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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <n5mctxqun4vsohpgy4o63cktohc6okj56qnzjakeqfdwzokndi@trpzynr7smwc>
Date: Mon, 20 Oct 2025 22:05:36 -0400
From: Aaron Tomlin <atomlin@...mlin.com>
To: Kees Cook <kees@...nel.org>
Cc: 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, Hans Verkuil <hverkuil+cisco@...nel.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 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@...tcorp.com.au>
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Luis Chamberlain <mcgrof@...nel.org>
> Cc: Petr Pavlu <petr.pavlu@...e.com>
> Cc: Daniel Gomez <da.gomez@...nel.org>
> Cc: Sami Tolvanen <samitolvanen@...gle.com>
> Cc: <linux-modules@...r.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@...mlin.com>

-- 
Aaron Tomlin