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

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Message-ID: <20251209001139.GA7982@quark>
Date: Mon, 8 Dec 2025 16:11:39 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: "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>,
	Daniel Gomez <da.gomez@...nel.org>,
	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 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/

- Eric