Re: [PATCH 3/3] module: Add compile-time check for embedded NUL characters

Petr Pavlu
2025-10-08T09:55:52Z

On 10/8/25 5:59 AM, 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 <rusty@rustcorp.com.au>
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
> Cc: Luis Chamberlain <mcgrof@kernel.org>
> Cc: Petr Pavlu <petr.pavlu@suse.com>
> Cc: Daniel Gomez <da.gomez@kernel.org>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: <linux-modules@vger.kernel.org>
> ---
> include/linux/moduleparam.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
> index 6907aedc4f74..160f1678fafa 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

Nit: I think it is better to use static_assert() over _Static_assert()
for consistency. Note also that C23 [1, 2] introduces static_assert()
with the message being optional, which essentially matches the
static_assert() macro in include/linux/build_bug.h, and deprecates
_Static_assert().

[1] https://en.cppreference.com/w/c/language/_Static_assert.html
[2] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf (C23 similar draft)

--
Thanks,
Petr