From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 214E52D323F; Fri, 19 Dec 2025 12:29:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766147370; cv=none; b=XGjrmJHMj4go22dznSB3Nhx+R99VcZ4LMUCO7hxcCGdm3jSXePB4DBVnBG08PxP9KRpPPVrOyZEQNue3w7O5dbMWIW5+vYzsx75LIx/dwSyoP0ob5rPqi3xGjee46htM02zaxTKFBr1ZdoodWQMIcWFcVYl6T+Tn0AAh1Q+0jOg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766147370; c=relaxed/simple; bh=5snV2hnts8wQy3pn7dDyRNapTGA2u4S2fB1CiR8N34w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HXvLUXYQOE2joMd/vxNArzeYXl1xixlOiIYaJiI9IHVJbyZ79ow4Qbmd6NdPfhavLYiX0zPZ4jHesmia7bxz7MyHen201z50GY6ZlPsjuPt1xjsZYNQ6Y8PVcqk8yt0S3qJKfINhivVUglpGM6SExMYdvFkhhHngQltwGwIYMOI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JntpCx6W; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JntpCx6W" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CBC4CC4CEF1; Fri, 19 Dec 2025 12:29:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766147369; bh=5snV2hnts8wQy3pn7dDyRNapTGA2u4S2fB1CiR8N34w=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=JntpCx6WVFB6NbpwjKrIZJJyUGJgVcpjuBm6pPla+HRgV+d+CEgB8+L0L5ybZY+Ai 4AugsyuDKYQvyUh00X51CcPlv4bNDtG1HYk8v96zkOh3K7Eyawb7CgdzNIGb4QjIUC gOS1b4QUEUzCzm9PKtsrMqB+Rp6yr72i6/Q/z+XRq43MSOdgvS7wfcggdOrBDOwmWl t2iFsjXmuAbKGQq4kzw3hu5txugYmvP3Ayq2m/QvASTAOTzjFJB+oe9ifJVoXVIjLJ rqMOXvFo1XiqW3zOPJ1Eh7O5nVRFiEk6t7y2jED6y/Qstaz6BhNhLPX+4raDkKvaTQ VfUelcX/B4wLg== Message-ID: <47a2f0c7-c25f-4734-840b-fdefc2f3c4a9@kernel.org> Date: Fri, 19 Dec 2025 13:29:21 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/3] module: Add compile-time check for embedded NUL characters Content-Language: en-GB, fr-BE To: Kees Cook , Dan Carpenter Cc: Rusty Russell , Petr Pavlu , Daniel Gomez , Sami Tolvanen , linux-modules@vger.kernel.org, Hans Verkuil , Malcolm Priestley , Mauro Carvalho Chehab , Hans Verkuil , =?UTF-8?Q?Uwe_Kleine-K=C3=B6nig?= , linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linux-hardening@vger.kernel.org, Luis Chamberlain , Chris Li , linux-sparse@vger.kernel.org References: <20251010030348.it.784-kees@kernel.org> <20251010030610.3032147-3-kees@kernel.org> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Kees, Dan, Sorry to react on an oldish patch, but I have a question about it, see below. 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") > > Link: https://lwn.net/Articles/82305/ [1] > Suggested-by: Rusty Russell > Signed-off-by: Kees Cook > --- > Cc: Luis Chamberlain > Cc: Petr Pavlu > Cc: Daniel Gomez > Cc: Sami Tolvanen > Cc: > --- > 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"); \ When checking MPTCP code on top of Linus tree, I get this new warning with all MPTCP KUnit tests (net/mptcp/*_test.c), e.g. $ touch net/mptcp/crypto_test.c && make C=1 net/mptcp/crypto_test.o CC [M] net/mptcp/crypto_test.o CHECK net/mptcp/crypto_test.c net/mptcp/crypto_test.c:72:1: error: bad integer constant expression net/mptcp/crypto_test.c:72:1: error: static assertion failed: "MODULE_INFO(license, ...) contains embedded NUL byte" net/mptcp/crypto_test.c:73:1: error: bad integer constant expression net/mptcp/crypto_test.c:73:1: error: static assertion failed: "MODULE_INFO(description, ...) contains embedded NUL byte" I'm using Sparse last development version with Dan's commit: $ sparse --version v0.6.4-73-gfbdde312 => fbdde312 ("builtin: implement __builtin_strlen() for constants") And the two lines causing the warnings don't have "\0": 72 MODULE_LICENSE("GPL"); 73 MODULE_DESCRIPTION("KUnit tests for MPTCP Crypto"); Am I missing something? Cheers, Matt -- Sponsored by the NGI0 Core fund.