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

[Sasha Levin <sashal@kernel.org>]

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