON_EXEC" part as Mickaël proposed.

But I'm struggling to see where the value is added over the prctl call.
Sure atomic enforcement with the ruleset is nice, but not a huge value added
on it's own.

Perhaps the setting of this bit with a -1 ruleset_fd could enable auditing
of the (failed or successful) privilege acquisition for the domain via the
fcaps/suid/guid methods w/o setting NNP? (otherwise this would be no
different from a Landlock specific prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)
call).

And setting it with a non -1 ruleset_fd would set NNP *and* enable the
same auditing.

That way we are making this a "value-added" flag instead of just a landlock-flavored
prctl call.

This would work well with the LANDLOCK_PERM_CAPABILITY_USE proposal,
which allows us to audit both methods of acquiring new capabilities.
(the clone/unshare path, and the fcaps/setuid/setgid path).

What do you think Mickaël?

Justin
>     smcv