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

[Michal Hocko <mhocko@suse.com>]

On Mon 11-05-26 09:39:32, Sasha Levin wrote:
> On Mon, May 11, 2026 at 03:07:51PM +0200, Michal Hocko wrote:
> > On Mon 11-05-26 04:41:38, Breno Leitao wrote:
> > > On Fri, May 08, 2026 at 05:47:04PM -0400, Sasha Levin wrote:
> > > > On Fri, May 08, 2026 at 01:56:30PM -0700, Andrew Morton wrote:
> > > > > On Thu,  7 May 2026 03:05:45 -0400 Sasha Levin <sashal@kernel.org> wrote:
> > > > >
> > > > > > When a (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
> > > > >
> > > > > It certainly sounds useful, but what would I know.  How do we hunt down
> > > > > suitable operations people (aka "target audience") to find out how
> > > > > useful this is to them?
> > > >
> > > > I'm not entierly sure here... If folks have suggestions on folks to loop in,
> > > > that'll be great!
> > > 
> > > I work with these issues at Meta, and this approach would address a real
> > > need we have.
> 
> Thanks for the feedback!
> 
> > > While livepatch could theoretically solve this problem, it's less suited
> > > for rapid mitigation for a couple of reasons:
> > > 
> > > 1) Livepatch rollout is inherently slower due to the blast radius if a
> > >    bug exists in the livepatch mechanism itself.
> > > 
> > > 2) It's common to run hundreds of different kernel versions across a
> > >    fleet. Since livepatch is kernel-specific, a single CVE suddenly
> > >    requires building and deploying hundreds of individual livepatches—
> > >    far less practical than a simple sysfs write.
> > 
> > LP is certainly a more laborous solution. I guess this is quite clear.
> > It is also much safer option as it deals with all implementation details
> > like consistency. All that is not done for fun. I am really wondering
> > how admins are expected to a) know which kernel functions are ok/safe to
> > disable and b) when it is safe to do so without introducing unsafe
> > kernel state or introduce an outright bug that way.
> 
> In a similar way to how they would know if a given livepatch is safe to apply -
> ideally it would be communicated by the vendor/distro/kernel team.

You have missed my point. KLP takes an extra steps to make sure patching
a particular function is safe to modify or to put the change into the
effect.

> "On Debian XX.YY, use the following command to mitigate CVE-AAAA-BBBB:
> 
>  echo "engage woops -1" > /sys/kernel/security/killswitch/control"
> 
> > Thiking about this I can see how waiting for an official LP can be time
> > consuming and sometimes creating those is far from trivial. But would it
> > make sense to have automated LP creation tooling available that would
> > allow to return early from a function and relly on the existing
> > infrastructure to do the right thing?
> 
> This would definitely help (and in light of how the last couple of weeks played
> out, the case for livepatches definitely increased), but not all
> vendors/distros provide livepatches.

The point I've tried to make is that you (as an admin) shouldn't depend
on your vendor to provide you with an official LP just to disable a
certain function(ality). That is/should be a trivial case where the LP
should be ideally generated automagically if you have a tooling
available. I might be wrong and overlook some complexity here.
-- 
Michal Hocko
SUSE Labs