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

[Sasha Levin <sashal@kernel.org>]

On Mon, May 11, 2026 at 06:14:27AM -0700, Breno Leitao wrote:
>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

This was a woopsie on my end: originally I planned to have a reason field, but
then decided to drop it to keep the patch simple. However, I forgot to fix up
the kconfig help text :(

If you think it'll be useful, I'm happy to add it back.

>> +#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)
>        |                ^~~~~~~~~

Will fix, thanks!

-- 
Thanks,
Sasha