On Thu, Oct 09, 2025 at 08:06:09PM -0700, Kees Cook wrote:
> Long ago, the kernel module license checks were bypassed by embedding a
> NUL character in the MODULE_LICENSE() string[1]. By using a string like
> "GPL\0proprietary text", the kernel would only read "GPL" due to C string
> termination at the NUL byte, allowing proprietary modules to avoid kernel
> tainting and access GPL-only symbols.
>
> The MODULE_INFO() macro stores these strings in the .modinfo ELF
> section, and get_next_modinfo() uses strcmp()-family functions
> which stop at the first NUL. This split the embedded string into two
> separate .modinfo entries, with only the first part being processed by
> license_is_gpl_compatible().
>
> Add a compile-time check using static_assert that compares the full
> string length (sizeof - 1) against __builtin_strlen(), which stops at
> the first NUL. If they differ, compilation fails with a clear error
> message.
>
> While this check can still be circumvented by modifying the ELF binary
> post-compilation, it prevents accidental embedded NULs and forces
> intentional abuse to require deliberate binary manipulation rather than
> simple source-level tricks.
>
> Build tested with test modules containing both valid and invalid license
> strings. The check correctly rejects:
>
> MODULE_LICENSE("GPL\0proprietary")
>
> while accepting normal declarations:
>
> MODULE_LICENSE("GPL")
>
> Link: https://lwn.net/Articles/82305/ [1]
> Suggested-by: Rusty Russell <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Luis Chamberlain <[email protected]>
> Cc: Petr Pavlu <[email protected]>
> Cc: Daniel Gomez <[email protected]>
> Cc: Sami Tolvanen <[email protected]>
> Cc: <[email protected]>
> ---
> include/linux/moduleparam.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
> index 6907aedc4f74..915f32f7d888 100644
> --- a/include/linux/moduleparam.h
> +++ b/include/linux/moduleparam.h
> @@ -26,6 +26,9 @@
>
> /* Generic info of form tag = "info" */
> #define MODULE_INFO(tag, info) \
> + static_assert( \
> + sizeof(info) - 1 == __builtin_strlen(info), \
> + "MODULE_INFO(" #tag ", ...) contains embedded NUL byte"); \
> static const char __UNIQUE_ID(modinfo)[] \
> __used __section(".modinfo") __aligned(1) \
> = __MODULE_INFO_PREFIX __stringify(tag) "=" info
> --
> 2.34.1
>
>
Nice!
Reviewed-by: Aaron Tomlin <[email protected]>
--
Aaron Tomlin
