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

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Message-ID: <d3dd8fc8-ab46-4cf1-87a9-0324685ba2e0@kernel.org>
Date: Tue, 9 Dec 2025 17:18:03 +0900
From: Daniel Gomez <da.gomez@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>, "Luck, Tony" <tony.luck@...el.com>
Cc: Kees Cook <kees@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>,
 Rusty Russell <rusty@...tcorp.com.au>, Petr Pavlu <petr.pavlu@...e.com>,
 Sami Tolvanen <samitolvanen@...gle.com>, linux-modules@...r.kernel.org,
 Malcolm Priestley <tvboxspy@...il.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>,
 Hans Verkuil <hverkuil@...nel.org>,
 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
 linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH 3/3] module: Add compile-time check for embedded NUL
 characters



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.