Re: [PATCH v3] killswitch: add per-function short-circuit mitigation primitive

[Sasha Levin <sashal@kernel.org>]

On Tue, May 19, 2026 at 02:13:26PM +0200, Daniel Borkmann wrote:
>On 5/19/26 1:59 AM, Song Liu wrote:
>>On Mon, May 18, 2026 at 6:33 AM Sasha Levin <sashal@kernel.org> wrote:
>>>On Sun, May 17, 2026 at 11:37:36PM -0700, Song Liu wrote:
>>>>On Sun, May 17, 2026 at 6:49 AM Sasha Levin <sashal@kernel.org> wrote:
>>>>>* fail_function (CONFIG_FUNCTION_ERROR_INJECTION) is disabled in
>>>>>   most production kernels. Even where enabled, it only works on
>>>>>   functions pre-annotated with ALLOW_ERROR_INJECTION() in source -
>>>>>   no help for a freshly-disclosed CVE. The debugfs UI is blocked by
>>>>>   lockdown=integrity and the override is probabilistic.
>>>>>
>>>>>* BPF override (bpf_override_return) honors the same
>>>>>   ALLOW_ERROR_INJECTION() whitelist, and BPF itself is off in many
>>>>>   production kernels. Even where on, the operator interface is
>>>>>   "load a verified BPF program," not a one-line write.
>>>>
>>>>If it is OK for killswitch to attach to any kernel functions, do we still
>>>>need ALLOW_ERROR_INJECTION() for fail_function and BPF
>>>>override? Shall we instead also allow fail_function and BPF override
>>>>to attach to any kernel functions?
>>>
>>>I don't think so. ALLOW_ERROR_INJECTION is not a security mechanism, it's an
>>>integrity/safety mechanism for both bpf and fault injection.
>>>
>>>It protects against a "developer or CI script doing legitimate fault injection
>>>accidentally panics the box" scenario, not an "attacker gets in" one.
>>
>>There really isn't a clear boundary between "security mechanism" and
>>"non-security mechanism". As we are making killswitch available
>>everywhere under root, users will soon learn to use it to do fault injection,
>>and potentially much more scary things. (Think about agents with sudo
>>access).
>
>Fully agree with Song here that there is no clear boundary, and that the
>killswitch could lead to arbitrary, hard to debug breakage if applied to
>the wrong function.. introducing worse bugs than the one being mitigated
>or even /short-circuit LSM enforcement/ (engage security_file_open 0,
>engage cap_capable 0, engage apparmor_* etc).

This is similar to livepatch, right? Do we need guardrails there too?

Or do we just trust root to do the right thing for it's systems without needing
to be it's babysitter?

>The ALLOW_ERROR_INJECTION() provides a curated white-list where you may
>return with an error without causing more severe damage (assuming the
>error handling code is right). The right thing would be to more widely
>apply ALLOW_ERROR_INJECTION() or to figure out a better way to safely
>enable the latter without explicit function annotation.

Sure, this would also work. How do you see this happening? Can we let a certain
user/pid/etc disable the allowlist if they choose to?

>Wrt BPF:
>
>>>>>* BPF override (bpf_override_return) honors the same
>>>>>   ALLOW_ERROR_INJECTION() whitelist, and BPF itself is off in many
>>>>>   production kernels. Even where on, the operator interface is
>>>>>   "load a verified BPF program," not a one-line write.
>
>The claim that BPF itself is off in many production kernels is not really
>true, where did you get that from? All the major distros and cloud providers
>have BPF enabled these days, and even systemd ships BPF programs for
>custom service firewalling etc.

The world is a bit bigger than home distros and cloud providers, but sure - bpf
is enabled widely enough at this point.

How do you see this working with the allowlist?

-- 
Thanks,
Sasha