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

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Message-Id: <20251008035938.838263-3-kees@kernel.org>
Date: Tue,  7 Oct 2025 20:59:35 -0700
From: Kees Cook <kees@...nel.org>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: Kees Cook <kees@...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: [PATCH 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@...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..160f1678fafa 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