From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-8fa8.mail.infomaniak.ch (smtp-8fa8.mail.infomaniak.ch [83.166.143.168]) (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 E2F35221FC8 for ; Sun, 24 Aug 2025 11:03:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=83.166.143.168 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756033408; cv=none; b=pmkK37G5PUx5FMYmPaaGU9xOr7S0JM/7JGEacBMaHy40Q14xcna/hY4RdrF8wMZHRSJoVKiLX8y0znYbbYiRL81c8U7w25VoBuZcz14QsExswX+RM4H7vpA54SRpjZvdbBB3lm1puu8fOU84l7xN3rGW/7K8uAAldedJt9tJfeM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756033408; c=relaxed/simple; bh=zhuxzzhCeIpMw+ZyZuGmrlzOFR/5Ur5sddoaNjY8BH4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Woa4cQCL90B+dL1Fqw1rismCOFWAMxOLxVl+sUmzsu8fPJqjFz8hdG8n64A0XU++3FTRttOpJtCYAodW1pPaHdYKQcUlwObF2P1ewHQXXIk/yjM7Ns9pnYlf2h5pijb9zEV9+rrtkUB4s5jjaJUi3+0m3/dXcaPsFxI47y1ak/Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net; spf=pass smtp.mailfrom=digikod.net; dkim=pass (1024-bit key) header.d=digikod.net header.i=@digikod.net header.b=1WXDHzqg; arc=none smtp.client-ip=83.166.143.168 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=digikod.net Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=digikod.net header.i=@digikod.net header.b="1WXDHzqg" Received: from smtp-3-0000.mail.infomaniak.ch (unknown [IPv6:2001:1600:4:17::246b]) by smtp-3-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4c8rd05FnbzRc5; Sun, 24 Aug 2025 13:03:16 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=digikod.net; s=20191114; t=1756033396; bh=npswCwX0a24vJxWw7gY+qI0TbgVywqqUB4cDV+3jp+w=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=1WXDHzqg/P9JB/nnDxxpueGJBM3RmJ6uqsbQWCIbvmI2W8btShCyr99j6/RnOTwsd zvp2JBaO4ueW7lSCf6nU1zM8s2cg1npfkPPbhf5gTLdumHsaPBFkmqP8jj9SHE39CQ MxIho8S6SakTYtQbqpej882GLb/vs7g55Ho0kuzI= Received: from unknown by smtp-3-0000.mail.infomaniak.ch (Postfix) with ESMTPA id 4c8rcw4v18zt0L; Sun, 24 Aug 2025 13:03:12 +0200 (CEST) Date: Sun, 24 Aug 2025 13:03:09 +0200 From: =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= To: Jann Horn Cc: Al Viro , Christian Brauner , Kees Cook , Paul Moore , Serge Hallyn , Andy Lutomirski , Arnd Bergmann , Christian Heimes , Dmitry Vyukov , Elliott Hughes , Fan Wu , Florian Weimer , Jeff Xu , Jonathan Corbet , Jordan R Abrahams , Lakshmi Ramasubramanian , Luca Boccassi , Matt Bobrowski , Miklos Szeredi , Mimi Zohar , Nicolas Bouchinet , Robert Waite , Roberto Sassu , Scott Shell , Steve Dower , Steve Grubb , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, Andy Lutomirski , Jeff Xu Subject: Re: [RFC PATCH v1 1/2] fs: Add O_DENY_WRITE Message-ID: <20250824.Ujoh8unahy5a@digikod.net> References: <20250822170800.2116980-1-mic@digikod.net> <20250822170800.2116980-2-mic@digikod.net> Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Infomaniak-Routing: alpha On Fri, Aug 22, 2025 at 09:45:32PM +0200, Jann Horn wrote: > On Fri, Aug 22, 2025 at 7:08 PM Mickaël Salaün wrote: > > Add a new O_DENY_WRITE flag usable at open time and on opened file (e.g. > > passed file descriptors). This changes the state of the opened file by > > making it read-only until it is closed. The main use case is for script > > interpreters to get the guarantee that script' content cannot be altered > > while being read and interpreted. This is useful for generic distros > > that may not have a write-xor-execute policy. See commit a5874fde3c08 > > ("exec: Add a new AT_EXECVE_CHECK flag to execveat(2)") > > > > Both execve(2) and the IOCTL to enable fsverity can already set this > > property on files with deny_write_access(). This new O_DENY_WRITE make > > The kernel actually tried to get rid of this behavior on execve() in > commit 2a010c41285345da60cece35575b4e0af7e7bf44.; but sadly that had > to be reverted in commit 3b832035387ff508fdcf0fba66701afc78f79e3d > because it broke userspace assumptions. Oh, good to know. > > > it widely available. This is similar to what other OSs may provide > > e.g., opening a file with only FILE_SHARE_READ on Windows. > > We used to have the analogous mmap() flag MAP_DENYWRITE, and that was > removed for security reasons; as > https://man7.org/linux/man-pages/man2/mmap.2.html says: > > | MAP_DENYWRITE > | This flag is ignored. (Long ago—Linux 2.0 and earlier—it > | signaled that attempts to write to the underlying file > | should fail with ETXTBSY. But this was a source of denial- > | of-service attacks.)" > > It seems to me that the same issue applies to your patch - it would > allow unprivileged processes to essentially lock files such that other > processes can't write to them anymore. This might allow unprivileged > users to prevent root from updating config files or stuff like that if > they're updated in-place. Yes, I agree, but since it is the case for executed files I though it was worth starting a discussion on this topic. This new flag could be restricted to executable files, but we should avoid system-wide locks like this. I'm not sure how Windows handle these issues though. Anyway, we should rely on the access control policy to control write and execute access in a consistent way (e.g. write-xor-execute). Thanks for the references and the background!