linux-hardening - Re: [PATCH v2 3/3] module: Add compile-time check for embedded NUL characters

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Message-ID: <1c3e9bc8-0d25-46b2-98a3-643157f78b21@kernel.org>
Date: Mon, 3 Nov 2025 09:58:15 +0100
From: Daniel Gomez <da.gomez@...nel.org>
To: Hans Verkuil <hverkuil+cisco@...nel.org>, Kees Cook <kees@...nel.org>,
 Luis Chamberlain <mcgrof@...nel.org>
Cc: 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 v2 3/3] module: Add compile-time check for embedded NUL
 characters

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@...tcorp.com.au>
>> Signed-off-by: Kees Cook <kees@...nel.org>

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