Re: [PATCH v2] killswitch: add per-function short-circuit mitigation primitive

[Sasha Levin]

On Sat, May 09, 2026 at 02:02:24PM +0200, Florian Weimer wrote:
> * Sasha Levin:
>
> > When a kernel (security) issue goes public, fleets stay exposed until a patched
> > kernel is built, distributed, and rebooted into.
> >
> > For many such issues the simplest mitigation is to stop calling the buggy
> > function. Killswitch provides that. An admin writes:
> >
> >     echo "engage af_alg_sendmsg -1" \
> >         > /sys/kernel/security/killswitch/control
> >
> > After this, af_alg_sendmsg() returns -EPERM on every call without
> > running its body. The mitigation takes effect immediately, and is dropped on
> > the next reboot -- by which point a patched kernel is hopefully in place.
>
> Do you expect this to be safe to enable in kernel lockdown mode (i.e.,
> with typical Secure Boot configurations in distributions)?

Yes: under lockdown, killswitch has to be configured on the cmdline. Runtime
engage is gated on the new LOCKDOWN_KILLSWITCH reason.

I do need to resend a v3 that also gates disengage and the retval write so a
cmdline-installed mitigation is fixed for the boot - I didn't think about that
scenario.

--
Thanks,
Sasha