[v2,3/3] module: Add compile-time check for embedded NUL characters

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(+)
  

On 10/10/25 5:06 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>

Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
  

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 <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..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 <atomlin@atomlin.com>
  

On 10/10/2025 05:06, 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")

Who will take this series? I can take the first two media patches and
someone else can take this last patch, or I can take all, or someone
else can take all patches. The media patches already have my 'Reviewed-by'.

Any preferences?

Regards,

	Hans

> 
> 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..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
  

On 03/11/2025 09.54, Hans Verkuil wrote:
> On 10/10/2025 05:06, 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")
> 
> Who will take this series? I can take the first two media patches and
> someone else can take this last patch, or I can take all, or someone
> else can take all patches. The media patches already have my 'Reviewed-by'.
> 
> Any preferences?

I will take patch 3 in modules' tree.


> 
> Regards,
> 
> 	Hans
> 
>>
>> Link: https://lwn.net/Articles/82305/ [1]
>> Suggested-by: Rusty Russell <rusty@rustcorp.com.au>
>> Signed-off-by: Kees Cook <kees@kernel.org>

Reviewed-by: Daniel Gomez <da.gomez@samsung.com>
  

On 03/11/2025 09:58, Daniel Gomez wrote:
> On 03/11/2025 09.54, Hans Verkuil wrote:
>> On 10/10/2025 05:06, 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")
>>
>> Who will take this series? I can take the first two media patches and
>> someone else can take this last patch, or I can take all, or someone
>> else can take all patches. The media patches already have my 'Reviewed-by'.
>>
>> Any preferences?
> 
> I will take patch 3 in modules' tree.

OK, then I'll take patches 1 and 2 in the media tree.

Regards,

	Hans

> 
> 
>>
>> Regards,
>>
>> 	Hans
>>
>>>
>>> Link: https://lwn.net/Articles/82305/ [1]
>>> Suggested-by: Rusty Russell <rusty@rustcorp.com.au>
>>> Signed-off-by: Kees Cook <kees@kernel.org>
> 
> Reviewed-by: Daniel Gomez <da.gomez@samsung.com>