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

[Breno Leitao <leitao@debian.org>]

helo Sasha,

First of all, Thanks for this feature, this is useful to me, and I am
interested in it. Feel free to copy me and I can test the next revisions

On Fri, May 08, 2026 at 03:57:48PM -0400, Sasha Levin wrote:

> +config KILLSWITCH
> +	bool "Killswitch: short-circuit a kernel function as a CVE mitigation"
> +	depends on SECURITYFS
> +	depends on KPROBES && HAVE_KPROBES_ON_FTRACE
> +	depends on HAVE_FUNCTION_ERROR_INJECTION
> +	select FUNCTION_ERROR_INJECTION
> +	help
> +	  Provide an admin-facing mechanism to make a chosen kernel function
> +	  return a fixed value without executing its body, as a temporary
> +	  mitigation for a security bug before a real fix is available.
> +
> +	  Operators write "engage <symbol> <retval> [reason]" to

Should [reason] be shown at "engaged" ? I was expecting it, and in fact find it
very useful, but I don't see it.

	# echo "engage __x64_sys_getuid 12 CVE-2026-99999-INCIDENT-4242" > /sys/kernel/security/killswitch/control
	# cat /sys/kernel/security/killswitch/engaged
	__x64_sys_getuid retval=12 hits=12

 
> +#if IS_ENABLED(CONFIG_KUNIT)
> +#include <kunit/test.h>
> +
> +/* Non-static so kallsyms resolves them without CONFIG_KALLSYMS_ALL. */
> +int ks_kunit_target_int(int x);
> +void *ks_kunit_target_ptr(int x);
> +
> +/* noipa keeps the call out-of-line and uneliminated. */
> +__attribute__((__noipa__)) int ks_kunit_target_int(int x)
> +{
> +	return x + 1;
> +}
> +
> +__attribute__((__noipa__)) void *ks_kunit_target_ptr(int x)
> +{
> +	return ERR_PTR(-EIO);
> +}

When compiling with LLVM=1, I get the following error:

        kernel/killswitch.c:708:16: error: unknown attribute '__noipa__' ignored [-Werror,-Wunknown-attributes]
        708 | __attribute__((__noipa__)) int ks_kunit_target_int(int x)
        |                ^~~~~~~~~
        kernel/killswitch.c:713:16: error: unknown attribute '__noipa__' ignored [-Werror,-Wunknown-attributes]
        713 | __attribute__((__noipa__)) void *ks_kunit_target_ptr(int x)
        |                ^~~~~~~~~

Thanks,
--breno