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

Daniel Gomez
2025-12-09T08:18:10Z

On 09/12/2025 09.11, Eric Biggers wrote:
> On Mon, Dec 08, 2025 at 01:05:55PM -0800, Luck, Tony wrote:
>> 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.
>
> Likewise, I just got the following kernel test robot report sent to me,
> where it's warning about MODULE_LICENSE("GPL"):
> https://lore.kernel.org/all/202512090359.7BkUaiC9-lkp@intel.com/

Can you both confirm which version of sparse are you using?

My understanding was that this patch fixed that problem:
https://lore.kernel.org/linux-sparse/CACePvbVG2KrGQq4cNKV=wbO5h=jp3M0RO1SdfX8kV4OukjPG8A@mail.gmail.com/T/#mf838b3e2e3245d88c30a801ea7473d5a5c0eb121

The patch is already merged into the sparse tree, and I was not able to
reproduce the issue.