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

Luck, Tony
2025-12-08T21:06:01Z

On Tue, Oct 07, 2025 at 08:59:35PM -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")

I did a "make W=1 C=1" and found that sparse is now unhappy with all MODULE_LICENSE(),
MODULE_PARM_DESC(), MODULE_DESCRIPTION(), MODULE_AUTHOR() defintions (with no NUL byte).

I see:

error: bad integer constant expression
error: static assertion failed: "MODULE_INFO(parmtype, ...) contains embedded NUL byte"

for every use.

-Tony